python for loop

In this post:

  • Basic loop - for x in range(3, 5):
  • for loop based on list
  • for loop list all files recursively
  • for loop and else clause
  • for loop with continue
  • for loop example get perfect numbers

Basic loop for in python

The syntax of for loop is straight forward and at the same time give you many options. You can see the some basic example below:

for loop from range

for x in range(3, 5):
    print ("%d" % (x))

result:

3
4

for x in range(5):
    print ("%d" % (x))

result:

0
1
2
3
4

for loop based on list

days = ["SUN", "MON", "TUE", "WED", "THU", "FRI", "SAT"]

for day in days:
    print (day)

result:

SUN
MON
TUE
WED
THU
FRI
SAT

for loop list all files recursively

You can use for loop to list all files and folders recursively by:

import os
indir = 'C:\\Users\\user\\'
for root, dirs, filenames in os.walk(indir):
                for f in filenames:
                        filename = indir + f
                        print filename

result:

test.txt

for loop and else clause

for x in xrange(3):
    print 'We start in %d' % (x)
else:
    print 'We are live'

result:

We start in 0
We start in 1
We start in 2
We are live

for loop with step

If you need to skip several steps you have the option of skipping n steps. Let say that you want odd numbers then you can do:

for x in range(1, 10, 2):
    print(x)

result:

1
3
5
7
9

for loop with continue

Continue can be used within for loop. The continue statement is used to continue with the next iteration of the loop. It's useful in several situations:

filtering an input

For loop and continue can be used to remove special characters from a string:

str = "find ;;;divisors of...... a ,,number without the number"

for x in str:
    if (x in [";",',','.']):
        continue
    print(x, end="")

result:

find divisors of a number without the number

preventing errors

Errors can be easily avoided with continue statement. In the example below error:

TypeError: unsupported operand type(s) for /: 'int' and 'str'

will be produced without continue statement:

for num in [1,4,5,'c',7,8,9,'b']:
    if  not isinstance(num, int): continue
    print (1/num)

for loop example get perfect numbers

By definition perfect number is a number that is equal to it's divisors sum. So 6 and 1, 2, 3 are the first perfect number. Below you can find example of for loop and continue in order to stop the second check if the number is perfect.

import math
from numpy import prod

#find divisors of a number without the number
def divisorGenerator(n):
    divs = [1]
    for i in range(2,int(math.sqrt(n))+1):
        if n%i == 0:
            divs.extend([i,n/i])
    divs.extend([])
    return list(set(divs))

for x in range(1, 30):
    divisors = list(divisorGenerator(x))
    suma = sum(divisors)
    product = prod(divisors)

    #check is it perfect number
    if(suma == x and divisors != [1]):
        print (x)
        print (divisors)
    continue # if we remove it we will get the second result
    # check if the sum is equal to product
    if (suma == product and divisors != [1]):
        print(x)
        print(divisors)

result:

6
[1, 2, 3]
28
[1, 2, 4, 14, 7]

second result without continue:

6
[1, 2, 3]
6
[1, 2, 3]
28
[1, 2, 4, 14, 7]

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