If, for, and while

• There are some control statements in Python.

if statement

• if statement can be written as follows

  if a > 0:
      print("a is lager than zero")
  else:
      print("a is smaller than zero")
  

• You can use else statement to add the instructions, as

  if a > 0:
      print("a is positive")
  elif a == 0:
      print("a is zero")
  else:
      print("a is negative")
  

for statement

• In Python, the code block is expressed by indent.
• Either tab or spaces are OK for indent but do not mix them.
• for statement can be written as follows:

  for i in [0, 1, 2]:
      print(i)
  

• range function is useful when using for statement.

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

• You can access the index and the value of the list with enumerate function.

  a = [0, 1, 2]
  for i, val in enumerate(a):
      print(i, val)
  

while statement

• If you want to repeat the procedure based on some condition, you can use while statement

  a = 10
  while a > 0:
      print(a)
      a -= 1
  

continue, break; the loop control

• If you want to skip the loop at some condition, you can use continue statement.

  for i in range(10):
      if i == 5:
      continue
  else:
      print(i)
  

• The for loop goes to the next step when it finds continue. So rest of the part is not executed.
• If you want to end the loop at some condition, you can use break statement.

  for i in range(10):
      if i == 5:
          break
      else:
        print(i)
  

• In this way, the loop after finding i == 5 is not executed.

Exercise

1. Write a program that checks if x is greater than y. If x is greater, print "x is greater than y"; otherwise, print "y is greater than or equal to x".
2. Create a list containing at least five elements (numbers or strings). Use a for loop to iterate through the list and print each element.

answer