The most commonly used operator in a computer language is the value assignment operator ( = ). For example, the statement

PHP

$x = 5;

Java, C++, C#

x = 5;

Visual Basic, Python

x = 5

assigns a value of 5 to variable x.

As you already know, this is equivalent to the left arrow used in flowcharts.

Probably the left arrow used in a flowchart is more convenient and clearer than the value assignment operator ( = ) sign because it shows in a more graphical way that the value or the result of an expression on the right is assigned to a variable on the left.

Be careful! The ( = ) sign is not equivalent to the one used in mathematics. In mathematics, the expression x = 5 is read as “x is equal to 5. However, in a computer language the expression x = 5 is read as “assign the value 5 to x” or “set x equal to 5.” They look the same but they act differently!

In mathematics, the following two lines are equivalent:

x = y + z

y + z = x

The first one can be read as “x is equal to the sum of y and z” and the second one as “the sum of y and z is equal to x.”

On the other hand, in a computer language, the following two statements are definitely not equivalent. In fact, the second one is considered wrong!

PHP


$x = $y + $z;
$y + $z = $z;

Java, C++, C#


x = y + z;
y + z = x;

Visual Basic, Python


x = y + z
y + z = x


The first statement seems quite correct. It can be read as “Set x equal to the sum of y and z” or “Assign the sum of y and z to x.

But what about the second one? Think! Is it possible to assign the value of x to y + z? The answer is obviously a big “NO!”

Remember! In a computer language, the variable on the left side of the ( = ) sign represents a region in main memory (RAM) where a value can be stored. Thus, on the left side only one single variable must exist! However, on the right side there can be a number, a variable, a string, or even a complex mathematical expression.

In Table 1 you can find some examples of value assignments.

PHP

Table 1 Examples of Value Assignments
$a = 9 Assign a value of 9 to variable a.
$b = $c Assign the content of variable c to variable b.
$d = "Hello Zeus" Assign the string Hello Zeus to variable d.
$d = $a + $b Calculate the sum of the contents of variables a and b and assign the result to variable d.
$b = $a + 1 Calculate the sum of the content of variable a and 1 and assign the result to variable b. Please note that the content of variable a is not altered.
$a = $a + 1 Calculate the sum of the content of variable a and 1 and assign the result back to variable a. In other words, increase variable a by one.

Java, C++, C#

Table 1 Examples of Value Assignments
a = 9 Assign a value of 9 to variable a.
b = c Assign the content of variable c to variable b.
d = "Hello Zeus" Assign the string Hello Zeus to variable d.
d = a + b Calculate the sum of the contents of variables a and b and assign the result to variable d.
b = a + 1 Calculate the sum of the content of variable a and 1 and assign the result to variable b. Please note that the content of variable a is not altered.
a = a + 1 Calculate the sum of the content of variable a and 1 and assign the result back to variable a. In other words, increase variable a by one.

Visual Basic, Python

Table 1 Examples of Value Assignments
a = 9 Assign a value of 9 to variable a.
b = c Assign the content of variable c to variable b.
d = "Hello Zeus" Assign the string Hello Zeus to variable d.
d = a + b Calculate the sum of the contents of variables a and b and assign the result to variable d.
b = a + 1 Calculate the sum of the content of variable a and 1 and assign the result to variable b. Please note that the content of variable a is not altered.
a = a + 1 Calculate the sum of the content of variable a and 1 and assign the result back to variable a. In other words, increase variable a by one.


Confused about the last one? Are you thinking about your math teacher right now? What would he/she say if you had written a = a + 1 on the blackboard? Can you personally think of a number that is equal to the number itself plus one? Are you nuts? This means that 5 is equal to 6 and 10 is equal to 11!

Obviously, things are different in computer science. The statement a = a + 1 is absolutely acceptable! It instructs the CPU to retrieve the value of variable a from main memory (RAM), to increase the value by one, and to assign the result back to variable a. The old value of variable a is replaced by the new one.

Still don’t get it? Let’s take a look at how the CPU and main memory (RAM) cooperate with each other in order to execute the statement A ← A + 1.

Let’s say that there is a region in memory, named A and it contains the number 13

When a program instructs the CPU to execute the statement

A ← A + 1

the following procedure is executed:

  • the number 13 is transferred from the RAM’s region named A to the CPU;
  • the CPU calculates the sum of 13 and 1; and
  • the result, 14, is transferred from the CPU to the RAM’s region named A replacing the existing number, 13.

After execution, the RAM looks like this.

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