How to create a function in C that prompts the user for positive integers only and call it in the main function?

Question

This is my code. I failed to use the break keyword after the printf function in order to break out of the loop. When I enter a negative number or zero, it doesn't prompt me again to re-enter.

#include <stdio.h>
#include <cs50.h>

int get_positive_int(void);

int main (void)
{
    get_positive_int();
}


int get_positive_int(void)
{   
  int i;
  i = get_int("Integer: ");

  while (true)
  {
    if (i<1)
    {
       return i;
    }
    else
    {
       printf("%i", i);  
    }
  }
}

You’ve wrote the code to exit the function when the number is less than one. If you want it to ask again you’ll have to tell it to do that. — Sami Kuhmonen, Apr 08 '20 at 11:42
Hint: your `while` is misplaced. Basically this is the algorithm you need: 1. get integer. 2. if it's positive, return it. 3. if it's negative go to step 1. — Jabberwocky, Apr 08 '20 at 11:43

RobertS supports Monica Cellio · Accepted Answer · 2020-04-08T17:05:23.757

2

The algorithm in the function get_positive_int() is wrong:

You need to place i = get_int("Integer: "); inside of the while loop.
Your if condition:

if (i < 1)

is wrong as that would return i if i is a negative integer or 0. If you want to return i when i is a positive integer or 0 you should use if(i >= 0).

Note that you can also place:

if (i == INT_MAX)
{
   // optional error handling.
   return INT_MAX;
}

after the call to maintain the occurrence of a read error. But if you want to only return INT_MAX then, you do not need to do so and can omit it since this would fit to the conditional statement ``if(i >= 0)` and its body.

The code is then:

int get_positive_int(void)
{   
  int i;

  while (true)
  {
    i = get_int("Integer: ");

    if (i == INT_MAX)
    {
        // optional error handling.
        return INT_MAX;
    }
    else if (i >= 0)
    {
       return i;
    }

    printf("%i", i);  
  }
}

Side note: If you don´t want to count 0 as positive integer, you need to have i >= 1 as the condition of the if statement.

As you said in the comments you only want to continue if i is a positive integer and exit if i is a negative characters or 0:

void get_positive_int(void)
{   
  int i;

  while (true)
  {
    i = get_int("Integer: ");

    if (i < 1)
    {
       return;
    }

    printf("%i\n", i);  
  }
}

Note that in this case, the return type of get_positive_int shall be void instead of int and it should omit to return i as it is not necessary to return any value from the function.

edited Apr 08 '20 at 17:05

answered Apr 08 '20 at 11:55

RobertS supports Monica Cellio

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What is the need for the `if (i == INT_MAX)` block? Looks discardable. – chux - Reinstate Monica Apr 08 '20 at 12:22
@chux-ReinstateMonica If `get_int()` can´t read, it returns `INT_MAX`. It´s a cs50-thing. – RobertS supports Monica Cellio Apr 08 '20 at 12:23
1

Interesting. [`get_int()`](http://dkui3cmikz357.cloudfront.net/library50/c/cs50-library-c-3.0/cs50.c) also returns `INT_MAX` when the value read is `INT_MAX`. Looks like `get_int()` does not provide for reading a valid `INT_MAX`. – chux - Reinstate Monica Apr 08 '20 at 12:36
@chux-ReinstateMonica So, now It should be all fixed. I often times don´t mind/overlook things when I have an idea about and the focus on another. Bad attribute of mine. – RobertS supports Monica Cellio Apr 08 '20 at 12:51
@chux-ReinstateMonica I´ve forget: Thank you for your hints. – RobertS supports Monica Cellio Apr 08 '20 at 12:56
[Unnecessary 'else' after 'return'. (No-else-return)](https://stackoverflow.com/q/46875442/2410359) applies here for your consideration. – chux - Reinstate Monica Apr 08 '20 at 13:18
@chux-ReinstateMonica Right. I changed that, too, but kept the `else if` because both are related. – RobertS supports Monica Cellio Apr 08 '20 at 13:27
@RobertSsupportsMonicaCellio Thank you so much! It actually not what I wanted, but you've just taught me a new way to deal with while loop. My problem was fixed after placing the i = get_int(".......") in the while loop... I only needed to keep getting prompt when I enter positive integers, but exit when I enter a zero or negative integer. Thanks again for your time :). – tareq91z Apr 08 '20 at 13:50
@tareq91z Is the added example an approach to what you want? – RobertS supports Monica Cellio Apr 08 '20 at 14:18
@RobertSsupportsMonicaCellio That's exactly what I want. You've just given me more approach to handle this problem! Thanks a lot :). – tareq91z Apr 08 '20 at 15:44

How to create a function in C that prompts the user for positive integers only and call it in the main function?

1 Answers1