Flow of control

The flow of control is the order in which statements run. By default that is top to bottom, but real programs need to make decisions and repeat work. Three structures cover everything: sequence, selection, and repetition.

Sequence

Statements run one after another, in order.

INPUT length
INPUT breadth
COMPUTE area = length * breadth
PRINT area

length = float(input("Length: "))
breadth = float(input("Breadth: "))
area = length * breadth
print("Area:", area)

Selection

Selection checks a condition and runs different code depending on whether it is true or false. The indented block under an if runs only when the condition holds.

INPUT number
IF number MOD 2 == 0 THEN
    PRINT "Even"
ELSE
    PRINT "Odd"

number = int(input("Enter a number: "))
if number % 2 == 0:
    print("Even")
else:
    print("Odd")

You can chain conditions with elif, and there is no limit on how many statements sit inside a block.

Repetition

Repetition, also called iteration or looping, runs statements again and again until a condition is met. Python has two loops.

The for loop

for walks over a sequence (a range, string, list, or tuple), running the body once per item.

for i in range(1, 6):      # 1, 2, 3, 4, 5
    print(i, i * i)

The while loop

while repeats as long as its condition stays true. The condition is checked before each pass, so if it starts false the body never runs.

total = 0
n = int(input("Number (negative to stop): "))
while n >= 0:
    total += n
    n = int(input("Number (negative to stop): "))
print("Sum:", total)

Changing the flow inside a loop

break ends the loop immediately and continues after it.

for n in range(100):
    if n * n > 50:
        break          # stop at the first square over 50
    print(n)

continue skips the rest of the current pass and jumps to the next one.

for n in range(7):
    if n == 3:
        continue       # print every value except 3
    print(n)

Nested loops

A loop inside another loop. The inner loop runs fully for each pass of the outer one.

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row}x{col}={row * col}", end="  ")
    print()

Exercises

In a Week_4 notebook:

1. Sequence: read a length and breadth, print area and perimeter.
2. Selection: read a number and report whether it is odd or even.
3. Repetition: calculate the factorial of a number the user enters.