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      How To Convert Data Types in Go


      Introduction

      In Go, data types are used to classify one particular type of data, determining the values that you can assign to the type and the operations you can perform on it. When programming, there are times when you will need to convert values between types in order to manipulate values in a different way. For example, you may need to concatenate numeric values with strings, or represent decimal places in numbers that were initialized as integer values. User-generated data is often automatically assigned the string data type, even if it consists of numbers; in order to perform mathematical operations in this input, you would have to convert the string to a numeric data type.

      Since Go is a statically typed language, data types are bound to variables rather than values. This means that, if you define a variable as an int, it can only be an int; you can’t assign a string to it without converting the data type of the variable. The static nature of data types in Go places even more importance on learning the ways to convert them.

      This tutorial will guide you through converting numbers and strings, as well as provide examples to help familiarize yourself with different use cases.

      Converting Number Types

      Go has several numeric types to choose from. Primarily they break out into two general types: integers and floating-point numbers.

      There are many situations in which you may want to convert between numeric types. Converting between different sizes of numeric types can help optimize performance for specific kinds of system architecture. If you have an integer from another part of your code and want to do division on it, you may want to convert the integer to a float to preserve the precision of the operation. Additionally, working with time durations usually involves integer conversion. To address these situations, Go has built-in type conversions for most numeric types.

      Converting Between Integer Types

      Go has many integer data types to pick from. When to use one over the other is typically more about performance; however, there will be times when you will need to convert from one integer type to another. For example, Go sometimes automatically generates numeric values as int, which may not match your input value. If your input value were int64, you would not be able to use the int and the int64 numbers in the same mathematical expression until you converted their data types to match.

      Assume that you have an int8 and you need to convert it to an int32. You can do this by wrapping it in the int32() type conversion:

      var index int8 = 15
      
      var bigIndex int32
      
      bigIndex = int32(index)
      
      fmt.Println(bigIndex)
      

      Output

      15

      This code block defines index as an int8 data type and bigIndex as an int32 data type. To store the value of index in bigIndex, it converts the data type to an int32. This is done by wrapping the int32() conversion around the index variable.

      To verify your data types, you could use the fmt.Printf statement and the %T verb with the following syntax:

      fmt.Printf("index data type:    %Tn", index)
      fmt.Printf("bigIndex data type: %Tn", bigIndex)
      

      Output

      index data type: int8 bigIndex data type: int32

      Since this uses the %T verb, the print statement outputs the type for the variable, and not the actual value of the variable. This way, you can confirm the converted data type.

      You can also convert from a larger bit-size integer to a smaller bit-size integer:

      var big int64 = 64
      
      var little int8
      
      little = int8(big)
      
      fmt.Println(little)
      

      Output

      64

      Keep in mind that when converting integers you could potentially exceed the maximum value of the data type and wraparound:

      var big int64 = 129
      var little = int8(big)
      fmt.Println(little)
      

      Output

      -127

      A wraparound happens when the value is converted to a data type that is too small to hold it. In the preceding example, the 8-bit data type int8 did not have enough space to hold the 64-bit variable big. Care should always be taken when converting from a larger number data type to a smaller number data type so that you do not truncate the data by accident.

      Converting Integers to Floats

      Converting integers to floats in Go is similar to converting one integer type to another. You can use the built-in type conversions by wrapping float64() or float32() around the integer you are converting:

      var x int64 = 57
      
      var y float64 = float64(x)
      
      fmt.Printf("%.2fn", y)
      

      Output

      57.00

      This code declares a variable x of type int64 and initializes its value to 57.

      var x int64 = 57
      

      Wrapping the float64() conversion around x will convert the value of 57 to a float value of 57.00.

      var y float64 = float64(x)
      

      The %.2f print verb tells fmt.Printf to format the float with two decimals.

      You can also use this process on a variable. The following code declares f as equal to 57, and then prints out the new float:

      var f float64 = 57
      fmt.Printf("%.2fn", f)
      

      Output

      57.00

      By using either float32() or float64(), you can convert integers to floats. Next, you will learn how to convert floats to integers.

      Converting Floats to Integers

      Go can convert floats to integers, but the program will lose the precision of the float.

      Wrapping floats in int(), or one of its architecture-independent data types, works similarly to when you used it to convert from one integer type to another. You can add a floating-point number inside of the parentheses to convert it to an integer:

      var f float64 = 390.8
      var i int = int(f)
      
      fmt.Printf("f = %.2fn", f)
      fmt.Printf("i = %dn", i)
      

      Output

      f = 390.80 i = 390

      This syntax would convert the float 390.8 to the integer 390, dropping the decimal place.

      You can also use this with variables. The following code declares b as equal to 125.0 and c as equal to 390.8, then prints them out as integers. Short variable declaration (:=) shortens up the syntax:

      b := 125.0
      c := 390.8
      
      fmt.Println(int(b))
      fmt.Println(int(c))
      

      Output

      125 390

      When converting floats to integers with the int() type, Go cuts off the decimal and remaining numbers of a float to create an integer. Note that, even though you may want to round 390.8 up to 391, Go will not do this through the int() type. Instead, it will drop the decimal.

      Numbers Converted Through Division

      When dividing integer types in Go the result will also be an integer type, with the modulus, or remainder, dropped:

      a := 5 / 2
      fmt.Println(a)
      

      Output

      2

      If, when dividing, any of the number types are a float, then all of the types will automatically be declared as a float:

          a := 5.0 / 2
          fmt.Println(a)
      

      Output

      2.5

      This divides the float 5.0 by the integer 2, and the answer 2.5 is a float that retains the decimal precision.

      In this section, you have converted between different number data types, including differing sizes of integers and floating-point numbers. Next, you will learn how to convert between numbers and strings.

      Converting with Strings

      A string is a sequence of one or more characters (letters, numbers, or symbols). Strings are a common form of data in computer programs, and you may need to convert strings to numbers or numbers to strings fairly often, especially when you are taking in user-generated data.

      Converting Numbers to Strings

      You can convert numbers to strings by using the strconv.Itoa method from the strconv package in the Go standard libary. If you pass either a number or a variable into the parentheses of the method, that numeric value will be converted into a string value.

      First, let’s look at converting integers. To convert the integer 12 to a string value, you can pass 12 into the strconv.Itoa method:

      package main
      
      import (
          "fmt"
          "strconv"
      )
      
      func main() {
          a := strconv.Itoa(12)
          fmt.Printf("%qn", a)
      }
      

      When running this program, you’ll receive the following output:

      Output

      "12"

      The quotes around the number 12 signify that the number is no longer an integer but is now a string value.

      You used the := assignment operator to both declare a new variable with the name of a and assign the value returned from the strconv.Itoa() function. In this case, you assigned the value 12 to your variable. You also used the %q verb in the fmt.Printf function, which tells the function to quote the string provided.

      With variables you can begin to see how practical it can be to convert integers to strings. Say you want to keep track of a user’s daily programming progress and are inputting how many lines of code they write at a time. You would like to show this feedback to the user and will be printing out string and integer values at the same time:

      package main
      
      import (
          "fmt"
      )
      
      func main() {
          user := "Sammy"
          lines := 50
      
          fmt.Println("Congratulations, " + user + "! You just wrote " + lines + " lines of code.")
      }
      

      When you run this code, you’ll receive the following error:

      Output

      invalid operation: ("Congratulations, " + user + "! You just wrote ") + lines (mismatched types string and int)

      You’re not able to concatenate strings and integers in Go, so you’ll have to convert the variable lines to be a string value:

      package main
      
      import (
          "fmt"
          "strconv"
      )
      
      func main() {
          user := "Sammy"
          lines := 50
      
          fmt.Println("Congratulations, " + user + "! You just wrote " + strconv.Itoa(lines) + " lines of code.")
      }
      

      Now, when you run the code, you’ll receive the following output that congratulates your user on their progress:

      Output

      Congratulations, Sammy! You just wrote 50 lines of code.

      If you are looking to convert a float to a string rather than an integer to a string, you follow similar steps and format. When you pass a float into the fmt.Sprint method, from the fmt package in the Go standard library, a string value of the float will be returned. You can use either the float value itself or a variable:

      package main
      
      import (
          "fmt"
      )
      
      func main() {
          fmt.Println(fmt.Sprint(421.034))
      
          f := 5524.53
          fmt.Println(fmt.Sprint(f))
      }
      

      Output

      421.034 5524.53

      You can test to make sure it’s right by concatenating with a string:

      package main
      
      import (
          "fmt"
      )
      
      func main() {
          f := 5524.53
          fmt.Println("Sammy has " + fmt.Sprint(f) + " points.")
      }
      

      Output

      Sammy has 5524.53 points.

      You can be sure your float was properly converted to a string because the concatenation was performed without error.

      Converting Strings to Numbers

      Strings can be converted to numbers by using the strconv package in the Go standard library. The strconv package has functions for converting both integer and float number types. This is a very common operation when accepting input from the user. For example, if you had a program that asked for a person’s age, when they type the response in, it is captured as a string. You would then need to convert it to an int to do any math with it.

      If your string does not have decimal places, you’ll most likely want to convert it to an integer by using the strconv.Atoi function. If you know you will use the number as a float, you would use strconv.ParseFloat.

      Let’s use the example of the user Sammy keeping track of lines of code written each day. You may want to manipulate those values with math to provide more interesting feedback for the user, but those values are currently stored in strings:

      package main
      
      import (
          "fmt"
      )
      
      func main() {
          lines_yesterday := "50"
          lines_today := "108"
      
          lines_more := lines_today - lines_yesterday
      
          fmt.Println(lines_more)
      }
      

      Output

      invalid operation: lines_today - lines_yesterday (operator - not defined on string)

      Because the two numeric values were stored in strings, you received an error. The operand - for subtraction is not a valid operand for two string values.

      Modify the code to include the strconv.Atoi() method that will convert the strings to integers, which will allow you to do math with values that were originally strings. Because there is a potential to fail when converting a string to an integer, you have to check for any errors. You can use an if statement to check if your conversion was successful.

      package main
      
      import (
          "fmt"
          "log"
          "strconv"
      )
      
      func main() {
          lines_yesterday := "50"
          lines_today := "108"
      
          yesterday, err := strconv.Atoi(lines_yesterday)
          if err != nil {
              log.Fatal(err)
          }
      
          today, err := strconv.Atoi(lines_today)
          if err != nil {
              log.Fatal(err)
          }
          lines_more := today - yesterday
      
          fmt.Println(lines_more)
      }
      

      Because it is possible for a string to not be a number, the strconv.Atoi() method will return both the converted type, as well as a potential error. When converting from lines_yesterday with the strconv.Atoi function, you have to check the err return value to ensure that the value was converted. If the err is not nil, it means that strconv.Atoi was unable to successfully convert the string value to an integer. In this example, you used an if statement to check for the error, and if an error was returned, you used log.Fatal to log the error and exit the program.

      When you run the preceding code, you will get:

      Output

      58

      Now try to convert a string that is not a number:

      package main
      
      import (
          "fmt"
          "strconv"
      )
      
      func main() {
          a := "not a number"
          b, err := strconv.Atoi(a)
          fmt.Println(b)
          fmt.Println(err)
      }
      

      You will get the following error:

      Output

      0 strconv.Atoi: parsing "not a number": invalid syntax

      Because b was declared, but strconv.Atoi failed to make a conversion, a value was never assigned to b. Notice that b has the value of 0. This is because Go has default values, referred to as zero values in Go. strconv.Atoi provides an error describing why it failed to convert the string as well.

      Converting Strings and Bytes

      Strings in Go are stored as a slice of bytes. In Go, you can convert between a slice of bytes and a string by wrapping it in the corresponding conversions of []byte() and string():

      package main
      
      import (
          "fmt"
      )
      
      func main() {
          a := "my string"
      
          b := []byte(a)
      
          c := string(b)
      
          fmt.Println(a)
      
          fmt.Println(b)
      
          fmt.Println(c)
      }
      

      Here you have stored a string value in a, then converted it to a slice of bytes b, then converted the slice of bytes back to a string as c. You then print a, b, and c to the screen:

      Output

      my string [109 121 32 115 116 114 105 110 103] my string

      The first line of output is the original string my string. The second line printed out is the byte slice that makes up the original string. The third line shows that the byte slice can be safely converted back into a string and printed back out.

      Conclusion

      This Go tutorial demonstrated how to convert several of the important native data types to other data types, primarily through built-in methods. Being able to convert data types in Go will allow you to do things like accept user input and do math across different number types. Later on, when you are using Go to write programs that accept data from many different sources like databases and APIs, you will use these conversion methods to ensure you can act on your data. You will also be able to optimize storage by converting data to smaller data types.

      If you would like a deeper analysis of data types in Go, check out our Understanding Data Types in Go article.



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      Understanding Data Types in Go


      Introduction

      Data types specify the kinds of values that particular variables will store when you are writing a program. The data type also determines what operations can be performed on the data.

      In this article, we will go over the important data types native to Go. This is not an exhaustive investigation of data types, but will help you become familiar with what options you have available to you in Go. Understanding some basic data types will enable you to write clearer code that performs efficiently.

      Background

      One way to think about data types is to consider the different types of data that we use in the real world. An example of data in the real world are numbers: we may use whole numbers (0, 1, 2, …), integers (…, -1, 0, 1, …), and irrational numbers (π), for example.

      Usually, in math, we can combine numbers from different types, and get some kind of an answer. We may want to add 5 to π, for example:

      5 + π
      

      We can either keep the equation as the answer to account for the irrational number, or round π to a number with an abbreviated number of decimal places, and then add the numbers together:

      5 + π = 5 + 3.14 = 8.14 
      

      But, if we start to try to evaluate numbers with another data type, such as words, things start to make less sense. How would we solve for the following equation?

      shark + 8
      

      For computers, each data type is quite different—like words and numbers. As a result we have to be careful about how we use varying data types to assign values and how we manipulate them through operations.

      Integers

      Like in math, integers in computer programming are whole numbers that can be positive, negative, or 0 (…, -1, 0, 1, …). In Go, an integer is known as an int. As with other programming languages, you should not use commas in numbers of four digits or more, so when you write 1,000 in your program, write it as 1000.

      We can print out an integer in a simple way like this:

      fmt.Println(-459)
      

      Output

      -459

      Or, we can declare a variable, which in this case is a symbol of the number we are using or manipulating, like so:

      var absoluteZero int = -459
      fmt.Println(absoluteZero)
      

      Output

      -459

      We can do math with integers in Go, too. In the following code block, we will use the := assignment operator to declare and instantiate the variable sum:

      sum := 116 - 68
      fmt.Println(sum)
      

      Output

      48

      As the output shows, the mathematical operator - subtracted the integer 68 from 116, resulting in 48. You’ll learn more about variable declaration in the Declaring Data Types for Variables section.

      Integers can be used in many ways within Go programs. As you continue to learn about Go, you’ll have a lot of opportunities to work with integers and build upon your knowledge of this data type.

      Floating-Point Numbers

      A floating-point number or a float is used to represent real numbers that cannot be expressed as integers. Real numbers include all rational and irrational numbers, and because of this, floating-point numbers can contain a fractional part, such as 9.0 or -116.42. For the purposes of thinking of a float in a Go program, it is a number that contains a decimal point.

      Like we did with integers, we can print out a floating-point number in a simple way like this:

      fmt.Println(-459.67)
      

      Output

      -459.67

      We can also declare a variable that stands in for a float, like so:

      absoluteZero := -459.67
      fmt.Println(absoluteZero)
      

      Output

      -459.67

      Just like with integers, we can do math with floats in Go, too:

      var sum = 564.0 + 365.24
      fmt.Println(sum)
      

      Output

      929.24

      With integers and floating-point numbers, it is important to keep in mind that 3 ≠ 3.0, as 3 refers to an integer while 3.0 refers to a float.

      Sizes of Numeric Types

      In addition to the distinction between integers and floats, Go has two types of numeric data that are distinguished by the static or dynamic nature of their sizes. The first type is an architecture-independent type, which means that the size of the data in bits does not change, regardless of the machine that the code is running on.

      Most system architectures today are either 32 bit or 64 bit. For instance, you may be developing for a modern Windows laptop, on which the operating system runs on a 64-bit architecture. However, if you are developing for a device like a fitness watch, you may be working with a 32-bit architecture. If you use an architecture-independent type like int32, regardless of the architecture you compile for, the type will have a constant size.

      The second type is an implementation-specific type. In this type, the bit size can vary based on the architecture the program is built on. For instance, if we use the int type, when Go compiles for a 32-bit architecture, the size of the data type will be 32 bits. If the program is compiled for a 64-bit architecture, the variable will be 64 bits in size.

      In addition to data types having different sizes, types like integers also come in two basic types: signed and unsigned. An int8 is a signed integer, and can have a value from -128 to 127. A uint8 is an unsigned integer, and can only have a positive value of 0 to 255.

      The ranges are based on the bit size. For binary data, 8 bits can represent a total of 256 different values. Because an int type needs to support both positive and negative values, an 8-bit integer (int8) will have a range of -128 to 127, for a total of 256 unique possible values.

      Go has the following architecture-independent integer types:

      uint8       unsigned  8-bit integers (0 to 255)
      uint16      unsigned 16-bit integers (0 to 65535)
      uint32      unsigned 32-bit integers (0 to 4294967295)
      uint64      unsigned 64-bit integers (0 to 18446744073709551615)
      int8        signed  8-bit integers (-128 to 127)
      int16       signed 16-bit integers (-32768 to 32767)
      int32       signed 32-bit integers (-2147483648 to 2147483647)
      int64       signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
      

      Floats and complex numbers also come in varying sizes:

      float32     IEEE-754 32-bit floating-point numbers
      float64     IEEE-754 64-bit floating-point numbers
      complex64   complex numbers with float32 real and imaginary parts
      complex128  complex numbers with float64 real and imaginary parts
      

      There are also a couple of alias number types, which assign useful names to specific data types:

      byte        alias for uint8
      rune        alias for int32
      

      The purpose of the byte alias is to make it clear when your program is using bytes as a common computing measurement in character string elements, as opposed to small integers unrelated to the byte data measurement. Even though byte and uint8 are identical once the program is compiled, byte is often used to represent character data in numeric form, whereas uint8 is intended to be a number in your program.

      The rune alias is a bit different. Where byte and uint8 are exactly the same data, a rune can be a single byte or four bytes, a range determined by int32. A rune is used to represent a Unicode character, whereas only ASCII characters can be represented solely by an int32 data type.

      In addition, Go has the following implementation-specific types:

      uint     unsigned, either 32 or 64 bits
      int      signed, either 32 or 64 bits
      uintptr  unsigned integer large enough to store the uninterpreted bits of a pointer value 
      

      Implementation-specific types will have their size defined by the architecture the program is compiled for.

      Picking Numeric Data Types

      Picking the correct size usually has more to do with performance for the target architecture you are programming for than the size of the data you are working with. However, without needing to know the specific ramifications of performance for your program, you can follow some of these basic guidelines when first starting out.

      As discussed earlier in this article, there are architecture-independent types, and implementation-specific types. For integer data, it’s common in Go to use the implementation types like int or uint instead of int64 or uint64. This will typically result in the fastest processing speed for your target architecture. For instance, if you use an int64 and compile to a 32-bit architecture, it will take at least twice as much time to process those values as it takes additional CPU cycles to move the data across the architecture. If you used an int instead, the program would define it as a 32-bit size for a 32-bit architecture, and would be significantly faster to process.

      If you know you won’t exceed a specific size range, then picking an architecture-independent type can both increase speed and decrease memory usage. For example, if you know your data won’t exceed the value of 100, and will only be a positive number, then choosing a uint8 would make your program more efficient as it will require less memory.

      Now that we have looked at some of the possible ranges for numeric data types, let’s look at what will happen if we exceed those ranges in our program.

      Overflow vs. Wraparound

      Go has the potential to both overflow a number and wraparound a number when you try to store a value larger than the data type was designed to store, depending on if the value is calculated at compile time or at runtime. A compile time error happens when the program finds an error as it tries to build the program. A runtime error happens after the program is compiled, while it is actually executing.

      In the following example, we set maxUint32 to its maximum value:

      package main
      
      import "fmt"
      
      func main() {
          var maxUint32 uint32 = 4294967295 // Max uint32 size
          fmt.Println(maxUint32)
      }
      

      It will compile and run with the following result:

      Output

      4294967295

      If we add 1 to the value at runtime, it will wraparound to 0:

      Output

      0

      On the other hand, let’s change the program to add 1 to the variable when we assign it, before compile time:

      package main
      
      import "fmt"
      
      func main() {
          var maxUint32 uint32 = 4294967295 + 1
          fmt.Println(maxUint32)
      
      }
      

      At compile time, if the compiler can determine a value will be too large to hold in the data type specified, it will throw an overflow error. This means that the value calculated is too large for the data type you specified.

      Because the compiler can determine it will overflow the value, it will now throw an error:

      Output

      prog.go:6:36: constant 4294967296 overflows uint32

      Understanding the boundaries of your data will help you avoid potential bugs in your program in the future.

      Now that we have covered numeric types, let’s look at how to store boolean values.

      Booleans

      The boolean data type can be one of two values, either true or false, and is defined as bool when declaring it as a data type. Booleans are used to represent the truth values that are associated with the logic branch of mathematics, which informs algorithms in computer science.

      The values true and false will always be with a lowercase t and f respectively, as they are pre-declared identifiers in Go.

      Many operations in math give us answers that evaluate to either true or false:

      • greater than
        • 500 > 100 true
        • 1 > 5 false
      • less than
        • 200 < 400 true
        • 4 < 2 false
      • equal
        • 5 = 5 true
        • 500 = 400 false

      Like with numbers, we can store a boolean value in a variable:

      myBool := 5 > 8
      

      We can then print the boolean value with a call to the fmt.Println() function:

      fmt.Println(myBool)
      

      Since 5 is not greater than 8, we will receive the following output:

      Output

      false

      As you write more programs in Go, you will become more familiar with how booleans work and how different functions and operations evaluating to either true or false can change the course of the program.

      Strings

      A string is a sequence of one or more characters (letters, numbers, symbols) that can be either a constant or a variable. Strings exist within either back quotes ` or double quotes " in Go and have different characteristics depending on which quotes you use.

      If you use the back quotes, you are creating a raw string literal. If you use the double quotes, you are creating an interpreted string literal.

      Raw String Literals

      Raw string literals are character sequences between back quotes, often called back ticks. Within the quotes, any character will appear just as it is displayed between the back quotes, except for the back quote character itself.

      a := `Say "hello" to Go!`
      fmt.Println(a)
      

      Output

      Say "hello" to Go!

      Usually, backslashes are used to represent special characters in strings. For example, in an interpreted string, n would represent a new line in a string. However, backslashes have no special meaning inside of raw string literals:

      a := `Say "hello" to Go!n`
      fmt.Println(a)
      

      Because the backslash has no special meaning in a string literal, it will actually print out the value of n instead of making a new line:

      Output

      Say "hello" to Go!n

      Raw string literals may also be used to create multiline strings:

      a := `This string is on 
      multiple lines
      within a single back 
      quote on either side.`
      fmt.Println(a)
      

      Output

      This string is on multiple lines within a single back quote on either side.

      In the preceding code blocks, the new lines were carried over literally from input to output.

      Interpreted String Literals

      Interpreted string literals are character sequences between double quotes, as in "bar". Within the quotes, any character may appear except newline and unescaped double quotes. To show double quotes in an interpreted string, you can use the backslash as an escape character, like so:

      a := "Say "hello" to Go!"
      fmt.Println(a)
      

      Output

      Say "hello" to Go!

      You will almost always use interpreted string literals because they allow for escape characters within them. For more on working with strings, check out An Introduction to Working with Strings in Go.

      Strings with UTF-8 Characters

      UTF-8 is an encoding scheme used to encode variable width characters into one to four bytes. Go supports UTF-8 characters out of the box, without any special setup, libaries, or packages. Roman characters such as the letter A can be represented by an ASCII value such as the number 65. However, with special characters such as an international character of , UTF-8 would be required. Go uses the rune alias type for UTF-8 data.

      a := "Hello, 世界"
      

      You can use the range keyword in a for loop to index through any string in Go, even a UTF-8 string. for loops and range will be covered in more depth later in the series; for now, it’s important to know that we can use this to count the bytes in a given string:

      package main
      
      import "fmt"
      
      func main() {
          a := "Hello, 世界"
          for i, c := range a {
              fmt.Printf("%d: %sn", i, string(c))
          }
          fmt.Println("length of 'Hello, 世界': ", len(a))
      }
      

      In the above code block, we declared the variable a and assigned the value of Hello, 世界 to it. The text assigned has UTF-8 characters in it.

      We then used a standard for loop as well as the range keyword. In Go, the range keyword will index through a string returning one character at a time, as well as the byte index the character is at in the string.

      Using the fmt.Printf function, we provided a format string of %d: %sn. %d is the print verb for a digit (in this case an integer), and %s is the print verb for a string. We then provided the values of i, which is the current index of the for loop, and c, which is the current character in the for loop.

      Finally, we printed the entire length of the variable a by using the builtin len function.

      Earlier, we mentioned that a rune is an alias for int32 and can be made up of one to four bytes. The character takes three bytes to define and the index moves accordingly when ranging through the UTF-8 string. This is the reason that i is not sequential when it is printed out.

      Output

      0: H 1: e 2: l 3: l 4: o 5: , 6: 7: 世 10: 界 length of 'Hello, 世界': 13

      As you can see, the length is longer than the number of times it took to range over the string.

      You won’t always be working with UTF-8 strings, but when you are, you’ll now understand why they are runes and not a single int32.

      Declaring Data Types for Variables

      Now that you know about the different primitive data types, we will go over how to assign these types to variables in Go.

      In Go, we can define a variable with the keyword var followed by the name of the variable and the data type desired.

      In the following example we will declare a variable called pi of type float64.

      The keyword var is the first thing declared:

      var pi float64
      

      Followed by the name of our variable, pi:

      var pi float64
      

      And finally the data type float64:

      var pi float64
      

      We can optionally specify an initial value as well, such as 3.14:

      var pi float64 = 3.14
      

      Go is a statically typed language. Statically typed means that each statement in the program is checked at compile time. It also means that the data type is bound to the variable, whereas in dynamically linked languages, the data type is bound to the value.

      For example, in Go, the type is declared when declaring a variable:

      var pi float64 = 3.14
      var week int = 7
      

      Each of these variables could be a different data type if you declared them differently.

      This is different from a language like PHP, where the data type is associated to the value:

      $s = "sammy";         // $s is automatically a string
      $s = 123;             // $s is automatically an integer
      

      In the preceding code block, the first $s is a string because it is assigned the value "sammy", and the second is an integer because it has the value 123.

      Next, let’s look at more complex data types like arrays.

      Arrays

      An array is an ordered sequence of elements. The capacity of an array is defined at creation time. Once an array has allocated its size, the size can no longer be changed. Because the size of an array is static, it means that it only allocates memory once. This makes arrays somewhat rigid to work with, but increases performance of your program. Because of this, arrays are typically used when optimizing programs. Slices, covered next, are more flexible, and constitute what you would think of as arrays in other languages.

      Arrays are defined by declaring the size of the array, then the data type with the values defined between curly brackets { }.

      An array of strings looks like this:

      [3]string{"blue coral", "staghorn coral", "pillar coral"}
      

      We can store an array in a variable and print it out:

      coral := [3]string{"blue coral", "staghorn coral", "pillar coral"}
      fmt.Println(coral)
      

      Output

      [blue coral staghorn coral pillar coral]

      As mentioned before, slices are similar to arrays, but are much more flexible. Let’s take a look at this mutable data type.

      Slices

      A slice is an ordered sequence of elements that can change in length. Slices can increase their size dynamically. When you add new items to a slice, if the slice does not have enough memory to store the new items, it will request more memory from the system as needed. Because a slice can be expanded to add more elements when needed, they are more commonly used than arrays.

      Slices are defined by declaring the data type preceded by an opening and closing square bracket [] and having values between curly brackets { }.

      A slice of integers looks like this:

      []int{-3, -2, -1, 0, 1, 2, 3}
      

      A slice of floats looks like this:

      []float64{3.14, 9.23, 111.11, 312.12, 1.05}
      

      A slice of strings looks like this:

      []string{"shark", "cuttlefish", "squid", "mantis shrimp"}
      

      Let’s define our slice of strings as seaCreatures:

      seaCreatures := []string{"shark", "cuttlefish", "squid", "mantis shrimp"}
      

      We can print them out by calling the variable:

      fmt.Println(seaCreatures)
      

      The output will look exactly like the list that we created:

      Output

      [shark cuttlefish squid mantis shrimp]

      We can use the append keyword to add an item to our slice. The following command will add the string value of seahorse to the slice:

      seaCreatures = append(seaCreatures, "seahorse")
      

      You can verify it was added by printing it out:

      fmt.Println(seaCreatures)
      

      Output

      [shark cuttlefish squid mantis shrimp seahorse]

      As you can see, if you need to manage an unknown size of elements, a slice will be much more versatile than an array.

      Maps

      The map is Go’s built-in hash or dictionary type. Maps use keys and values as a pair to store data. This is useful in programming to quickly look up values by an index, or in this case, a key. For instance, you may want to keep a map of users, indexed by their user ID. The key would be the user ID, and the user object would be the value. A map is constructed by using the keyword map followed by the key data type in square brackets [ ], followed by the value data type and the key value pairs in curly braces.

      map[key]value{}
      

      Typically used to hold data that are related, such as the information contained in an ID, a map looks like this:

      map[string]string{"name": "Sammy", "animal": "shark", "color": "blue", "location": "ocean"}
      

      You will notice that in addition to the curly braces, there are also colons throughout the map. The words to the left of the colons are the keys. Keys can be any comparable type in Go. Comparable types are primitive types like strings, ints, etc. A primitive type is defined by the language, and not built from combining any other types. While they can be user-defined types, it’s considered best practice to keep them simple to avoid programming errors. The keys in the dictionary above are: name, animal, color, and location.

      The words to the right of the colons are the values. Values can be comprised of any data type. The values in the dictionary above are: Sammy, shark, blue, and ocean.

      Let’s store the map inside a variable and print it out:

      sammy := map[string]string{"name": "Sammy", "animal": "shark", "color": "blue", "location": "ocean"}
      fmt.Println(sammy)
      

      Output

      map[animal:shark color:blue location:ocean name:Sammy]

      If we want to isolate Sammy’s color, we can do so by calling sammy["color"]. Let’s print that out:

      fmt.Println(sammy["color"])
      

      Output

      blue

      As maps offer key-value pairs for storing data, they can be important elements in your Go program.

      Conclusion

      At this point, you should have a better understanding of some of the major data types that are available for you to use in Go. Each of these data types will become important as you develop programming projects in the Go language.



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