I have been playing around with what would be considered valid C syntax and what wouldn't,
I tried 1<2<1, and to my surprise, it is a valid C syntax. Does anyone know what that would do or represent ?

I tried figuring it out by testing the value this kind of expressions take and changing the integer values but found no pattern.

CodePudding user response：

In C, when using a relational operator such as <, the result will always be 1 if the condition is true, or 0 if the condition is false.

Due to the rules on operator precedence and associativity, the expression

1<2<1

is equivalent to:

(1<2)<1

The sub-expression (1<2) is true, so it will evaluate to 1, so the entire expression is equivalent to:

1<1

Since this expression is false, it will evaluate to 0.

Therefore, writing 1<2<1 is equivalent to writing 0.

CodePudding user response：

Because of the associativity of the < operator, the expression 1 < 2 < 1 is the same as (1 < 2) < 1.

So you're checking if the boolean result of 1 < 2 is less than 1. And the result of 1 < 2 is true which will be converted to the integer value 1 (false would become 0).

In other words, 1 < 2 < 1 is transformed into 1 < 1 which is false.