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.
if BOOLEAN EXPRESSION:
STATEMENTS_1 # executed if condition evaluates to Trueelse:
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.
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 (:).
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).
Approximating Pi with Simulation Section 21.7 In this guided lab exercise we will work through a problem solving exercise related to approximating the value of pi using random numbers.
Python will always execute either the if-block (if the condition is true) or the else-block (if the condition is false). It would never skip over both blocks.
Although TRUE is printed after the if-else statement completes, both blocks within the if-else statement print something too. In this case, Python would have had to have skipped both blocks in the if-else statement, which it never would do.