C: print the longest common prefix - c

Question

Beginner programmer here, trying to figure out how to find and print the longest common prefix in C.
I have a base here for the program.
#include <stdio.h>
void findprefix(char *str1, char *str2, char *found);
int main(void) {
char str1[100];
char str2[100];
char found[10] = { '\0' };
printf("\nGive string 1: ");
scanf("%99s", str1);
printf("\nGive string 2: ");
scanf("%99s", str2);
//Print prefix
findprefix(str1, str2, found);
printf("%s", found);
return 0;
}
//Function to find the longest common prefix
void findprefix(char *str1, char *str2, char *found) {
int i, j;
for () {
if () {
}
}
}
The initial idea is to use a for loop and an if statement in the function but I'm not sure how.

Answer 1

This declaration
char found[10] = { '\0' };
is redundant and does not make a sense.
Also the function findprefix should return the length of the common prefix.
The function should be declared and defined the following way
size_t findprefix( const char *str1, const char *str2 )
{
size_t n = 0;
for ( ; *str1 && *str1 == *str2; ++str1, ++str2 )
{
++n;
}
return n;
}
And in main you can write
size_t n = findprefix( str1, str2 );
if ( n != 0 ) printf( "%.*s\n", ( int )n, str1 );
Here is a demonstration progarn.
#include <stdio.h>
size_t findprefix( const char *str1, const char *str2 )
{
size_t n = 0;
for ( ; *str1 && *str1 == *str2; ++str1, ++str2 )
{
++n;
}
return n;
}
int main( void )
{
const char *str1 = "Hello Word!";
const char *str2 = "Hello Kallum Smith";
size_t n = findprefix( str1, str2 );
if ( n != 0 ) printf( "\"%.*s\"\n", ( int )n, str1 );
return 0;
}
The program output is
"Hello "
Using the return value of the function you also can dynamically allocate an array or declare a variable length array where you can copy the prefix if it is required.

Answer 2

You have a good base, except you should define prefix with a length of 100 for pathological cases.
In the function, you should iterate with an index i starting at 0, comparing the characters from str1 and str2 at offset i and stop when they differ or when either one is a null byte (a char with the value 0), otherwise copying the byte to the found array at the same offset i.
After the loop. you would store a null byte in found at the offset where you stopped the iteration.
Finally, you would return to the caller.
Here is an example:
#include <stdio.h>
//Function to extract the longest common prefix
int findprefix(const char *str1, const char *str2, char *found) {
int i;
for (i = 0; str1[i] == str2[i] && str1[i] != '\0'; i++) {
found[i] = str1[i];
}
found[i] = '\0';
return i;
}
int main(void) {
char str1[100];
char str2[100];
char prefix[100];
printf("\nGive string 1: ");
if (scanf("%99s", str1) != 1)
return 1;
printf("\nGive string 2: ");
if (scanf("%99s", str2) != 1)
return 1;
//Print prefix
findprefix(str1, str2, prefix);
printf("%s\n", prefix);
return 0;
}