CODEWITHSUNDEEP
csyntax
lbenedetto
lbenedetto

Reputation: 2114

What is the behavior of multiple unseparated equality comparisons in C?

I made a typo while I was writing some C code and noticed that CLion did not see anything wrong with doing

while(line != NULL != 0)

In fact, it actually compiles and runs without error (though it does always return true)

I did some further testing, and found that it actually does return false sometimes

//true
if(1 != 0 == 1)
//false
if(1 != 0 == 0)
//true
if(1 != 0 > 2)

At first I thought it was essentially doing

if((1!=0) == true)

but that last check has me completely lost.

Upvotes: 2

Views: 47

Answers (1)

Stephen Docy
Stephen Docy

Reputation: 4788

They are evaluated via correct order of operations, and remember false evaluates to 0 and true to 1

!= and == have equal precedence, evaluate left to right

> has greater precedence than !=, evaluate it first

//true
if(1 != 0 == 1)  => if ((1 != 0) == 1)  => if (1 == 1)  => true

//false
if(1 != 0 == 0)  => if ((1 != 0) == 0)  => if (1 == 0)  => false

//true
if(1 != 0 > 2)  => if (1 != (0 > 2))  => if (1 != 0)  =>  true

http://en.cppreference.com/w/c/language/operator_precedence

Upvotes: 3

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