Thread: ++ Operator Misunderstanding

I think that you are trying to do this:

Code:

putchar( *(++some_string) );

You are incrementing the pointer after the line has completed, instead of before.

[edit]

Code:

banana = apple++ + mango;
//is the same as
banana = apple + mango;
apple = apple + 1;

banana = ++apple + mango;
//is the same as
apple = apple + 1;
banana = apple + mango;

[/edit]

Originally Posted by hex_dump

doesn't the ++ operator have higher precedence... As it stands it starts printing from (string + 0).

It does have higher precedence, but this is the correct behaviour for the code you posted. Remember that with postfix ++ (i.e. ++ after the operand like"some_string++") its operand is first used and then incremented - the increment only happens *after* its value is taken. With prefix ++, it is first incremented, then the (newly incremented) value is used.

As a more concrete example:

Code:

int post_a = 2;
int post_b = post_a++;  // post_a++ yields value of post_a *before* it was incremented.
// At this point, post_a = 3, post_b = 2.

int pre_a = 2;
int pre_b = ++pre_a;  // ++pre_a yields value of pre_a *after* it gets incremented.
// At this point, pre_a = 3, pre_b = 3.

Take a look here: C Operator Precedence Table. Pay particular attention to 'Note 2' at the bottom.

To clarify more, yes, it has higher precedence, and your parentheses do ensure the ++ happens first, but you are misunderstanding the semantics of the ++ postfix operator.

First, it may help to understand the concept of a sequence point (link). Basically, the increment happens at the end of a sequence point. A sequence point is the end of a full expression. Your parenthetical is not a full expression. You have a case more similar to rule #4: the call to putchar is the sequence point at which the increment happens, however the original value of some_string is passed to putchar.

The result of the parenthetical expression

Code:

(some_string++);

is the current value of some_string, i.e. some_string+0. That value is used for the dereference operator *. Then, when you reach a sequence point, the increment happens. Basically what you have is equivalent to:

Code:

putchar( *(some_string) );
some_string += 1;

Click_here gave you a possible solution if you wish to skip the first char, using the ++ prefix operator.

Hope that clears it up a bit.