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I have some question regarding the for-else loop in another for loop. So here is an example:

primelist = []
for p in range (2, x+1):
    print 'in first for'
    raw_input()
    for i in range(2, p):
        print 'in second for'
        if p%i == 0:
            print 'in if'
            raw_input()
            break
    else:
        print 'in else'
        raw_input()
        primelist = primelist + [p]
return primelist

As you see this is for determining the prime numbers and store them into a list so that they can be retrieved later. So my problem is with the first for loop. When I run the program, it enters like this: First FOR loop, ELSE, First FOR loop, Second FOR loop and so on. Why does it skip to ELSE the first time? Also if I ask for the 1000th number I get : 7919 with the 

for i in range(2, p)

and 7907 with the

for i in range(2, p/2).

Why is that? Hope you will help me with this, but please do not give other methods for implementing the Prime Number algorithm!

tshepang
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BlackTM
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  • As you've written it, this code has a syntax error. Did you mean to indent the `else` statement (and its children) to be inline with the `if` statement? – Hooked Feb 20 '14 at 15:53
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    @Hooked You can `else` a `for` in Python. The condition gets entered if the loop terminates without `break`ing. – Silas Ray Feb 20 '14 at 15:54
  • @SilasRay wow, I didn't realize that. My mistake BlackTM. I'll leave my comment up for others to learn from. – Hooked Feb 20 '14 at 15:55

1 Answers1

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Python's range doesn't include the ending value.

range[start, end)

So, In the first iteration, value of p will be 2 and range(2, 2) is an empty list. That is why the else part is entered.

print range(2, 2)  # []
thefourtheye
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