Question

When do I need to insert/don’t insert & for scanf() in C? Thank you.

int main()
{
    char s1[81], s2[81], s3[81];

    scanf("%s%s%s", s1, s2, s3);

    // If replace scanf() with the expression below, it works too.
    // scanf("%s%s%s", &s1, &s2, &s3);

    printf("\ns1 = %s\ns2 = %s\ns3 = %s", s1, s2, s3);

    return 0;
}

//programming is fun
//
//s1 = programming
//s2 = is
//s3 = fun

Answer 1

scanf puts the scanned values in the address pointed by it’s arguments. The & is the address operator, and it’s used to take the address of a variable.

But, you are using arrays, and arrays are downgrade to pointers when used as functions arguments. So, you don’t need to use the & operator in the case of arrays.

Example:

char s[81];
int n;
int* nptr;
//Initialize nptr to some meaningful value
scanf("%s %d %d",s,&n,nptr);

In this case, we need to use the & operator to get the address were n is kept. We don’t need to use it with nptr, because it is already a pointer to someplace in the memory, neither with s, because an array gets downgrade to a pointer when it’s passed to a function.