C program to find the Majority Element of Array using a user defined array size returns unexpected results

Question

I am trying to write a c program that prompts the user to input the size of an array (N) between 1 and 100. Then prompt the user to input array elements to the size of N. Following that I want to display the aforementioned elements to the user, however, in run-time random, or what seems to be random numbers are displayed. I am not sure if this is due to using malloc for user defined array size or what. If you have any insight to what the problem may be I would really appreciate it.

Program:

int main()
{
    int N;
    int dW;
    int *inputArray;
    int counter;

do {
    // Prompts user to input array size up to 100
    printf("\nEnter the value of N up to 100:   \n");
    scanf("%d", &N);

    // Uses pointer "inputArray" to pacify compiler and allocates size of N in memory for array
    inputArray = (int *)malloc(N * sizeof(int));

    // Checks if temp is greater than 1 and less than 100, if yes prompts user to reenter N
    if (N >= 1 & N <= 100)
    {
        
        // Prompts user to input array numbers to the size of N
        for (counter = 0; counter < N; counter++)
        {
            printf("\nEnter the element %d: \n", counter);
            scanf("%d", &inputArray[counter]);
        }

        // displays numbers provided by user
        for (counter = 0; counter < N; counter++);
        {
            printf("%d\n", inputArray[counter]);
        }
        dW = 0;
    }
    else
    {
        printf("\nIllegal Entry, enter a value between 1 and 100\n");
        dW = 1;
    }
    

} while (dW >= 1);
return 0;

}

Output:

Enter the value of N up to 100: 5

Enter the element 0: 1

Enter the element 1: 2

Enter the element 2: 3

Enter the element 3: 4

Enter the element 4: 5

00200000

Also, do the check before you `malloc`. With your program as written, I can still type `2147483647` for `N` and cause your program to suddenly grab 8 whole GiB of RAM, even if you do bail right after. — Charles Srstka, Oct 08 '17 at 20:44

score 3 · Answer 1 · answered Oct 08 '17 at 20:50

Your problem is on this line:

for (counter = 0; counter < N; counter++);

See that semicolon there? That essentially causes this line to do the same thing as:

for (counter = 0; counter < N; counter++) { /* do nothing */ }

Your printf block then runs, but only once, and the counter is equal to N, so you read whatever random data happens to be in memory directly past the end of inputArray in the heap.

Solution: Get rid of that semicolon.

That was it! Thank you for your response! – Sam Lieber Oct 08 '17 at 21:07 — Sam Lieber, Oct 08 '17 at 21:07

frslm · Accepted Answer · 2017-10-08T20:55:35.743

There are a few issues with your code:

You should move inputArray = (int *)malloc(N * sizeof(int)); into the if-block so that you allocate memory only when you enter the block (and N is guaranteed to be between 1 and 100). Also there's no need to cast malloc() as (int *).
Your second for-loop has a semi-colon at the end: for (counter = 0; counter < N; counter++);. If you leave it there, the for-loop will increment counter until counter == N, and then printf("%d\n", inputArray[counter]); will be called afterwards, but only once and with the wrong index (since counter == N, which would take you out of bounds).
You forget to free(inputArray) after you're done using it. You could do that right after dW = 0;.

C program to find the Majority Element of Array using a user defined array size returns unexpected results

2 Answers2