Question

Having char double[] = "1.2345678"; , I have to trim all digits in 4nd place and above after the . in this char * i.e make it to "1.234" .

Answer 1

One obvious solution is to replace a character by NUL, like so:

char *foo = strdup("1.2345678");   // FIXME: check return value
foo[5] = '\0';

Note that the exact position might differ, depening on how many digits appear before the ‘.’ character.

Iterate over the string foo, change the state in passed_dot if you come across ‘.’, and insert a NUL after 4 more characters:

char *p = foo;
int i = 0;
int passed_dot = 0;
while (p && *p) {
        if (*p == '.') passed_dot = 1;
        if (passed_dot) i++;
        if (i == 4) {
                *p = '\0';
                break;
        }

        p++;
}

If you can’t afford to buy more RAM, you may strdup() the resulting string and free() the old one in order to save memory:

new_str = strdup(foo);   // and don't forget to check for NULL
free(foo);