Question

I am reading C Primer Plus these days and here is the code I wrote for the programming practice No.4 in Chapter 10, finding the index of the largest number in a double-typed array. I used variable length array in order to manually specify the array size:

#include <stdio.h>
int findmax(const double array[], int s);
//find the index of the largest number in the array
int main(void)
{
    int size = 0; //size of the array
    int index = 0; //index of the largest number
    double num[size]; //the array holding double-type numbers

    printf("Enter the size of the array: ");
        scanf("%d", &size);
    printf("Enter %d numbers: ", size);
    for (int i = 0; i < size; i++)
        scanf("%lf", &num[i]);

    index = findmax(num, size);
    printf("The index of the max number in the array is: %d\n", index);
    return 0;
}

int findmax(const double array[], int s)
{
    int index = 0;
    double max = array[0];
    for (int i = 0; i < s; i++)
            if (array[i] > max)
            {
                max = array[i];
                index = i;
            }
    return index;
}

This piece of program compiles normally, using MinGW (assume the program file name is prog.c):

gcc prog.c -o prog.exe -std=c99

The program works fine when the “size” varialbe is less than 5. But when I enter 6 or larger numbers for the “size” varialbe, the program crashes during runtime.

Loosely translated, the error message is:

the memory 0x00000038 used by 0x77c1c192 could not be "written".

I tried to eliminate the use of variable length array, the program seems to work fine. But I still couldn’t get where is wrong with the original one.

Answer 1

Size is 0 when you allocate num. You get access violation later on because you try to acces num[0] which has not been allocated.

EDIT: I propose to use dynamic memory or declare num after size is read.