7.4. Conditional Execution: Binary Selection¶

In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Selection statements, sometimes also referred to as conditional statements, give us this ability. The simplest form of selection is the if statement. This is sometimes referred to as binary selection since there are two possible paths of execution.

x: int

x = 15

​

if x % 2 == 0:

    print(str(x) + " is even")

else:

    print(str(x) + " is odd")

​

The syntax for an if statement looks like this:

if BOOLEAN EXPRESSION:
    STATEMENTS_1        # executed if condition evaluates to True
else:
    STATEMENTS_2        # executed if condition evaluates to False

The boolean expression after the if statement is called the condition. If it is true, then the immediately following indented statements get executed. If not, then the statements indented under the else clause get executed.

Flowchart of a if statement with an else

As with the function definition from the last chapter and other compound statements like for, the if statement consists of a header line and a body. The header line begins with the keyword if followed by a boolean expression and ends with a colon (:).

The more indented statements that follow are called a block.

Each of the statements inside the first block of statements is executed in order if the boolean expression evaluates to True. The entire first block of statements is skipped if the boolean expression evaluates to False, and instead all the statements under the else clause are executed.

There is no limit on the number of statements that can appear under the two clauses of an if statement, but there has to be at least one statement in each block.

Each compound statement includes a heading and all the following further-indented statements in the block after the heading. The if - else statement is an unusual compound statement because it has more than one part at the same level of indentation as the if heading, (the else clause, with its own indented block).

Check your understanding

if 4 + 5 == 10:
    print("TRUE")
else:
    print("FALSE")

if 4 + 5 == 10:
    print("TRUE")
else:
    print("FALSE")
print("TRUE")

a. TRUE

b.
   TRUE
   FALSE

c.
   FALSE
   TRUE
d.
   TRUE
   FALSE
   TRUE