CODEWITHSUNDEEP
c++pointerswhile-loopincrement
Ming An
Ming An

Reputation: 155

Increment a char pointer in c++

I am working on a question from Cracking the Coding Interview. The question is: Write code to reverse a C-Style String. And here is the answer I got that is written in c++, but I don't understand the meaning behind increment a char pointer.

void swap(char &a, char &b){
    a = a^b;
    b = a^b;
    a = a^b;
}

void reverse1(char *s){
    if(!s) return;
    char *p = s, *q = s;
    while(*q) ++q;
    --q;
    while(p < q)
        swap(*p++, *q--);
}

The part that I don't understand is in the second function from the first while loop. What is the condition *p? And why increment q will chop off the starting character in the string?

For example, if

char s[] = "1234567890";

After you pass s into function reverse1, why will ++q give you a result like "234567890"?

Upvotes: 0

Views: 2019

Answers (4)

Pouya Tehrani
Pouya Tehrani

Reputation: 96

p and q are pointer to char and both are initialized as the address of the first char in the string s[] (as the name of an array in C is a pointer to its first element). while (*q) q++; increases q (the position of char) until it reaches the end of the string ( NULL character '\0'). and --q decreases the position to point to one before NULL char which is the last element (char) of the string.

That is why after passing an string to reverse1() and doing ++q, the result would start from the second char rather than the first one.

Upvotes: 1

Chaka
Chaka

Reputation: 387

Thats the question for '\0' which terminates the string.

and the result is: 

0987654321

as expected from a function called reverse (in Visual Studio 2013)

Upvotes: -1

user2477
user2477

Reputation: 997

Strings are arrays. q is pointing to the first character of the string. So when you increase q. q will point to the next character. enter image description here

Upvotes: 2

Vagish
Vagish

Reputation: 2547

C style strings are null(0x00 or /0) terminated.

char s[] = "1234567890"; will be stored in memory like this: 0x31 0x32 0x33 0x34 0x35 0x36 0x37 0x38 0x39 0x30 0x00

The first while loop is searching for end of string.

At start of while loop q points to 0x31.

In each iteration q is incremented to point to next memory location.

When q points to 0x00 loop breaks.

Upvotes: 0

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