If—elif—else statements

8. If—elif—else statements#

Sometimes you want greater control over the sequence of code execution, and that’s where control flow comes in. Here we’ll work with the if, elif (else if), and else constructs for conditional statements.

Conditional statements#

In Python, the general structure is like this:

if condition1:
    do something only when condition1 is True
elif condition2:
    otherwise, do something if condition2 is True (and condition1 was False)
else:
    otherwise, do something

Note

if is required, and the other two are optional.

Note

In MATLAB, control flow blocks have an end to conclude. No need for that in Python!

Part 1#

Suppose you are buying apples at a supermarket. If you buy more than \(5\) apples, you get a \(10\%\) discount. Write a program computing the amount owed given the price per apple (assume it to be \(\$1\)) and the number of apples purchased.

n = 6       # number of apples purchased
price = 1   # price of one apple

if n > 5:
    cost = 0.9 * price * n
else:
    cost = price * n

""" this is a block comment, showing another solution
cost = price * n
if n > 5:
    cost *= 0.9
"""
print(f"Amount owed for {n} apples: ${cost:.2f}")

Amount owed for 6 apples: $5.40

In the above, we used f-strings, which Enze thinks is one of the coolest things since sliced bread. 🍞

Part 2#

Suppose now the store offers a super discount of \(40\%\) if you buy more than \(20\) apples. Modify the code in Part 1 to implement this new condition using the elif statement. Note that when programming conditional statements, we generally put equality conditions first, followed by bounded inequalities, and finally unbounded conditions.

n = 21       # number of apples purchased
price = 1    # price of one apple

if n > 5 and n <= 20:
    cost = 0.9 * price * n
elif n > 20:
    cost = 0.6 * price * n
else:
    cost = price * n

print(f"Amount owed for {n} apples: ${cost:.2f}")

Amount owed for 21 apples: $12.60

Important

In Python, logical “and” uses the actual word and instead of the symbols && as it’s commonly done in other languages. Similarly, logical “or” uses the actual word or. Finally, please don’t confuse these keywords with the literal strings 'and' and 'or'!

Contents