Traversal of C string through pointer incrementation

Question

I'm reading through Bjarne Stroustrup's A Tour of C++ and I'm having trouble understanding an early example. In the code below, a while loop is used to increment a pointer over a C style string until it hits the null character. What I don't get is why pointing to a null character would cause the pointer to take on the value of nullptr. It seems like it should be a perfectly good pointer that happens to point to a null value. I'd be surprised if it were an author error since the book is included in stackexchange's reccommendations here: The Definitive C++ Book Guide and List

int count_x(char∗ p, char x)
    // count the number of occurrences of x in p[]
    // p is assumed to point to a zero-terminated array of char (or to nothing)
{
    if (p==nullptr) return 0;
    int count = 0;
    for (; p!=nullptr; ++p)
        if (∗p==x)
            ++count;
        return count;
}

The sample in the book is different. The loop is `while(*p)`. Which edition do you have? — imreal, Oct 07 '14 at 05:56
Well, I have a version that I found by googling "*A Tour of C++* pdf" (first result). Serves me right I guess, although interestingly it purports to be copywrited 2014. — agd, Oct 07 '14 at 06:03
According to the [errata](http://www.stroustrup.com/Tour_printing2.html), "The code for count_if() is wrong", in the 1st printing, fixed in the 2nd. — Mike Seymour, Oct 07 '14 at 06:06

score 4 · Accepted Answer · answered Oct 07 '14 at 05:51

4

To check for null character, you should test *p != '\0', not p != nullptr.

answered Oct 07 '14 at 05:51

timrau

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score 2 · Answer 2 · answered Oct 07 '14 at 05:58

2

It segs for the reasons you mention. Try:

  for (; *p; ++p)

answered Oct 07 '14 at 05:58

Adrian May

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Traversal of C string through pointer incrementation

2 Answers2