Summing an Array of Numbers but Receiving Error when Run [closed] - c

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I am trying to sum an array of numbers. The array has a length determined by an input and then the user gives the array. There were no compilation errors and I am able to run other programs. On the immediate start of running the program I am given a message that program has stopped working and that windows is searching for solution.
#include <stdio.h>
int main()
{
int sum, length, count;
int array[length];
sum=0;
scanf("%d",&length);
scanf("%d",&sum);
for(count=0; count<length-1; count++)
{
sum = sum + array[count];
}
printf("%d", sum);
return 0;
}

Answer 1

When you declare your array it depends on length but you ask the user for length after.
A solution could be to ask the user for length (scanf("%d",&length);) before declaring your actual array (int array[length];).

Answer 2

you should move int array[length] to after scanf("%d", &length). But it is not allowed in C to declare variables after the first non-declaration (it is however possible if you compile this program as C++).
In fact, in standard C you can't have a non-const length definition for an array variable. gcc on the other hand for example allows this nevertheless.
In your case, the problem is that length has an undefined value at the declaration of int array[length];. If you are lucky, your data segment has been initialized to zero (there is no guarantee for that) but otherwise, it may be any value, including a value which leads the program to exceed your physical memory.
A more standard way of doing this is:
int *array = NULL;
scanf("%d",&length);
...
array = (int*) malloc(sizeof(int) * length);
...
free(array);
By the way, even after fixing that, you will most likely get random numbers because you never actually assign the contents of the elements of array.

Answer 3

Local variable are initialized to 0. Hence value of length is 0. So you array is of length. You are then reading length, say 10, from stdin and expect the array to be of length 10. This can't be. Since this is a stack variable, the size is determined in time of pre-processing and not in run time. If you want to define the array length in run time then use malloc.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
int sum, length, count;
int *array;
sum=0;
scanf("%d", &length);
scanf("%d",&sum);
array = (int *)malloc(sizeof(int) * length);
if (array == NULL) return 0;
memset(array, length, 0);
for(count=0; count<length-1; count++)
{
sum = sum + array[count];
}
printf("%d", sum);
return 0;
}
Thanks.

Answer 4

first problem:
the length variable is being used to set the number of entries in the array[], before the variable length is set. Therefore, length will contain what ever trash happens to be on the stack when the program starts so the number of entries defined in array[] is an unknown.
This results in undefined behaviour and could lead to a seg fault event, depending on what was on the stack and what the user entered for length.
second problem:
The array array[] is never initialized so will contain what ever trash is on the stack at program startup. This means the value being printed could be anything. And the 'sum' could overflow, depending on the trash values in array[]

Answer 5

OP program lacks the part of data input, it's asking for sum instead of the values to sum, which is weird. The only inputs requested are also never checked (the return value of scanf must always be checked).
In C (at least C99 and optionally C11) Variable Length Arrays, like the one defined by int array[length], can be used, but the variable length here is used uninitialized and before it is even asked to the user.
Moreover, the loop where the sum is calculated stops before the last element of the array (not really a big deal in this case, considering that all those variables are uninitialized...).
A better way to perform this task could be this:
#include <stdio.h>
// helper function to read an integer from stdin
int read_int( int *value ) {
int ret = 0;
while ( (ret = scanf("%d", value)) != 1 ) {
if ( ret == EOF ) {
printf("Error: Unexpected end of input.\n");
break;
}
scanf("%*[^\n]"); // ignore the rest of the line
printf("Please, enter a number!\n");
}
return ret;
}
int main(void) {
int sum = 0,
length = 0,
count,
i;
printf("Please, enter the number of values you want to add: ");
if ( read_int(&length) == EOF )
return -1;
// Use a VLA to store the numbers
int array[length];
// input the values
for ( count = 0; count < length; ++count ) {
// please, note ^^^^^^^^ the range check
printf("Value n° %2d: ", count + 1);
if ( read_int(&array[count]) == EOF ) {
printf("Warning: You entered only %d values out of %d.\n",
count, length);
break;
}
// you can sum the values right here, without using an array...
}
// sum the values in the array
for ( i = 0; i < count; ++i ) {
// ^^^^^^^^^ sum only the inputted values
sum += array[i];
}
printf("The sum of the values is:\n%d\n", sum);
return 0;
}