Increment function with pointers not working

Question

I am trying to find the number of digits in a given number through the use of pointers.

This code below will give the correct output,

void numDigits(int num, int *result) {
  *result = 0;

  do {
    *result += 1;
    num = num / 10;
  } while (num > 0);
}

However, if I change the *result += 1; line to *result++;, the output will no longer be correct and only give 0.

What is going on here?

Check this will helps, stackoverflow.com/questions/1068849/… — yazan
– yazan, Commented Oct 14, 2020 at 6:46

Jasmeet · Accepted Answer · 2020-10-14 06:57:44Z

2

In C/C++, precedence of Prefix ++ (or Prefix --) has higher priority than dereference (*) operator, and precedence of Postfix ++ (or Postfix --) is higher than both Prefix ++ and *.

If p is a pointer then *p++ is equivalent to *(p++) and ++*p is equivalent to ++(*p) (both Prefix ++ and * are right associative).

edited Oct 14, 2020 at 6:57

answered Oct 14, 2020 at 6:49

Jasmeet

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Comments

hotoku · Accepted Answer · 2020-10-14 06:50:52Z

1

*result++ is interpreted as *(result++).

answered Oct 14, 2020 at 6:50

hotoku

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Comments

Gilad · Accepted Answer · 2020-10-14 06:48:50Z

0

the order of evaluation is the increment is first, then you dereference the pointer, the value of the pointer stays the same for the line in the expression if it's a postfix. what you are trying to do is to increment the dereference of the pointer so you need to put the parenthesis on the dereference then increment it, like this: (*result)++;

answered Oct 14, 2020 at 6:48

Gilad

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