1.9 while loops

Important

This lesson is still under development.

Suppose we have a list of names of people who have ruled Pakistan with one name in it who has not been the ruler and we want to find out at which position the name of this person is located. One way to solve this problem is to use while loops.

looters = ['sikandar mirza', 'ayub khan', 'yahya khan', 'zulfiqar bhutto', 'shahid afridi',
           'zia-ul-haq', 'benazir', 'nawaz sharif', 'musharaf',   'zardari']

acc_id = 0
while acc_id<len(looters):   # this condition must become False at some point.
    if looters[acc_id] == 'shahid afridi':
        print('Found a normal person at position', acc_id)
    else:
        print(looters[acc_id], 'was a thug')
    acc_id += 1

sikandar mirza was a thug
ayub khan was a thug
yahya khan was a thug
zulfiqar bhutto was a thug
Found a normal person at position 4
zia-ul-haq was a thug
benazir was a thug
nawaz sharif was a thug
musharaf was a thug
zardari was a thug

The basic syntax of while statement in python is:

while (condition):
    do something

The condition after while must become False after some time otherwise the loop will continue indefinitely. Consider not increasing the value of acc_id in upper example and the print statement will continue forever until we have to stop it forcefully by terminating the program. (In case you do do this, you can stop this by going to Runtime –> Interrupt execution.)

while with else

while condition:
  do something
else:
  do something at last

looters = ['sikandar mirza', 'ayub khan', 'yahya khan', 'zulfiqar bhutto', 'shahid afridi',
           'zia-ul-haq', 'benazir', 'nawaz sharif', 'musharaf', 'zardari']

acc_id = 0
while acc_id<len(looters):
    if looters[acc_id] == 'shahid afridi':
        print('Found a normal person at position', acc_id)
    else:
        print(looters[acc_id], 'was a thug')
    acc_id += 1
else:
    print("Search finished from the whole list of 'looters'")

sikandar mirza was a thug
ayub khan was a thug
yahya khan was a thug
zulfiqar bhutto was a thug
Found a normal person at position 4
zia-ul-haq was a thug
benazir was a thug
nawaz sharif was a thug
musharaf was a thug
zardari was a thug
Search finished from the whole list of 'looters'

looters = ['sikandar mirza', 'ayub khan', 'yahya khan', 'zulfiqar bhutto', 'shahid afridi',
           'zia-ul-haq', 'benazir', 'nawaz sharif', 'musharaf', 'zardari']

acc_id = 0
while acc_id<len(looters):
    if looters[acc_id] == 'shahid afridi':
        print('Found a normal person at position ', acc_id)
    else:
        print(looters[acc_id], 'was a thug')
    acc_id += 1

print("Search finished from the whole list of 'looters'")

sikandar mirza was a thug
ayub khan was a thug
yahya khan was a thug
zulfiqar bhutto was a thug
Found a normal person at position  4
zia-ul-haq was a thug
benazir was a thug
nawaz sharif was a thug
musharaf was a thug
zardari was a thug
Search finished from the whole list of 'looters'

We may think what is the use of else statement? Of course we can achieve same thing by just placing the code inside else statement, without making use of else.

Wouldn’t it be better if we just stop the search after we found the thief. Here comes the benefit of break statement along with else keyword.

looters = ['sikandar mirza', 'ayub khan', 'yahya khan', 'zulfiqar bhutto', 'shahid afridi',
           'zia-ul-haq', 'benazir', 'nawaz sharif', 'musharaf', 'zardari']

acc_id = 0
while acc_id<len(looters):
    if looters[acc_id] == 'shahid afridi':
        print('Found a normal person at position', acc_id, '. No need to continue searching anymore')
        break
    else:
        print(looters[acc_id], 'was a thug')
    acc_id += 1
else:
    print("Search finished from the whole list of 'looters'")

sikandar mirza was a thug
ayub khan was a thug
yahya khan was a thug
zulfiqar bhutto was a thug
Found a normal person at position 4 . No need to continue searching anymore