CS61A: Conditional Statements

Statements

• Statements are executed by the interpreter to perform an action
• A Compound Statement is a statement that can span over multiple lines.
  • A Clause is a single header followed by a series of statements
    • The statements within a clause is known as the Suite
• An example of a compound statement is a function declaration.
  • The suite of a function is known as the function body.

<header>:           // The first header determines the type of a statement
    <statement>
    <statement>
    ...
<separating header>:    // Headers of a clause "controls" the suite that follows
    <statement>
    <statement>
    ...
...

• When we execute a suite, we run the sequence of the statements within the suite in order.
• Execution Rule:
  • Execute the first statement
  • Unless instructed otherwise, execute the remaining statements.

Conditional Statement

• Execution rule for conditional statements: 1) Evaluate the header’s expression
  2) If the header’s expression returns true, execute the suite and then skip the remaining clauses.

• Only one suite gets executed in a conditional statement.
• George Boole: Logician.
  • Boolean contexts are about expressions that either evaluate to true or false.
    • Ex: x < 0 or x == 0
• False values in Python: False, 0, '', [], None
• True values in Python: True, Anything else

def absolute_value(x):
    """Return the absolute value of x."""
    if (x<0):
        return -x
    elif x == 0:
        return 0
    else:
        return x

absolute_value(-2)

2