Does if (!(-1)) evaluate to true or false in C?

c if-statement negative-number negation
codenaugh picture codenaugh · Jun 17, 2016 · Viewed 9.6k times · Source

I know that 0 and NULL evaluate to FALSE on their own and I know that a negative integer or a positive integer evaluate to TRUE on their own.

My understanding is that the NOT operation will happen after evaluating the expression, so if (-1) will evaluate to TRUE, then applying the ! operand will mean NOT TRUE which equals FALSE. Is this the correct order of operations and is it correct that if (!(-1)) will evaluate to FALSE?

Answer

Vality picture Vality · Jun 17, 2016

The evaluation of if (!(-1)) can be worked out by thinking about the operator precedences.

Firstly the unary - is applied to 1 which produces an integral -1. Then this value is logically negated by the !. This involves collapsing -1 into a logical value. The rule for this in C is nice and simple for integral types: 0 is falsy and everything else is truthy.

Therefore -1 is truthy and when the logical negation happens we get false.

Therefore this statement is portably false.

Related questions

Checking for NULL pointer in C/C++

In a recent code review, a contributor is trying to enforce that all NULL checks on pointers be performed in the following manner: int * some_ptr; // ... if (some_ptr == NULL) { // Handle null-pointer error } else { // Proceed } instead of int * some_ptr; // ... …

Read More
#ifdef vs #if - which is better/safer as a method for enabling/disabling compilation of particular sections of code?

This may be a matter of style, but there's a bit of a divide in our dev team and I wondered if anyone else had any ideas on the matter... Basically, we have some debug print statements which we turn …

Read More
IF-ELSE statement shortcut in C

C has the following syntax for a shorthand IF-ELSE statement (integer == 5) ? (TRUE) : (FALSE); I often find myself requiring only one portion (TRUE or FALSE) of the statement and use this (integer == 5) ? (TRUE) : (0); I was just wondering if there was a …

Read More