C: Loop through struct and concat to string - c

Question

I have C program with the following struct:
struct {
char *ext;
char *filetype;
} extensions [] = {
{"gif", "image/gif" },
{"jpg", "image/jpg" },
{"jpeg","image/jpeg"},
{"png", "image/png" },
{0,0}
};
How do I create a function that returns a string that contains only the extensions separated by new lines? Basically this is what to be able to do this:
printf("\nThe following extensions are supported:\n%s",GetExtensions());
And have it output this:
The following extensions are supported:
.gif
.jpg
.jpeg
.png
I think I've got the looping part correct, but I'm not understanding how to concat each ext + \n to a string:
#include <leaving these off for brevity...>
struct {
char *ext;
char *filetype;
} extensions [] = {
{"gif", "image/gif" },
{"jpg", "image/jpg" },
{"jpeg","image/jpeg"},
{"png", "image/png" },
{0,0}
};
char *getExtensions(void) {
char* exts;
int i;
for(i=0;extensions[i].ext != 0;i++){
// What do I do here??
}
return exts;
}
int main(int argc, char **argv){
printf("\nThe following extensions are supported: \n%s",GetExtensions());
}

Answer 1

It seems you mean the following.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct extension
{
char *ext;
char *filetype;
};
char * getExtensions( const struct extension a[] )
{
size_t n = 0;
for ( size_t i = 0; a[i].ext != NULL; i++ )
{
n += strlen( a[i].ext ) + 1;
}
char *s = malloc( n + 1 );
if ( s != NULL )
{
s[0] = '\0';
for ( size_t i = 0; a[i].ext != NULL; i++ )
{
strcat( s, a[i].ext );
strcat( s, "\n" );
}
}
return s;
}
int main(void)
{
struct extension extensions [] =
{
{"gif", "image/gif" },
{"jpg", "image/jpg" },
{"jpeg","image/jpeg"},
{"png", "image/png" },
{ NULL, NULL }
};
char *s = getExtensions( extensions );
printf( "The following extensions are supported:\n%s", s );
free( s );
return 0;
}
The program output is
The following extensions are supported:
gif
jpg
jpeg
png

Answer 2

#include <stdio.h>
#include <string.h>
// Your struct
struct extensionInfo {
char *ext;
char *filetype;
};
struct extensionInfo extensions [] = {
{"gif", "image/gif" },
{"jpg", "image/jpg" },
{"jpeg","image/jpeg"},
{"png", "image/png" },
{0,0}
};
int main(int argc, char **args, char **env) {
char buffer[1024];
struct extensionInfo *ext;
// Initialize the buffer
memset(buffer, 0, sizeof(buffer));
// Insert your first text.
strncat(buffer, "The following extensions are supported:", sizeof(buffer) - 1);
// Loop through your array and append everything
for (ext = extensions; ext->ext != 0; ext++) {
strncat(buffer, "\n", sizeof(buffer) - 1);
strncat(buffer, ext->ext, sizeof(buffer) - 1);
}
// Show your result
printf("%s\n", buffer);
return 0;
}
Here is a commented example which works. If you have any questions, feel free to ask.

Answer 3

I think the following is what you wish. You can read the comments to grasp how it works.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct {
char *ext;
char *filetype;
} extensions [] = {
{"gif", "image/gif" },
{"jpg", "image/jpg" },
{"jpeg","image/jpeg"},
{"png", "image/png" },
{0,0}
};
char *getExtensions(void) {
size_t count = 0;
// let's know how many characters are needed for all extensions
for(int i = 0; extensions[i].ext != 0; ++i) {
size_t j = 0;
for (; extensions[i].ext[j] != 0; ++j) ;
count += j + 1; // +1 for every newline
}
// +1 for null terminator
char *str = calloc(count + 1, sizeof(char));
// let's concatenate
for(size_t i = 0; extensions[i].ext != 0; ++i) {
strcat(str, extensions[i].ext);
strcat(str, "\n");
}
return str;
}
int main(int argc, char **argv){
char * const result = getExtensions();
printf("\nThe following extensions are supported: \n%s", result);
free(result);
}