how to decrement and increment loop range 'i' variable in the execution of loop in python

Question

I am new in python and I don't know why this program is not updating the 'i' variable when I am incrementing it in the execution of for loop. len(a) = 10 you can see in the code that I am incrementing 'i' time after time but after the increment when I comes into loop iteration it nullifies the update made in the body of loop. Why is this is ? It should be updated in general and loop execution should be less than 10. Please help.

final_result = 0
a= '3 4  4 5 6'
for i in range(0,len(a)):
    print('iteration')
    print('i is = ')      
    print(i)
    if a[i] is ' ' and a[i+1] is not ' ':
        if i-1 is 0:
            final_result = int(a[i-1]) + int(a[i+1])
            i += 2                           //here goes the increment
            print('1a- m here')
            print(final_result)
            print('i is = ')
            print(i)
        else:
            final_result = final_result + int(a[i+1])
            i += 2                              //here goes the increment
            print('1b- m here')
            print(final_result)
    elif a[i] is ' ' and a[i+1] is ' ':
        if i-1 is 0: 
            final_result = int(a[i-1]) - int(a[i+1])
            i += 3                           //here goes the increment
            print('2a- m here')
            print(final_result)
        else:
            final_result = final_result - int(a[i+2])
            i += 3                                 //here goes the increment
            print('2b- m here')
            print(final_result)
            print('i is = ')
            print(i)
print(final_result)

Because in the next iteration i takes up the next value from the list returned by the range — nu11p01n73R
– nu11p01n73R, Commented Jun 4, 2015 at 10:18
You should be using while loops for these purposes as the increment are conditional — nu11p01n73R
– nu11p01n73R, Commented Jun 4, 2015 at 10:19
What are you trying to do? There is almost certainly a better way to do it. — mkrieger1
– mkrieger1, Commented Jun 4, 2015 at 11:06

James Mills · Accepted Answer · 2015-06-04 10:37:09Z

So several things:

Use a while loop.
Initialize i = 0
Increment i += 1 when none of your conditions match
Comments in Python are written with # comment, not // comment.

Example:

final_result = 0
a = '3 4  4 5 6'
i = 0
while i < len(a):
    print('iteration')
    print('i is = ')
    print(i)
    if a[i] is ' ' and a[i + 1] is not ' ':
        if i - 1 is 0:
            final_result = int(a[i - 1]) + int(a[i + 1])
            i += 2  # here goes the increment
            print('1a- m here')
            print(final_result)
            print('i is = ')
            print(i)
        else:
            final_result = final_result + int(a[i + 1])
            i += 2  # here goes the increment
            print('1b- m here')
            print(final_result)
    elif a[i] is ' ' and a[i + 1] is ' ':
        if i - 1 is 0:
            final_result = int(a[i - 1]) - int(a[i + 1])
            i += 3  # here goes the increment
            print('2a- m here')
            print(final_result)
        else:
            final_result = final_result - int(a[i + 2])
            i += 3  # here goes the increment
            print('2b- m here')
            print(final_result)
            print('i is = ')
            print(i)
    else:
        i += 1
print(final_result)

Output:

$ python3.4 foo.py
iteration
i is = 
0
iteration
i is = 
1
1a- m here
7
i is = 
3
iteration
i is = 
3
2b- m here
3
i is = 
6
iteration
i is = 
6
1b- m here
8
iteration
i is = 
8
1b- m here
14
14

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