Convert integer input to digit using array in C

Question

#include <stdio.h>

void seperate(int intNum);

int main()
{
   int x;
   
   printf("Please enter an integer: \n");
   scanf("%d",&x);
   seperate(x);
}

void seperate(int intNum)
{
    int i,count=0;
    while (intNum!=0)
    {
        intNum/=10;
        ++count;  //to calculate the number of digits for user input = size of array
    }
    int array[count];   
    
    printf("The number on seperate line as follows:\n");

    for(i=count-1; i>=0; i--)
    {
        array[i]= intNum%10;
        intNum /= 10;
    }           
    for(i=0; i<=count-1; i++)
    {
        printf("%d\n", array[i]);
    }
}

The expected output(it works when the array size is fixed):

Please enter an integer:
42568
The number on separate line as follows:
4
2
5
6
8

The output:

Please enter an integer:
42568
The number on separate line as follows:
0
0
0
0
0

The code works only if the array sized is fixed, how to solve this problem? Does it not work if I create an array without declaring the size?

EDIT: the value for variable intNum changed in while loop, nothing to do with the declaration of the array

Do basic debugging. Use a debugger or even just debug print statements to trace the execution of the program. It has nothing to do with the declaration of the array. Consider what the value of `intNum` is at the start of the `for` loop. That's something you should be able to easily spot if you do any basic debugging. — kaylum, Dec 10 '20 at 03:06
Here are a couple of really good links on debugging [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/), consider a chat with the duck, and see [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/q/25385173/3422102) — David C. Rankin, Dec 10 '20 at 03:19
`while (intNum!=0){ ... }` Guess the value of `intNum` after that loop. — Lundin, Dec 10 '20 at 09:16

Adrian Mole · Accepted Answer · 2020-12-10T15:56:31.987

Your while loop, in which you count the number of digits, is changing the given intNum variable, which will be zero at the end of that loop.

You should make a copy of intNum and use/modify that in the first (while) loop, like this:

void seperate(int intNum)
{
    int i, count = 0, temp = intNum; // Make a copy to use in the "count" loop
    while (temp != 0) {
        temp /= 10;
        ++count;  //to calculate the number of digits for user input = size of array
    }
    if (count == 0) count = 1; // To take care of the case when intNum == 0
    int array[count];

    printf("The number on seperate line as follows:\n");

    for (i = count - 1; i >= 0; i--) {
        array[i] = intNum % 10;
        intNum /= 10;
    }
    for (i = 0; i <= count - 1; i++) {
        printf("%d\n", array[i]);
    }
}

Alternatively, you could save the value of intNum and restore that after the while loop has completed:

void seperate(int intNum)
{
    int i, count = 0, save = intNum; // Save the original value of intNum ...
    while (intNum != 0) {
        intNum /= 10;
        ++count;  //to calculate the number of digits for user input = size of array
    }
    intNum = save;                   // ... and restore it after counting the digits
    if (!count) count = 1;
    int array[count];
//...

EDIT: There is a much simpler and shorter solution, with only one loop, using the sprintf function to get the digits (in the right order) for you:

void seperate(int intNum)
{
    char output[100];// Should be long enough for the largest integer.
    sprintf(output, "%d", intNum);
    printf("The number on seperate line as follows:\n");
    for (char* cp = output; *cp; ++cp) {
        printf("%c\n", *cp);
    }
}

@chux Edited. There is also an issue with negative numbers, but I'll leave that as an "exercise for the reader." For now. — Adrian Mole, Dec 10 '20 at 15:57
Good fix. Alternate: If code used `do { } while` loop instead of `while`, then the `seperate(0)` case would not need `if (count == 0) count = 1;` — chux - Reinstate Monica, Dec 10 '20 at 16:01

score 1 · Answer 2 · answered Dec 10 '20 at 03:14

You could simplify this problem with a little math. First,

#include <math.h>

Then, the count of digits can be determined by calculating log₁₀intNum. And you only need a single loop. Something like,

void seperate(int intNum)
{
        printf("The number on seperate line as follows:\n");
        double log10 = log(10);
        while (intNum > 0) {
                int count = (int) (log(intNum) / log10);
                int pow10 = pow(10, count);
                int digit = (int) (intNum / pow10);
                printf("%d\n", digit);
                intNum -= digit * pow10;
        }
}

This approach fails when `intNum <= 0` or `intNum` near a power of 10 and `(log(intNum) / log10)` is xxx.99999... due to the usual FP concerns. — chux - Reinstate Monica, Dec 10 '20 at 15:53

Convert integer input to digit using array in C

2 Answers2