Iterating and for function - Python

Question

I can't seem to get my head around this. I would appreciate some help. It looks to me like the for function is iterating at a rate of n+1 for every new iteration, although I can't figure out why.

Here is my Python code:

for n in range(2, 10):
    print "This is %d" % n
    for x in range(2, n):
        print "%d" % n

And here is the output in Powershell:

This is 2
This is 3
3
This is 4
4
4
This is 5
5
5
5
This is 6
6
6
6
6
This is 7
7
7
7
7
7
This is 8
8
8
8
8
8
8
This is 9
9
9
9
9
9
9
9

Why is this happening?

range accepts three arguments. range(start, stop, step). u are passing start and stop. so its printing what it should be — Taxellool
– Taxellool, Commented Jan 28, 2015 at 12:39
@Raz debug code and see how it works step by step. You will get much more information. — Alexander Perechnev
– Alexander Perechnev, Commented Jan 28, 2015 at 12:40
@Raz debuger is an utility which allows you to run your code line-by-line. And on each step you can see what variables contain. Google it. — Alexander Perechnev
– Alexander Perechnev, Commented Jan 28, 2015 at 12:43

Martijn Pieters · Accepted Answer · 2015-01-28 12:45:32Z

1

The end value for a range is not included, so the range from 2 to 2 doesn't include any values. You then create a range from 2 to 3, which only includes the value 2, etc.

As such, you are producing loops that'll loop n - 2 times, where n loops from 2 to 9 inclusive, and each inner loop loops from 2 to n - 1, inclusive.

For each nested loop, you are printing n again, which doesn't change in the inner loop. Try printing x instead, or better still, print it together:

for n in range(2, 10):
    print "This is %d" % n
    for x in range(2, n):
        print "n=%d, x=%d" % (n, x)

For each n this'll print the values for x from 2 to n - 1, inclusive:

>>> for n in range(2, 10):
...     print "This is %d" % n
...     for x in range(2, n):
...         print "n=%d, x=%d" % (n, x)
... 
This is 2
This is 3
n=3, x=2
This is 4
n=4, x=2
n=4, x=3
This is 5
n=5, x=2
n=5, x=3
n=5, x=4
This is 6
n=6, x=2
n=6, x=3
n=6, x=4
n=6, x=5
This is 7
n=7, x=2
n=7, x=3
n=7, x=4
n=7, x=5
n=7, x=6
This is 8
n=8, x=2
n=8, x=3
n=8, x=4
n=8, x=5
n=8, x=6
n=8, x=7
This is 9
n=9, x=2
n=9, x=3
n=9, x=4
n=9, x=5
n=9, x=6
n=9, x=7
n=9, x=8

edited Jan 28, 2015 at 12:45

answered Jan 28, 2015 at 12:32

Martijn Pieters

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6 Comments

Raz Over a year ago

But why is 4 printed twice, and 5 three times, and 6 four times, etc?

Raz Over a year ago

Why is it looping n - 2 times?

Matthias Over a year ago

4 is printed twice because you coded it. You have an inner loop for x in range(2, n): and if n is 4 it loops over the values 2 and 3. Did you want to print x instead?

Martijn Pieters Over a year ago

@Raz: you are simply printing n. Did you mean to print x instead?

Raz Over a year ago

Martin I posted another answer to this addressed to you - I can't seem to fit it all in to this comments thread but if you scroll down you will see it.

|

Sylvain Leroux · Accepted Answer · 2015-01-28 12:45:34Z

for n in range(2, 10):
    for x in range(2, n):
        ...

You have nested loops. The upper bound of the inner loop is set to the current value of the outer loop.

The outer loop will iterate from 2 (incl.) to 10 (excl.):

At first iteration, n is 2. So, the inner loop will iterate from 2 (incl.) to 2 (excl.). As the upper bound is in fact equal to the lower bound, the inner loop does not iterate.
At second iteration of the outer loop, n is 3. So, the inner loop will iterate from 2 (incl.) to 3 (excl.). So there is one iteration of the inner loop with x = 2
At third iteration of the outer loop, n is 4. So, the inner loop will iterate from 2 (incl.) to 4 (excl.). There is two iterations of the inner loop (x = 2 and the x = 3).

And so on...

Raz · Accepted Answer · 2015-01-28 12:46:09Z

0

I think I understand now. Within every single iteration of the first for loop, the second foor loop will iterate once for every 'x' value between 2 and n.

So if n is 7, the first for loop will iterate once for the number 7, and the second for loop will iterate once for every value between 2 and 7 (i.e. 5 times).

answered Jan 28, 2015 at 12:46

Raz

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