How can I define and call this code as a function? - c

Question

I'm new to programming.Im learning from CS50. I'm not sure on how to define this code as a function.I want the function to return key[n].And how to call the function?
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, string argv[])
{
int n = strlen(argv[1]);
int key[n];
int k =0;
for( int i = 0; i < strlen(argv[1]); i++)
{
if(islower(argv[1][i]) != 0)
{
for(int j = 97; j < argv[1][i]; j++)
{
k++;
}
key[i] = k;
k = k*0;
}
if(isupper(argv[1][i]) != 0)
{
for(int j = 65; j < argv[1][i]; j++)
{
k++;
}
key[i] = k;
k = k*0;
}
}
}

Answer 1

The function can look for example the following way as it is shown in the demonstrative program below.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int * f( const char *s )
{
int *key = NULL;
size_t n = strlen( s );
if ( n != 0 && ( key = calloc( n, sizeof( int ) ) ) != NULL )
{
for ( size_t i = 0; i < n; i++ )
{
unsigned char c = s[i];
if ( islower( c ) ) key[i] = c - 'a';
if ( isupper( c ) ) key[i] = c - 'A';
}
}
return key;
}
int main( int argc, char * argv[] )
{
if ( argc == 2 )
{
int *key = f( argv[1] );
if (key != NULL)
{
// some other code
free( key );
}
}
return 0;
}

Answer 2

A function is a block of code with an "in" and "out". You define a function to receive values from the outside with syntax like int key (int example){} and pass in values to your function call with key(1);.
Your main() is also a function. The values that main() receives are being used in your code right now.
Your own function (which you define outside of main()) can be done in many ways, but the easiest would be to simply keep almost all of the same code within main() while figuring out how to transport what main() received into yours.