3

this is automatically generated C code, so please don't mind my method of scrolling though a char pointer

I'm using a char pointer to store a string:

char str[] = "Hello!";
char *ptr = str;

And I use pointer arithmetic to scroll through it:

++ptr;
++ptr;
...
...
--ptr;

I'd like to know at some point to exactly what index it's pointing (eg. 1 for 'e' in "Hello"). How do I do this?

Yu Hao
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Mark
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4 Answers4

11

Appropriately enough, std::distance(str, ptr) or just ptr - str.

Don Reba
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2

You can Simply print that corresponding value with printf or cout with %c to get the character where that is pointing.

printf("%c",*p);

If you want to know the position then 

printf("%td\n",ptr-str);
Karthikeyan.R.S
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    the correct format is [`%td`](http://stackoverflow.com/questions/7954439/c-which-character-should-be-used-for-ptrdiff-t-in-printf), not `%d` – phuclv Dec 09 '14 at 11:07
  • @LưuVĩnhPhúc Thank you I done that in editing. But normally use the %d it is working. – Karthikeyan.R.S Dec 09 '14 at 12:43
  • no, stricly speaking, it's undefined behavior, esp. in systems with pointers larger than 32 bits like 64-bit architectures – phuclv Dec 09 '14 at 12:45
0

Just do 

   printf("\ndistance[%d]",ptr-str);
Saravanan
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    the result of the subtraction between 2 pointers isn't an int, so using `%d` invoke undefined behavior – phuclv Dec 09 '14 at 11:07
0

To answer your question, simplest way is use std::distance. This will return the number of elements between the given iterators. In your case, std::distance(str, ptr) return the number of elements between str and ptr. Say str is pointing to start and ptr is somewhere at 'o', then the value returned by this API is 4.

Jagadeesh Nellimarla
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