Finding number of occurrence of character in sub-string

Question

What is the fastest method to find the number of occurrence of character in a sub-string of string?

I have tried the normal scanning each character but the test data is too large.

Give us your code. Is the problem related to the fact that there's too much data to load into memory at once? This is very vague right now. — BaseZen, Mar 06 '15 at 19:54
If you want to count how many times a particular character appears, then you have no choice but to look at every byte. Otherwise you might be interested by [Boyer-Moore](https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore_string_search_algorithm) and in general [what grep does](https://lists.freebsd.org/pipermail/freebsd-current/2010-August/019310.html) — tux3, Mar 06 '15 at 19:56
Does this have to be C or can you use C++? If so then this answer is properly the most efficient way. https://stackoverflow.com/questions/3867890/count-character-occurrences-in-a-string — wybourn, Mar 06 '15 at 19:56
@wybourn why do you ask? I think there would be a c++ tag otherwise. — Iharob Al Asimi, Mar 06 '15 at 19:57

Iharob Al Asimi · Answer 1 · 2015-03-06T20:13:28.297

1

It's very simple

unsigned int countChar(const char *string, char characterOfInterest)
 {
    unsigned int count; 

    count = 0;
    while ((string = strchr(string, characterOfInterest)) != NULL)
     {
        count  += 1;
        string += 1;
     }
    return count;
 }

edited Mar 06 '15 at 20:13

answered Mar 06 '15 at 19:57

Iharob Al Asimi

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2

This code is calling `strchr` for each character of the string, there is no need to do that. – David Ranieri Mar 06 '15 at 20:04
1

Maybe something like `while ((p = strchr(string, characterOfInterest)) != NULL) { count++; string = p + 1; }` Note: `characterOfInterest == 0` is a special case. – chux - Reinstate Monica Mar 06 '15 at 20:09
or maybe `char *p=string; while ((p = strchr(p, characterOfInterest)) != NULL) { count++; p++; }` (to preserve `string` unchanged) – David C. Rankin Mar 06 '15 at 20:28
1

@DavidC.Rankin good suggestion although there is no benefit in that, IMO. – Iharob Al Asimi Mar 06 '15 at 20:31
@DavidC.Rankin is there any real difference with this answer, except that **iharob** wrote to `const char *string` as the argument as opposed to its target string? – Weather Vane Mar 06 '15 at 20:45
@iharob don't be so antagonistic. I was supporting your comment, altering the pointer provided is ok, **DavidC.Rankin** proposed the same thing with another pointer var. Why can't you just forget that I downvoted you once, but have upvoted since many times? – Weather Vane Mar 06 '15 at 20:51
@WeatherVane I didn't understand your comment at first, and then ... I deleted mine. And I didn't know that you downvoted one of my answers. – Iharob Al Asimi Mar 06 '15 at 20:54
@iharob you once complained about my downvote when I copped to it. If I was out of order, I apologise. – Weather Vane Mar 06 '15 at 21:06
@WeatherVane - first it is a **comment**, not an **answer**. The reason for it (generally, not with `strchr`) is to preserve the pointer to the original string. So `string` remains `string`. All commenting and answering know, that **it does not matter for this function**, but the poster may not know that failing to do so with other string functions, like `strtok`, or where `string` is dynamically allocated will cause headaches. That's why it was worth a comment. – David C. Rankin Mar 07 '15 at 03:58

Finding number of occurrence of character in sub-string

1 Answers1