Displaying pointer substraction

Question

Okay, say I have a sequence "abcdefg" and I do

char* s = strdup("abcdefg");
char* p;
char* q;
p = strchr(s, 'c');// -> cdefg
q = strchr(p, 'd');// -> defg

And I want to display s - p basically abcdefg - cdefg = ab, can I do this using pointer arithmetic?

Is modifying `s` an option? You duplicated it, so I suspect that shouldn't be an issue. — StoryTeller - Unslander Monica, Dec 25 '18 at 11:23

score 7 · Accepted Answer · answered Dec 25 '18 at 11:28

7

You can do:

printf("%.*s", (int)(p - s), s);

This prints s with a maximum length of p - s which is the number of characters from s to p.

answered Dec 25 '18 at 11:28

M.M

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Fascinating. I had no idea the percision affected strings like that. – StoryTeller - Unslander Monica Dec 25 '18 at 11:30
Is there any chance to save it in a `char*` pointer, something lile: `const char *ptr = (int)(p - s), s;` ? – Michi Dec 25 '18 at 19:15
@Michi a `char *` points to a character, the only thing you can store in a pointer is the address of a character – M.M Dec 25 '18 at 21:02
@M.M You right. I really do not know where I was thinking. Probably `sprintf( buf, "%.*s", (int)(p - s), s );` will do the job. Thank you :). – Michi Dec 26 '18 at 11:14

score 1 · Answer 2 · answered Dec 25 '18 at 11:20

You cannot. The string is saved as a sequence of letters ending in a zero byte. If you print a pointer as a string, you will tell it to print the sequence up to the zero byte, which is how you get cdefg and defg. There is no sequence of bytes that has 'a', 'b', 0 - you would need to copy that into a new char array. s - p will simply give you 2 (the distance between the two) - there is no amount of pointer arithmetic that will copy a sequence for you.

To get a substring, see Strings in C, how to get subString - copy with strncpy, then place a null terminator.

M.M said `You can` and you said: `You cannot`. :)) – Michi Dec 25 '18 at 16:04 — Michi, Dec 25 '18 at 16:04

score 1 · Answer 3 · answered Dec 25 '18 at 11:27

Okay, if you need to cut p from s, the following code should work:

char s[100];  // Use static mutable array
strcpy(s, "abcdefg");  // init array (ok, ok I dont remember the syntax how to init arrays)
char* p = strchr(s, 'c');// -> cdefg
*p = 0;  // cut the found substring off from main string
printf("%s\n", s); // -> ab

Displaying pointer substraction

3 Answers3