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Looping

• Executing a given block of code a number of times is achieved using either the for or while statements

For … else

• The syntax to loop with a for statement is

for ''target'' in sequence:
    ''block-of-code-to-execute''

where target gets the value of the current number in sequence.

• A sequence from the loop can be a predefined sequence that has been constructed previously within the code or the range(start, end, step) function can be used to generate a sequence of numbers from start to end-1 in steps of step. The default step value is 1. E.g.

for i in range(0,10):
    print(i)

gives the output

0
1
2
3
4
5
6
7
8
9

for i in range(0,10,2):
    print(i)

gives the output

0
2
4
6
8

• Extra loop control is provided by the break and continue statements. break causes the loop to terminate at that point without executing any other code in the block,

nums = [1,2,3,-1,5,6]
list_ok = True
for i in nums:
    if i < 0:
        list_ok = False
        break

if list_ok == False:
    print('The list contains a negative number')

gives the output

The list contains a negative number

• continue causes execution to immediately jump to the next iteration of the loop,

nums =  [1,2,3,-1,5,6]
pos_sum = 0
for i in nums:
    if i < 0:
        continue
    pos_sum += i     # compound assignment means pos_sum = pos_sum + i

print('Sum of positive numbers is ' + str(pos_sum))

gives the output

Sum of positive numbers is 17

• An optional else clause can be added after the loop that will only get executed if the whole loop executes successfully,

for i in range(0,10):
    print(i)
else:
    print('done')     # Prints numbers 0-9 and the 'done'

gives the output

0
1
2
3
4
5
6
7
8
9
done

for i in range(0,10):
    if i == 5:
        break
    print(i)
else:
    print('done')     # Prints numbers 0-4

gives the output

0
1
2
3
4

While

• While is another looping statement that simple executes until a given statement is False,

sum = 0
while sum < 10:
    sum += 1   # ALWAYS remember to update the loop test or it will
                       # run forever!!

print(sum)      # Gives value 10

gives the output

10

• The while loop also supports the else syntax in the same manner as the for loop

Category:Tested Examples