Section 4.2 Conditional Execution: Binary Selection
Now that we know how to express “True or False” questions in Python, it’s time to put them to use. 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, 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.
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.
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 indented statements that follow are called a block.
The statements inside the first block of statements are executed when the boolean expression evaluates to True. The entire first block of statements is skipped when 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.
We will see more compound statements later in this course. In general, all compound statements include a heading and all the following further-indented statements in the block after the heading.
Checkpoint 4.2.1.
How many statements can appear in each block (the if and the else) in a conditional statement?
Just one.
- Each block may also contain more than one.
Zero or more.
- Each block must contain at least one statement.
One or more.
- Yes, a block must contain at least one statement and can have many statements.
One or more, and each must contain the same number of statements.
- The blocks may contain different numbers of statements.
Checkpoint 4.2.2.
What does the following code print (choose from output a, b, c or nothing)?
if 4 + 5 == 10:
print("Math is broken")
else:
print("Math is correct")
Math is broken
- “Math is broken” is printed by the
if block, which only executes if the conditional (in this case, 4 + 5 == 10) is true. However, 4 + 5 is not equal to 10.
Math is correct
- Since 4 + 5 == 10 evaluates to
False, Python will skip over the if block and execute the statement in the else block.
“Math is broken” on one line and “Math is correct” on the next
- Python would never print both sentences because it will only execute either the
if block or the else block, but not both.
Nothing will be printed
- Python will always execute either the
if block (when the condition is true) or the else block (when the condition is false). It would never skip over both blocks.
Checkpoint 4.2.3.
What does the following code print?
if 5 % 2 == 1:
print("odd")
else:
print("even")
print("finished")
a.
finished
b.
odd
finished
c.
odd
even
finished
Output a
Although finished is printed after the if-else statement completes, both blocks within the if-else statement print something too. For this choice to be correct, Python would have to skip both blocks in the if-else statement, which it never would do.
Output b
Python will print odd from within the else block (because 5 % 2 does equal 1), and then print finished after the if-else statement completes.
Output c
To print these three lines, Python would have to execute both blocks in the if-else statement, which it can never do.
