Deleting characters from string

[myk45]

Hello.

i wrote a program to do the following:

Given two strings S1 and S2. Delete from S2 all those characters which occur in S1 also and finally create a clean S2 with the relevant characters deleted.

This is what i tried:

void trimString()
{
	char s1[] = "hello world";
	char s2[] = "el";
	int i, j;

	for (i = 0; i < (signed)strlen(s1); i++) {
		for (j = 0; j < (signed)strlen(s2); j++) {
			if (s1[i] == s2[j]) {
				s1[i] = -1;
				break;
			}
		}
	}
	for (i = 0, j = 0; i < (signed)strlen(s1); i++) {
		if (s1[i] != -1) {
			s1[j] = s1[i];
			j++;
		}
	}
	s1[j] = '\0';
	printf("\nString is%s", s1);
}

Can i further improve it in any way?

Thanks.

[Ancient Dragon]

One way to do it is to use another character array to copy all the characters in s1 that are not in s2.

void trimString()
{
   char s1[] = "hello world";
   char s2[] = "el";
   char s3[255] = {0};
   size_t sz1 = strlen(s1);
   size_t j = 0;
   for(size_t i = 0; i < sz1; i++)
   {
       if( !strchr(s2,s1[i]) )
          s3[j++] = s1[i];
   }
}

[myk45]

@AncientDragon

Thanks for the reply.

Well, doesnt

if( !strchr(s2,s1[i]) )

take same time as

for (j = 0; j < (signed)strlen(s2); j++) {
	if (s1[i] == s2[j]) {
		s1[i] = -1;
		break;
	}
}

i was hoping to make the running time better.

[WaltP]

Can i further improve it in any way?

Thanks.

Yes. See the comments I added:

void trimString()
{
	char s1[] = "hello world";
	char s2[] = "el";
	int i, j;

	/* Do not use strlen() as a loop parameter.  
	   Use it to load a variable before the loop and use 
	   the variable in the loop definition */
	/* Why are you casting the value to signed? Define the loop 
	   parameter variables as unsigned if you think it's necessary */
	for (i = 0; i < (signed)strlen(s1); i++) {
		for (j = 0; j < (signed)strlen(s2); j++) {
			if (s1[i] == s2[j]) {
				s1[i] = -1;   // this is good
				break;
			}
		}
	}
	for (i = 0, j = 0; i < (signed)strlen(s1); i++) {
		if (s1[i] != -1) {
			s1[j] = s1[i];
			j++;
		}
	}
	s1[j] = '\0';
	printf("\nString is%s", s1);
}

[Ancient Dragon]

>>take same time as

Not necessarily, depending on how the compiler implements strchr() function. calling strlen() on every loop iteration is very very time consuming and I doubt any compiler will implement strchr() with that. On Intel compatible x86 processors compilers can use a processor instruction to do it.

[abhimanipal]

You could take the hash map route

[myk45]

Thanks for the replies.