Using relational operators

What you'll learn

• How relational operators compare values and produce True or False.
• The Python 3 symbols for equal to, less than, greater than, and their variants.
• How to use comparisons inside if, elif, else and while.
• How to avoid common boundary and equality mistakes.

Start point: values, variables and Boolean results

A value is a piece of data, such as 42, "yes" or True.

A variable is a named storage location for a value, such as score or username.

A data type is the kind of data a value has. For example:

• an integer is a whole number, such as 17
• a string is text, usually written in quotation marks, such as "Sam"
• a Boolean value can only be True or False

An expression is a piece of code that works out to a value. For example, score + 5 is an expression because Python can calculate its result.

What is a relational operator?

A comparison such as score >= 50 is called a relational expression. It asks a yes/no question: “Is score greater than or equal to 50?”

The six relational operators you need

For Pearson Edexcel GCSE Computer Science, you need to be able to write programs using all six of these relational operators.

Translating English into operators

Some common wording in questions maps neatly to operators:

• “exactly”, “is equal to” → ==
• “different from”, “is not” → !=
• “less than”, “under” → <
• “more than”, “over” → >
• “at most”, “no more than”, “up to and including” → <=
• “at least”, “minimum”, “or more” → >=

For example, “the user must be at least 13” means age >= 13, not age > 13, because 13 itself is allowed.

Equality is not assignment

In Python, assignment means storing a value in a variable. Assignment uses one equals sign:

score = 50

Testing whether two values are equal uses two equals signs:

score == 50

Correct equality check:

pin = input("Enter PIN: ")

if pin == "2468":
    print("Access granted")
else:
    print("Access denied")

Here, pin == "2468" is the Boolean condition. It will be either True or False.

Using relational operators in selection

Selection is when a program chooses which path to take. In Python, selection usually uses if, elif and else.

A condition is an expression that is tested by if or while. It must evaluate to True or False.

mark = int(input("Enter mark: "))

if mark >= 80:
    print("Distinction")
elif mark >= 50:
    print("Pass")
else:
    print("Fail")

elif means “else if”. Python only checks an elif if the previous if condition was False.

The order of tests matters. If you test mark >= 50 before mark >= 80, then a mark of 90 would be accepted as a pass before the program ever reaches the distinction test.

Boundary values: choosing < or <=

A boundary value is a value at the edge of a range, such as 0 or 100 for a percentage mark.

An inclusive boundary includes the edge value. Use <= or >=.

An exclusive boundary does not include the edge value. Use < or >.

For example, if valid scores are from 0 to 100 inclusive, then both 0 and 100 are allowed.

score = int(input("Enter score: "))

if score >= 0 and score <= 100:
    print("Valid score")
else:
    print("Invalid score")

The word and is Python’s version of the logical operator AND. It means both comparisons must be True.

Using relational operators in iteration

Iteration means repeating code. A while loop repeats while its condition is True.

The != operator is especially useful when a program should keep going until a particular value is entered.

choice = input("Enter A to add or Q to quit: ")

while choice != "Q":
    print("Adding item")
    choice = input("Enter A to add or Q to quit: ")

Data types matter when comparing input

In Python, input() always returns a string. If you want to do a numerical comparison, convert the input first.

age_text = input("Enter your age: ")
age = int(age_text)

if age >= 18:
    print("Adult")
else:
    print("Under 18")

String comparisons are fine when you are checking exact text, such as a menu option, username or PIN. Remember that strings are case-sensitive: "Q" and "q" are different values.

Comparing calculated values

Each side of a relational operator can be a calculation, not just a single variable.

correct_answers = int(input("Correct answers: "))
marks = correct_answers * 2

if marks >= 10:
    print("Pass")
else:
    print("Try again")

Python calculates marks first, then compares it with 10. The comparison marks >= 10 still produces a Boolean result.