Python "while" Statement for Execution Loop

Computer History Notes - Herong's Tutorial Notes

∟1991 - Python Language Designed by Guido van Rossum

∟Python "while" Statement for Execution Loop

This section provides a quick introduction of 'while' statement, which executes a block of sub-statements repeatedly while the given condition is valid.

One unique feature of Python is that it uses indentation to define sub-statement blocks instead of curly brackets.

For example, a "while" statement is a compound statement that executes a block of sub-statements repeatedly while the given condition is valid.

The "while" statement syntax in Python is shown below:

while condition:
  sub-statement
  sub-statement
  ...
else:
  sub-statement
  sub-statement
  ...

Here is a Python sample code, while_test.py, that shows you how to use "while" statements.

#  while_test.py
#- Copyright 2011 (c) HerongYang.com. All Rights Reserved.

upperLimit = 20
i = 3
while i<upperLimit:
   isPrime = True
   j = 2
   while j<=i//2:
      isPrime = i%j > 0
      if not isPrime:
         break
      j = j + 1

   if isPrime:
      print("   "+str(i)+" is a prime number.")
   i = i + 1

print("Reached the upper limit "+str(upperLimit))

If you run this sample code, you should get:

herong> python while_test.py
   3 is a prime number.
   5 is a prime number.
   7 is a prime number.
   11 is a prime number.
   13 is a prime number.
   17 is a prime number.
   19 is a prime number.
Reached the upper limit 20

Notice that we have a "while" statement nested inside another "while" statement. The "break" statement inside the inner "while" statement only breaks the immediate containing "while" statement, not the outer "while" statement.