Decision structures and boolean logic

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Relational operators and boolean expressions

Relational operators are essentials to create decision structures. A decision structure can be represented by a simple flow chart. In this example, we test the logical statement “the lamp is ON”. if the statement is False we do something (“turn lamp ON”). if the statement is True we don’t do anything. In computer science, a statement that can be either True or False is called a boolean expression (from George Boole, a mathematician).

A simple flow chart

In Python there are seven relational (or comparison) operators. These same operators are also used in many other computer languages. They are used to compare the values of two operands: one at the left of the operator and one at its right. It is now easy to think at the operands as numerical variables (let’s say a = 10 and b = 20) but in Python also strings can be compared.

Relational operator	Meaning	Boolean expression
>	Greater than	a > b is False
<	Less than	a < b is True
>=	Greater than or equal to	a >= b is False
<=	Less than or equal to	a <= b is True
==	Equal to	a == b is False
<>	Not equal to	a <> b is True
!=	Not equal to	a != b is True
Relational operators and exaples with a = 10 and b = 20.
Note: != is similar to <>, but != is more common

The if statement

Relational operators can be used in an if statement. In Python, an if statement is defined with a block of at least two lines. The first line contains the condition - a logical statement - that will be tested by the interpreter. The statements in the lines below the first line that are indented will be executed only if the condition in the first line is True.

if condition:
    statement 1
    statement 2
    etc.

The if-else statement

The if-else statement simply details what statements to execute if the condition in the first line of the block is False.

if condition:
    statement 1
    statement 2
    etc.
else:
    statement 3
    statement 4
    etc.

In the previous example, statement 3 and statement 4 are executed by the intepreter only if the condition in the first line is False, in which case statement 1 and statement 2 are not executed. Simple. Just be careful to the indentation!

Exercises

In the following cells, replace ____ so that the intepreter will always print yes.

a = ____
b = ____
if a == b:
    print('yes')
else:
    print('no')

a = ____
b = ____
if a <= b:
    print('yes')
else:
    print('no')

a = ____
b = ____
if a != b:
    print('yes')
else:
    print('no')

Additional resources

• Conditional structures on Lynda.com.