Using `for` in `range(x)` loop

Question

Shouldn't both blocks of code print similar results? Why is the range function inside of the inner loop reevaluated each time the inner for statement is reached while the range function in the outerloop is only evaluated once?

x = 4
for j in range(x)
   for i in range(x)
       print i
       x = 2

Results
0
1
2
3
0
1
0
1
0
1

I know the first 4 integers printed ( 0 - 3) are a result of the code for j in range(x): code but why are the the following also printed?
0
1
0
1
0
1
The code

x = 4
for j in range(x):
    print i
    x = 5

Prints
0
1
2
3

Additional Info Python 2.7 in IDLE

Is this really a practical, answerable question based on actual problems that you face? (faq) — Adam
– Adam, Commented Mar 28, 2013 at 16:38
The code to the right of in (the range(x) call in this case) is evaluated each time the loop is entered from above. Therefore, the range in the outer loop only gets evaluated once, but in the inner loop it gets evaluated for each iteration of the outer loop. Think of range as returning a list, and what that list would hold, each time it is called. (In python2 it does return a list. In python3 it's more efficient, and returns a range type that's like an iterator.) — Jeff Learman
– Jeff Learman, Commented Nov 13, 2019 at 22:40

sberry · Accepted Answer · 2013-03-28 16:40:59Z

4

I can only explain by walking through the iterations of the loops, so here goes:

x = 4
for j in range(x)
   for i in range(x)
       print i
       x = 2

First time through.

x = 4
for j in [0, 1, 2, 3]
    for i in range [0, 1, 2, 3]
        print i
        x = 2

prints

Now x is set as 2, but the outer loops range has already been executed, so it is not reevaluated.

Code now becomes:

for j in [0, 1, 2, 3]:
    for i in [0, 1]:
        print i
        x = 2

prints

0
1

And this continues two more times.

answered Mar 28, 2013 at 16:40

sberry

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Comments

Jakub M. · Accepted Answer · 2013-03-28 16:45:49Z

4

Function range(x) produces a list of [0,1,2,3,4]. In for loop you iterate over this list.

Your code is equivalent to:

for j in [0,1,2,3]:
    for i in [0,1,2,3]:
        print i
    for i in [0,1]:
        print i
    for i in [0,1]:
        print i
    for i in [0,1]:
        print i

edited Mar 28, 2013 at 16:45

answered Mar 28, 2013 at 16:39

Jakub M.

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5 Comments

Jose Rodriguez Over a year ago

I'm just trying to get a better understanding of how for loop functions work. Can't seem to wrap my head around why the range function in the inner loop is reevaluated while the range function in the outer loops is only evaluated once

Jakub M. Over a year ago

"range function in the inner loop" - as you noticed, it is in the inner loop, therefore it will be run in each iteration of the outer loop. What other behavior did you expect?

Jose Rodriguez Over a year ago

I expected the same results if I were to have ran the code <br> x = 4 for j in range(x): print j x = 5 How come this code only prints 0 1 2 3 instead of 0 1 2 3 0 1 2 3 4

Karl Knechtel Over a year ago

The range function in the outer loop in your example is evaluated every time the outer for statement is reached. But that only happens once. Similarly in your example from the comment with a single loop: print j is reached multiple times, but for j in range(x) is only reached once.

Jeff Learman Over a year ago

The range is evaluated whenever entering the loop. The outer loop is entered once. The inner loop is entered once per outer loop iteration.

stevedes · Accepted Answer · 2013-03-28 16:42:09Z

0

range(x) is evaluated only once i.e. when the loop begins, that is why modifying x inside the loop has no effect.

However, in the first code clock, you change x to 2 in the inner, so the next time the inner loop is executed, range only gives (0,1).

Edit: your first block of code is equivalent to

x = 5
for i in range(x)
    print i
    x = 2

answered Mar 28, 2013 at 16:42

stevedes

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Comments

Stephen Rauch · Accepted Answer · 2018-04-08 01:42:06Z

0

Here is how I view it:

Inner loop is executed for each iteration of outer loop

x = 4
j = 0
for i in range (x):
    print(i)
    x = 2
j = 1
for i in range(x) # this x is no longer 4 but it is 2
    print(i)
    x = 2
j = 2
for i in range (x): # this x is, of course, 2
    print(i)
    x = 2
j = 3
for i in range (x): # this x is, of course, 2
    print(i)
    x = 2

edited Apr 8, 2018 at 1:42

Stephen Rauch♦

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answered Apr 8, 2018 at 1:17

Sharma

1

Comments

fiona fino · Accepted Answer · 2019-01-07 23:45:10Z

-2

j is not printed, insert " print('xx', j, 'oo') " before " for i in range(x): "

then change to "print('xx', x, 'oo')

or change 'for j in range(x):' to 'for j in range(225,200,-5):' follow with 'print(j)' before 'for i in range(x):'

edited Jan 7, 2019 at 23:45

answered Jan 7, 2019 at 23:05

fiona fino

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