why index goes until value 12 inside loop

[glao]

Hello , I have the following code:

count = 0
phrase = "hello, world"
for iteration in range(5):
    index = 0
    while index < len(phrase):
        count += 1
        index += 1
        print "index: " +str(index)
    print "Iteration " + str(iteration) + "; count is: " + str(count)

I can't understand why index goes until value 12.When index=11 ,the condition index<len(phrase) (11<12) is true ,
but when index becomes 12 ,the condition is false.Why then the value of index is 12?

Thanks!

[rrashkin]

You print the index after the value is incremented:

        index += 1
        print "index: " +str(index)

So the loop is iterated with the value of "index" equal to 11, then the value of index is increased but the loop is not evaluated so the processing doesn't cease.

[glao]

Sorry , I didn't understand..

You say

then the value of index is increased but the loop is not evaluated so the processing doesn't cease

but the loop is evaluated when index=11 shouldn't it be evaluated when index=12 ,so it will be evaluated as false and won't execute?

[vegaseat]

Rewrite your code a little ...

count = 0
phrase = "hello, world"
for iteration in range(5):
    index = 0
    while index < len(phrase):
        count += 1
        print "index: " + str(index)
        index += 1
    print "Iteration " + str(iteration) + "; count is: " + str(count)

[glao]

Ok , but the print statement is inside the loop.Why does it matter if it is before or after?

[vegaseat]

The statements in the loop will complete unless you insert a break before some statements.

Something like this ...

count = 0
phrase = "hello, world"
for iteration in range(5):
    index = 0
    while index < len(phrase):
        count += 1
        index += 1
        if index >= len(phrase):
            break
        print "index: " + str(index)

    print "Iteration " + str(iteration) + "; count is: " + str(count)

Actually I find this a better style (the logic is clearer) ...

count = 0
phrase = "hello, world"
for iteration in range(5):
    index = 0
    while True:
        count += 1
        index += 1
        if index >= len(phrase):
            break
        print "index: " + str(index)

    print "Iteration " + str(iteration) + "; count is: " + str(count)

[glao]

Ok , I understand completely the two codes.

But still I can't get it (regarding my code) sorry..

The while loop will be executed until the condition hits false.Why it is continuing?

[HiHe]

If you put your exit condition in the while line, then whatever is in the while block of statements will execute during that final loop.

[glao]

If I understood well ,

when index goes 12 , the while loop isn't executed (because its false) but any variables that are inside the loop and correlated with index are affected?

If is this the case it seems a little weird..

[ddanbe]

In a while loop, the increment of an index, counter, etc. should always be the last statement. At least, that's what I mostly do.

[vegaseat]

Write the while loop in C or C++ and it will behave the same way.

[vegaseat]

// while loop in C++
//

#include <iostream>

int main()
{
  int k=0;

   while (k < 10)
   {
      ++k;
      std::cout << k << std::endl;  // cout after increment
   }
  std::cin.get();  // console wait
  return 0;
}

/* result ...
1
2
3
4
5
6
7
8
9
10
*/

[vegaseat]

k = 0
while k < 10:
    k += 1
    print(k)  # print after increment

''' result ...
1
2
3
4
5
6
7
8
9
10
'''

[vegaseat]

k = 0
while k < 10:
    print(k)  # print before increment 
    k += 1

''' result ...
0
1
2
3
4
5
6
7
8
9
'''

[glao]

Οκ , thanks for the answers.I think I understood.

The loop is been evaluated alright but the variable we have inside may increment.

Thanks!