How to count the appearances of a string within a string?

Question

I have to count the appearances of a specific C string within a bigger one. I am using strstr() function to count and advance with a char pointer through the big C string. The problem is that it seems to count too little, like the reduction of the text is increasing to fast, jumping over big chunks of characters. I am using a type int variable, called 'x', to 'reduce' text from left to right, but I don't know were is the problem with assigning its new value.

int x=0;
while((p=strstr(text+x,triG))!=NULL)
{
    v[i]++;
    x+=x+3+(p-(text+x));
}

text is of type char*, dynamically alocated. triG is of type char[4], and p is of type char*.

Are there any restrictions on the content of text and triG besides the maximum lenght of triG? Do you need to find for example all occurrences of "aaa" in "aaaaaaaaaa"? ;-) — alk
– alk, Commented Oct 12, 2012 at 9:04

simonc · Accepted Answer · 2012-10-12 09:06:49Z

5

You're increasing the value of x by too much after each match.

int x=0;
while((p=strstr(text+x,triG))!=NULL)
{
    v[i]++;
    x = p - text + strlen(triG);
}

You can also simplify the calculation of x. You were both adding and removing x - these cancel out. Its also a bit more flexible to use strlen(triG) rather than hard-coding the assumption it'll always be 4.

edited Oct 12, 2012 at 9:06

answered Oct 12, 2012 at 8:59

simonc

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2 Comments

Ionut Goldan Over a year ago

Thank you so much...It's seems so silly to me now. To much debugging over C code, I guess. A wish you all a nice day:D

simonc Over a year ago

@IonutGoldan Glad this has helped. If you're satisfied with this as an answer, can you consider accepting it?

Peter Miehle · Accepted Answer · 2012-10-12 09:05:52Z

1

you have

x+=x+...

you should have either

x = x+...

or

x += ...

answered Oct 12, 2012 at 9:05

Peter Miehle

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