Easy Programing

ลำดับตอนที่ #8 : Go I - Using Variables

Basics of Variables

What are Variables?

Now we will learn about variables. You can think of a variable as a box where you can put data (values). Also, each variable has a name like variableName, and values can be accessed using the name. Variables are needed for almost any program, so it's important to understand how to use them.

Variables and Data Types

Variables must be defined before you can use them. To define a variable, you can write var valueName dataType as shown below. Here, var is an abbreviation of the word variable. By writing var, you give the computer an instruction to create a variable.

Data Types

So far, we have learned the types of values using words like "strings" and "integers". These are actually called data types. First, remember the two data types written below; strings and integers. Strings are a series of characters, while ints are integer values.

Using Variables

The code n = 100 will assign the value 100 to the variable n. Unlike the "=" used in math, this does not mean the left and right sides are equal. In programming, it is a symbol used for inserting a value in a variable "box". Also, writing println(n) will print the value of n to the console.

Definition and Assignment

By writing var n int = 100, you can define a variable and assign it a value simultaneously.

Updating Variables

Variables can be rewritten (updated) as well. In order to update variables, new values must be assigned using the = operator. Below, the variable n is printed 2 times and the values 100 and 200 are displayed in the console. This is because n gets updated with the value 200 before the second println.

Variable Types

Abbreviating Data Types

In Go, when variable definitions and value assignment are done at the same time, like var a int = 100, the data type can be abbreviated. In the example, "int" is left out so that only var a = 100 is written. This is because as 100 is obviously an integer, it can be safely assumed that the data type assigned to the variable a is an integer.

Abbreviating Variable Definitions

Variable definitions can be written without a data type by using a shortened syntax. As shown below, var b int = 200 and b := 200 are the same. Note the colon before the = symbol (:=). This can be very convenient, so give it a try!

Other Ways to Use println

There is another useful way to use println. In the example, the variables a and b used with println are separated by a comma in the parentheses. This allows you to print two values at the same time. Note that the comma is not printed and that they are printed on a single line with a space.

Variables & Errors

Variable Definitions and Assignment

The same variable cannot be defined multiple times. On the left, a is defined as a := 200 right after being defined using a := 100 (:= is a variable definition). Here, an error meaning "there is no new variable on the left of :=" appears in the console.

Where Variables Can Be Used

Variables can only be used after they are defined. In the example below, n is used before it is defined. The console shows an error message meaning "variable n is undefined".

Unused Variables

In Go, when variables are defined but are left unused, errors are displayed. In the example on the left, b is defined but not used. Because of this, an error is displayed, as shown on the right. Since the unused variable is often times a mistake, Go prevents bugs from appearing by showing these kinds of error messages.

Type Mismatch

Values that do not match variable data types cannot be assigned. When a variable defined with the int type (n := 100) is assigned a string type value, the error message meaning "string type value cannot be assigned to int type" is shown. You can find and fix mistakes by reading the error messages!

Self-Assignment

Calculations with Variables

Since variables can be used in the same way as values, calculations can be made using variables. As in the example below, addition between variables and integers, variables and variables can be seen.

Self-Assignment

Below, on the left, n = n + 20 may seem strange when you think of it in a mathematical way. However, since = means "assignment" in Go, it should make sense. The code n = n + 20 adds 20 to the original value of n (10) and assigns that value to n (this is called self-assignment). The final value of n will be 30, as shown in the console.

Shortened  Self-Assignment

Self-assignment means to assign the result of a calculation using the variable's original value back into the same variable. This technique is convenient and commonly used in programming. As you can see below, n = n + 10 can be abbreviated as n += 10, and this is true the symbols -, *, /, and %.

When you want to add or subtract 1 from a variable like n = n + 1 or n = n - 1, you can write it in an even shorter way. ++ means "add 1 to the variable". Also, -- means "subtract 1 from the variable. This is very commonly used, so be sure to remember this!