CODEWITHSUNDEEP
c
JHFVM
JHFVM

Reputation: 11

Why does the last element of an array change?

I have the following code:

void generate_random_array(int8_t array[], int array_size){
    srand(time(NULL));

    for(int i = 0; i < array_size + 1; i++){

        array[i] = rand() % 21 + (-10);
        printf("%d, %d\n", array[i], i); 
    }
}


int main(){

    int8_t some_array[100];

    generate_random_array(some_array, 100);

    for(int i = 0; i < 101; i++){
        printf("%d %d\n", some_array[i], i); 
    }

    return 0;
}

The program generates random elements of a given array in the range from -10 to 10 and then it displays them. The problem is that the last element of an array changes to 100 when I print elements the second time. I would like to know why.

Upvotes: 1

Views: 99

Answers (2)

Amit Sharma
Amit Sharma

Reputation: 2067

In your function generate_random_array you are iterating the i upto array_size + 1. And you have declared an array of size 100; int8_t some_array[100]; OS will reserve the 100*sizeof(int8_t) bytes memory for you.

Indexes of your array would be lying in range [0,100) ie. excluding the 100th location.

Now, what you are doing is that you modifying the some_array[100] which you have not claimed in your declaration. Its being possible because C doesn't do any out-of-bound access checking.

But key point to note is that you are trying to modifying/reading the unclaimed memory. So this is undetermined behavior. It might be possible that you might get different value other than 100, sometimes.

But all story short, this behavior is undetermined because you are accessing the out-of-bound index of array.

Upvotes: 1

dave234
dave234

Reputation: 4945

Indexing of an array starts at zero. 

int array[3];//  an int array with 3 elements
array[0];    //first element
array[1];    //second element
array[2];    //third element
array[3];    //undefined because it is beyond the end of the array

Upvotes: 3

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