Python Control Flow

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

• 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")

while loops

• 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.

for loops

• 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.

break and continue

• 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=' ')

Loops with an else Block

• 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)