Int array empty - C - c

Question

I am attempting to write a program that will take two sets of strings N and Q. The goal of the program is to print out the number of times each string in Q occurs in N. However, I am struggling to manage strings and pointers in C and in particular I believe my issue stems from attempting to have an array of strings. I get a segmentation fault when the code below is executed. I have commented out my attempts at debugging using printf(). I believe the issue occurs when I try to assign S into the N_array.
int main() {
int N, Q;
char *N_array[1000], *Q_array[1000];
scanf("%d", &N);
for (int N_i = 0; N_i < N; N_i++) {
//printf("made it through A for loop %d times\n", N_i+1);
scanf("%s", N_array[N_i]);
}
scanf("%d", &Q);
//Does the array contain any information?
//for (int N_i = 0; N_i < N; N_i++) { printf("N_array[%d] == %d\n", N_i, N_array[N_i]);}
for (int Q_i = 0; Q_i < Q; Q_i++) {
//printf("Made it to B for loop\n");
int occurs = 0, result;
char s[21];
scanf("%s", &s[21]);
strcpy(Q_array[Q_i], s);
for (int N_i2 = 0; N_i2 < N; N_i2++) {
//printf("Made it to C for loop\n");
result = strcmp(Q_array[Q_i], N_array[N_i2]);
if (result == 0) occurs++;
}
printf("%d", occurs);
}
return 0;
}

Answer 1

One problem is here
for (int N_i = 0; N_i < N; N_i++) {
//printf("made it through A for loop %d times\n", N_i+1);
scanf("%s", N_array[N_i]);
}
N_Array contains 1000 pointers to char, but each of those pointers points to, well.. nowhere. It's an uninitialized pointer which points to a random memory location you don't own. This is undefined behavior. You have to allocate memory before scanf.
N_Array[N_i] = malloc(max_length_of_string + 1);
Another problem is this line
char s[21];
scanf("%s", &s[21]);
The second parameter of scanf should be just s, not &s[21], which is just outside your array.
And one line below you have the same problem as described in my first point
strcpy(Q_array[Q_i], s);
Q_array[Q_i] doesn't yet point to any memory which you're allowed to write to. You should allocate memory here as well.

Answer 2

scanf("%s", N_array[N_i]);
This should lead to SegFault as expected as N_array[N_i] doesn't contain a valid memory address.
What you need is to allocate memory using malloc, calloc to point N_array[N_i] to a valid memory location.
Correct code would be:
for (int N_i = 0; N_i < N; N_i++)
{
if(!(N_array[N_i] = malloc((MAX_STRING_LENGTH + 1) * sizeof(char))))
{
printf("malloc failed!\n);
exit(1);
}
scanf("%s", N_array[N_i]);
}
scanf("%s", &s[21]);
This is incorrect as %s expects char * but type of &s[21] is char **.
You need to use scanf("%20s", s); instead.
This way scanf only read at most 20 chars (as we have char s[21]. So leaving last byte for \0 character) and thus this will avoid accidental buffer overrun (where the length of string entered by user is greater than 20).
strcpy(Q_array[Q_i], s);
Again here Q_array[Q_i] points to invalid memory address as it contains garbage value. Use malloc, calloc etc. to allocate memory first.
Correct code could be:
if(!(Q_array[Q_i] = malloc((MAX_STRING_LENGTH + 1) * sizeof(char))))
{
printf("malloc failed!\n);
exit(1);
}
strcpy(Q_array[Q_i], s);