Lecture 3

if, elif and else

if statements let a program make decisions.

Where this is taught

Lecture slides

Notebooks (open in Colab)

One chain, one branch

Python checks the conditions from the top and runs the first branch that is True; the rest of the chain is skipped. else takes no condition. Indentation decides which lines belong to each branch.

ph = 6.5
if ph < 7:
    print("acid")
elif ph == 7:
    print("neutral")
else:
    print("base")

Output

acid

Decisions inside a loop

Combine a loop with an if to count or collect only the items that pass a test.

evens = 0
for v in [2, 7, 4, 9]:
    if v % 2 == 0:
        evens += 1
print(evens)

Output

Separate ifs are not a chain

Two separate if statements are each checked, so both can run. Use elif when only one branch should.

x = 10
if x > 5:
    print("over 5")
if x > 8:
    print("over 8")

Output

over 5
over 8

Going further

From the advanced notebook — this is what the advanced questions test.

Conditional expressions

value_if_true if condition else value_if_false picks one of two values in a single line.

x = 7
print("even" if x % 2 == 0 else "odd")

Output

odd

Filtering with comprehensions

An if at the end of a comprehension keeps only the items that pass; an if/else at the front labels every item. not in checks that something is absent.

values = [3, 8, 1, 9]
print([v for v in values if v > 2])
print(["H" if v > 5 else "L" for v in values])
print([b for b in "ANTG" if b not in "N"])

Output

[3, 8, 9]
['L', 'H', 'L', 'H']
['A', 'T', 'G']

Practise these questions →All concepts