Need String initialization advice - c

Question

I got an assignment for wich i have to write an program that will take the letters in the first parameter string, and find them in the second parameter string like so:
./a.out "lolabab" "ablcocllcab"
the program needs to print "loab", because each letter should only be printed once.
here's the main part of my program
char *do_stuff(char *s1, char *s2)
{
int i, j, k;
char *out;
out = malloc(sizeof(char) * str_len(s1));
i = 0;
j = 0;
k = 0;
while (s2[j] != '\0' && s1[i] != '\0')
{
if (s2[j] == s1[i])
{
if (check_char(out, s1[i]) == 0)
{
out[k] = s1[i];
k++;
}
i++;
j = -1;
}
j++;
}
return (out);
}
my question is: if I dont initialize "out" i have a problem.
i initialize it with malloc at the moment, but i am not allowed to use malloc :).
any other way i tried, seems to not work for me (segmentation fault).
So how do i initialize a string without using malloc?
It's probably obvious, but i'm new at this so pls help. Thanks!

Answer 1

You can always pass the output buffer as a parameter
void do_stuff(char *s1, char *s2, char *out /* some large enough char [] */)
{
int i, j, k;
i = 0;
j = 0;
k = 0;
while (s2[j] != '\0' && s1[i] != '\0')
{
if (s2[j] == s1[i])
{
if (check_char(out, s1[i]) == 0)
{
out[k] = s1[i];
k++;
}
i++;
j = -1;
}
j++;
}
}
and in the calling function
char result[SOME_REASONABLE_SIZE] = {0} /* initialize it for the check_char function */;
do_stuff(argv[1], argv[2], result);
you should check that the function recieved the 2 arguments of course.
One more thing, try not to use strlen in the check char function, pass the current string length k to it, that way your program would be more efficient.

Answer 2

Use the fact that the number of characters is constant (and relatively small):
#include <limits.h>
#define CHAR_NUM (1<<CHAR_BIT)
#define FLAG(x) (1<<(x))
void get_common_chars(char* s1,char* s2,char out[CHAR_NUM])
{
int i,n;
int flags[CHAR_NUM] = {0};
for (i=0; s1[i]!=0; i++)
flags[(unsigned char)s1[i]] |= FLAG(1);
for (i=0; s2[i]!=0; i++)
flags[(unsigned char)s2[i]] |= FLAG(2);
n = 0;
for (i=0; i<CHAR_NUM; i++)
if (flags[i] == FLAG(1)|FLAG(2))
out[n++] = (char)i;
out[n] = 0;
}
If you're only interested in non-capital letters, then you can further improve it:
#define MIN_CHAR 'a'
#define MAX_CHAR 'z'
#define CHAR_NUM (MAX_CHAR-MIN_CHAR+1)
#define FLAG(x) (1<<(x))
void get_common_chars(char* s1,char* s2,char out[CHAR_NUM])
{
int i,n;
int flags[CHAR_NUM] = {0};
for (i=0; s1[i]!=0; i++)
if (MIN_CHAR <= s1[i] && s1[i] <= MAX_CHAR)
flags[s1[i]-MIN_CHAR] |= FLAG(1);
for (i=0; s2[i]!=0; i++)
if (MIN_CHAR <= s2[i] && s2[i] <= MAX_CHAR)
flags[s2[i]-MIN_CHAR] |= FLAG(1);
n = 0;
for (i=0; i<CHAR_NUM; i++)
if (flags[i] == FLAG(1)|FLAG(2))
out[n++] = (char)(MIN_CHAR+i);
out[n] = 0;
}
Here is a usage example:
#include <stdio.h>
int main(int argc,char* argv[])
{
char common_chars[CHAR_NUM];
if (argc >= 3)
{
get_common_chars(argv[1],argv[2],common_chars);
printf("%s\n",common_chars);
}
return 0;
}

Answer 3

If I understand correctly what you need, you should not create a new string, but use the command-line parameters, which are available in the arguments of main().
When you write
int main(int argc, char** argv) {
The compiler will arrange so that argc is the number of command-line arguments, and argv is an array of strings with the arguments. The first, argv[0], is the program name, and the rest are arguments passed to the program.
So this is one way to get your assignment done (high-level description only -- the rest is yours!)
Take the first argument, argv[1], and loop over it, character by character. For each character, try to find it in the other argument, argv[2]. If you find it, print the single character.
No need to allocate memory at all!
edit: if you don't want to print doubles, then one way would be to keep a static array that you could use as an index of already printed characters:
static int printed[26] = { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 };
When you print c, set its position to 1. And only print if the character's position is zero.
It's up to you to find out how to find the index of an arbitrary character (and to decide wether you want to differentiate between upper and lower case).