I need to find common prefix between two strings in C++ - c++

Question

My strncpy function is not working, shows argument of type "cons char" is in compatible with parameter type "char"
And when I call out the prefix function in the main function it says i must have a pointer to function type
#include <iostream>
#include <string>
#include <cstring>
using namespace std;
void prefix(const char s1[], const char s2[], char prefix[]);
int main()
{
char s1[30];
char s2[30];
char prefix[30];
cout << "Enter two sentences to store in two different strings" << endl;
cin.getline(s1, 30);
cin.getline(s2, 30);
prefix(s1, s2, prefix);
}
void prefix(const char a[], const char b[], char prefix[])
{
int size;
if (strlen(a) < strlen(b))
{
size = strlen(a);
}
else if (strlen(a) > strlen(b))
{
size = strlen(b);
}
else
{
size = strlen(a);
}
for (int i = 0; i < size; i++)
{
if (a[i] != b[i])
{
strncpy(a, b, size);
}
}
}

Answer 1

Not sure on your exact error, but it is probably like "error C2064: term does not evaluate to a function taking 3 arguments" or "error: ‘prefix’ cannot be used as a function".
The issue here is you declared a local variable with the name prefix, so it will take precedence over the global function prefix. Some types of variable may be callable (e.g. function pointers, std::function, etc.).
The best solution for that is generally to rename your local, but you can explicitly tell it to use the global scope if desired: ::prefix(s1, s2, prefix);.
There are further errors within the prefix function itself however, as strncpy(a, b, size); tries to copy to a "const" string, which is not allowed, presumably you meant to copy to the prefix string instead, and probably end the loop there.
However, for C++ it would also generally be better to use the std::string type. You can use std::getline(std::cin, my_std_string) to read lines, and prefix = my_std_string.substr(0, i) would be a way to copy part of a string.

Answer 2

For starters this declaration in main
char prefix[30];
hides the function with the same name declared in the global name space.
Either rename the function or the variable or use a qualified name for the function.
This loop
for (int i = 0; i < size; i++)
{
if (a[i] != b[i])
{
strncpy(a, b, size);
}
}
does not make sense and in this call
strncpy(a, b, size);
you are trying to change the constant array pointed to by the pointer a.
And there are many redundant calls of the function strlen.
The function can be declared and defined the following way as it is shown in the demonstrative program below.
#include <iostream>
char * common_prefix( const char s1[], const char s2[], char prefix[] )
{
char *p = prefix;
for ( ; *s1 != '\0' && *s1 == *s2; ++s1, ++s2 )
{
*p++ = *s1;
}
*p = '\0';
return prefix;
}
int main()
{
const size_t N = 30;
char s1[N];
char s2[N];
char prefix[N];
std::cout << "Enter two sentences to store in two different strings" << '\n';
std::cin.getline( s1, N );
std::cin.getline( s2, N );
std::cout << "The common prefix is \"" << common_prefix( s1, s2, prefix )
<< "\"\n";
return 0;
}
Its output might look like
Enter two sentences to store in two different strings
Hello C#
Hello C++
The common prefix is "Hello C"