Logical Expressions in C misunderstanding

So, I was writing some code and I was getting an unexpected output in one part of my program which disrupted the entire system.

I managed to extract and simplify the problem to a basic logical expression. Let's say:

int i = 1, j = 1, k = 0;

printf("%d\n", ++i || ++j && k);
printf("%d, %d, %d\n", i, j, k);

return 0;

The output for this program is:

`1`

`2` `1` `0`

I am thinking that the value of `j` was not incremented to `2` due to the short circuit nature of the `||` operator. However I am confused how the value of the first `"%d"` is `1`. Shouldn't the value of `k` be non-zero for the `&&` statement to return `1`? Or isn't this statement executed at all since `++i || ++j` is not `0` and thus returns a `1`?

- I know that `&&` is a logical *and*, and *expr1* && *expr2* has the value 1 if values of *expr1* && *expr2* are **both** non-zero.

Any clarifications would be appreciated and please excuse the basic nature of this question.

`++i || ++j && k` is evaluate to 1 (true) because i equals 2 (++1), (++j && k) isn't evaluated because short circuit.

From the C Standard (6.5.14 Logical OR operator)

> 3 The || operator shall yield 1 if either of its operands compare
> unequal to 0; otherwise, it yields 0. The result has type int.

This expression

++i || ++j && k

is equivalent to

++i || ( ++j && k )

and according to the quote from the Standard the expression returns integer value 1 because `++i` is not equal to zero. The subexpression `( ++j && k )` is not evaluated.

Operators precedence. `&&` has higher precedence than `||`.

Your expression is the same as: `++i || (++j && k)`

`++i` is `TRUE`, the parenthesis is not evaluated anymore.

`&&` has higher precedence than `||`.
(See

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)
so

++i || ++j && k
is

++i || (++j && k)

and `||` shortcircuits if the first operator is truthy, as per [6.5.14p4][1] .

If you're on `gcc` or `clang` and compile your code with `-Wall`, the compiler will nudge you to put those parentheses there. It's probably a good idea to heed that advice, as some people get confused by the precedences (I hear).


[1]:

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