Understanding pointers in C/C++

Question

I'm trying to understand how pointers work and I'm stuck at this line

for (p = s + strlen(s) - 1; s < p; s++, p--)

I don't understand p what it's equated to. can anyone help me?

here's the full code.

void Reverse(char *s){
    char c, *p;

    for (p = s + strlen(s) - 1; s < p; s++, p--){
        c = *s;
        *s = *p;
        *p = c;
    }
}


int main(){
    char ch[] = "!dlroW olleH";
    Reverse(ch);
    printf("%s", ch);

    return 0;

}

Read up on "pointer arithmetic". [Here](https://stackoverflow.com/questions/394767/pointer-arithmetic) is a question on that topic. Once you understand that then you will see that `p = s + strlen(s) - 1` sets `p` to point to the last (non-NUL) character in the `s` string. — kaylum, Feb 19 '16 at 00:52

CodeArtist · Answer 1 · 2016-02-19T01:35:36.457

0

On the statement: for (p = s + strlen(s) - 1; s < p; s++, p--)

`p` contains the pointer to the end of the `s` string.
`s` contains the pointer to the beginning of the string.
`strlen()` calculates the length of the string

So in the beginning p = s + strlen(s) is assigned only once and translates to p = <starting point of the pointer of the s string> + <length of s string>

Then s increases his position and p decreases his own.

Inside the for loop position they just swapping characters using c like temporary variable.

Example for the word hello:

1) hello
2) oellh
3) olleh

edited Feb 19 '16 at 01:35

answered Feb 19 '16 at 00:55

CodeArtist

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I'm confused, with s + strlens(s). What is s ? an address? . and the function strlen(s) -1 is the number of char? – seito Feb 19 '16 at 01:00
I Updated my answer. It's understandable? – CodeArtist Feb 19 '16 at 01:05
yes! thank you very much! – seito Feb 19 '16 at 01:07
Also can you not say "reference" when you mean "pointer", please. It's very confusing, and indeed wrong on a question tagged [tag:c++]. – Lightness Races in Orbit Feb 19 '16 at 01:29
@PreferenceBean strictly talking you are right. I have a broader background i'm not using only `c` but either way i modified the answer to comply with the terminology. – CodeArtist Feb 19 '16 at 01:38
@CodeArtist: Many of us have a broader background! I doubt anyone here knows _just_ C and C++, but the question is specifically about those languages. Thanks for the update. – Lightness Races in Orbit Feb 19 '16 at 02:24

score 0 · Answer 2 · answered Feb 19 '16 at 00:56

In this example the for loop is going backwards through s as the idea is to reverse the data held in s. P is being assigned the last memory location of s and though pointer arithmetic (p-- in the code) is going backwards through s. To access the data in each memory location the pointers s and p are dereferenced by putting * in front of each variable (*p).

Does this make it any clearer?

Understanding pointers in C/C++

2 Answers2