C Program crashes at For Loop

Question

I'm new to C programming (I have some very basic experience with programming via vb.NET), and I'm attempting to write a program for the Project Euler Problem #1. https://projecteuler.net/problem=1

Algorithm

The challenge requires the programmer to find the sum of all multiples of 3 or 5 (inclusive) below 1000 (I used intInput to allow the user to enter an integer in place of 1000).

My current solution takes the input, and decrements it by 1 until (intInput - n) % 3 = 0, that is, until the next nearest multiple of 3 under the input integer is found.

The program then cycles through all integers from 1 to ((intInput - n) / 3), adding each integer to the sum of the previous integers, so long as the current integer is not a multiple of 5, in which case, it is skipped.

The resultant sum is then stored in intThreeMultiplier.

The above process is then repeated, using 5 in place of 3 to find the highest multiple of 5 under intInput, and then cycles through integers 1 to ((intInput - n) / 5), not skipping multiples of 3 this time, and stores the sum in intFiveMultiplier.

The output sum is then calculated via sum = (3 * intThreeMultiplier) + (5 * intFiveMultiplier).

The Problem

Whenever I compile and run my code, the user is allowed to input an integer, and then the program crashes. I have determined that the cause has something to do with the first For loop, but I can't figure out what it is.

I have commented out everything following the offending code fragment.

Source Code:

#include <stdio.h>
#include <stdlib.h>

void main()
{
    int intInput = 0;   /*Holds the target number (1000 in the challenge statement.)*/
    int n = 0;
    int count = 0;
    int intThreeMultiplier = 1;
    int intFiveMultiplier = 1;

    printf("Please enter a positive integer.\n");
    scanf("%d",intInput);

    for( ; (((intInput - n) % 3) != 0) ; n++)  
        {}

    /*for(; count <= ((intInput - n) / 3); count++)
        {
            if ((count % 5) != 0)
            {
                intThreeMultiplier += count;
            }
        }

    count = 0;
    for(n = 0 ; ((intInput - n) % 5) != 0 ; n++)
    {}

    for(; count <= ((intInput - n) / 5) ; count++)
    {
        intFiveMultiplier += count;
    }

    int sum = (3 * intThreeMultiplier) + (5 * intFiveMultiplier);
    printf("The sume of all multiples of 3 or 5 (inclusively) under %d is %d.",intInput, sum);*/
}

This is my first time posting on StackOverflow, so I apologize in advance if I have broken any of the rules for asking questions, and would appreciate any feedback with respect to this.

In addition, I am extremely open to any suggestions regarding coding practices, or any rookie mistakes I've made with C.

Thanks!

The first loop can not be able to crash unless you have a very buggy compiler generating faulty code. Have you tried running the program in a debugger to catch the crash in action, and then locate where it actually happen? — Some programmer dude, Sep 27 '16 at 18:15
Why do you need 2 loops? Just use 1 loop, and when the current number is a multiple of 3 or 5 add it to the total. — Barmar, Sep 27 '16 at 18:18
`void main()` is an invalid **and** deprecated signature. The minimum correct signature is `int main(void)` for hosted environments (you have one). — too honest for this site, Sep 27 '16 at 18:28

score 1 · Accepted Answer · edited May 23 '17 at 10:34

1

scanf("%d",intInput);

might be

scanf("%d", &intInput);  // note the ampersand

scanf need the address the variable where the content is to be stored. Why scanf must take the address of operator

For debugging only, print the input to verify that the input is accepted correctly, something like

printf("intInput = %d\n", intInput);

edited May 23 '17 at 10:34

Community

1
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answered Sep 27 '16 at 18:14

Arun

19,750
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This fixed it, thanks! Now I just need to work on the algorithm itself. :) – M. Boyes Sep 27 '16 at 18:32

score 0 · Answer 2 · edited Sep 27 '16 at 19:33

0

The first thing you need when you are inputting intInput you should use:

 scanf("%d", &intInput);

Because scanf() need as an argument of a pointer to your variable. You are doing this by just putting the & sign before your int.

In addition I think that you should double check your algorithm, because you are summing up some numbers more than once. :)

edited Sep 27 '16 at 19:33

buczek

2,011
7
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answered Sep 27 '16 at 18:27

I. Milev

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C Program crashes at For Loop

Algorithm

The Problem

Source Code:

2 Answers2