Conditional Statements

Conditional Statements

Statements normally run in sequence. A conditional statement chooses which block runs. Python has if, if-else and if-elif-else; a block is marked by its indentation.

n = int(input())
if n < 0:
    n = -n          # absolute value without abs()
print(n)

A classic program sorts three numbers using only comparisons. Each swap puts a smaller value in front:

a, b, c = 30, 10, 20
if a > b:
    a, b = b, a
if b > c:
    b, c = c, b
if a > b:
    a, b = b, a
print(a, b, c)   # 10 20 30

After the first two comparisons the largest value has moved to c; the third comparison puts the remaining two in order. Draw the flowchart: each if is one diamond.

a, b, c = map(int, input().split())
if a > b:
    a, b = b, a
# two more comparisons are needed
print(a, b, c)