Python Control Flow

Control flow is the order in which statements are evaluated.

There are two main ways to alter sequential control flow:

• Selection: conditional statements

• Iteration: loop statements

Conditional statements select blocks of code to execute based on some Boolean condition.

x = 42
if x == 0:
    print(x, "is zero")
elif x > 0:
  print(x, "is positive")
elif x < 0:
  print(x, "is negative")
else:
  print("this should not happen")

A while loop iterates until some condition is met

i = 0
while i < 10:
    print(i, end=' ')
    i += 1

A while loop is executed until the Boolean expression evaluates to False.

Loops are a way to repeatedly execute a block of code

The Python for loop is for iterating through a sequence:

for N in [2, 3, 5, 7]:
    print(N, end=' ')

The range object generates a sequence of numbers

for i in range(10):
    print(i)

The arguments to range are integers (start, stop, step) where stop is exclusive and the start and step are optional.

There are two statements that can alter how loops are executed:

• The break statement breaks out of the loop entirely

• The continue statement skips the remainder of the current iteration

for n in range(20):
    if n == 10:
        break # exit the loop if n equals 10
    if n % 2 == 0:
        continue # skip the rest of the loop
    print(n, end=' ')

Python allows a loop to have an else statement which is executed if the loop does not encounter a break statement.

L = []
nmax = 30

for n in range(2, nmax):
    for factor in L:
        if n % factor == 0:
            break
    else: # no break
        L.append(n)