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.
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.
As with the function definition from the last chapter and other compound
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
else statement is an unusual compound statement because it
has more than one part at the same level of indentation as the
else clause, with its own indented block).
Approximating Pi with Simulation In this guided lab exercise we will work through a problem solving exercise related to approximating the value of pi using random numbers.
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