Why Does the selection of 0 end this while loop?

Question

With this code I am trying to build an array with integer values entered by the user. the variable "int selection" is an int, so if the value entered is an int, the while loop should keep going, but the value 0 seems to end it and I can't figure out why. Thank you for your help guys.

int main()
{
  //data to be entered
  int selection;

  //array size
  int const array_size = 100;

  //array
  int integers[array_size];

  //array index
  int index = 0;

  //prompt
  std::cout << "Enter integer ('x' to quit): " << std::endl;

  //get the data
  std::cin >> selection;

  //while data is int
  while (selection)
  {
    //put it in the array
    integers[index] = selection;

    //increment index
    index += 1;

    //get new data point
    std::cin >> selection;

  }

  return 0;

}

because value `0` is convertible to _bool_ `false`. As value `!= 0` is convertible to _bool_ `true`. — NetVipeC, Sep 06 '14 at 17:03
Setting `selection` to 0 ends your loop because you told it so: `while (selection)`. — Jongware, Sep 06 '14 at 17:04

score 2 · Answer 1 · answered Sep 06 '14 at 17:05

2

This code won't do what the comment says that it would do:

//while data is int
while (selection)

The data will always be an int, it's not possible to store anything else in an int variable.

What the code actually does is to loop while the value is non-zero.

answered Sep 06 '14 at 17:05

Guffa

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Oh ok I see. How should I approach this then. I do not need code just a better way to think about it. – Duly Kinsky Sep 06 '14 at 17:14

score 1 · Answer 2 · edited Sep 06 '14 at 17:04

1

Because 0 in Boolean context is interpreted as false.

edited Sep 06 '14 at 17:04

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answered Sep 06 '14 at 17:04

Degustaf

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Ranald Lam · Accepted Answer · 2014-09-06T17:28:57.187

0

false is interpreted as 0 in C++. Similarly, 0 is interpreted as false . Hence, when selection is 0, the loop effectively becomes:

while ( false )
{
...
}

which does not run.

On the other hand, when selection is not 0, C++ treats it as true and the loop will run.

EDIT: If you want to loop while the input is an integer, try

while (std::cin >> selection)
{
...
}

edited Sep 06 '14 at 17:28

answered Sep 06 '14 at 17:05

Ranald Lam

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I actually want to be able to take 0 in, How do I actually get this done. I don't need code, just a better way to think about it – Duly Kinsky Sep 06 '14 at 17:25
Edited, please try :) – Ranald Lam Sep 06 '14 at 17:29
Thank you Ranald. Just to make sure I understand, the line while (cin >> selection){} means while we can get selection that is an int do what is in the braces – Duly Kinsky Sep 06 '14 at 17:33
Yeap, you can check it out here: http://stackoverflow.com/questions/6791520/if-cin-x-why-can-you-use-that-condition – Ranald Lam Sep 06 '14 at 17:40
"`false` is interpreted as `0` in C++. `0` is interpreted as `false`". So, how does this infinite recursion end? – AnT stands with Russia Sep 06 '14 at 17:51
@AndreyT doesn't `while (false)` not run in the first place? – Ranald Lam Sep 06 '14 at 17:52
@RanaldLam: Correct. He was trying to simplify matters by showing post-evaluation code, but ended up inadvertently confusing them. – Lightness Races in Orbit Sep 06 '14 at 18:09

Duly Kinsky · Answer 4 · 2014-09-06T19:41:25.393

0

This is how the code stands now and it works. Thank you guys for all your help

int main()
{
  //data to be entered
  int selection;

  //array size
  int const array_size = 100;

  //array
  int integers[array_size];

  //array index
  int index = 0;

  //prompt
  std::cout << "Enter integer ('x' to quit): " << std::endl;

  //while data is int
  while (std::cin >> selection)
  {
    //put it in the array
    integers[index] = selection;

    //increment index
    index += 1;

  }

  return 0;

}

edited Sep 06 '14 at 19:41

answered Sep 06 '14 at 17:46

Duly Kinsky

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Nope, this won't compile, because `std::din` does not exist. – Lightness Races in Orbit Sep 06 '14 at 17:49
@LightnessRacesinOrbit I think it is a typo error and he meant std::cin – Ranald Lam Sep 06 '14 at 18:06

score 0 · Answer 5 · answered Sep 06 '14 at 17:49

0

while (selection) doesn't stop when selection is no longer an int; selection is always an int.

while (selection) stops when selection is not equal to 0.

You should instead test the result of the >> operation.

answered Sep 06 '14 at 17:49

Lightness Races in Orbit

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Why Does the selection of 0 end this while loop?

5 Answers5