Why is if (2 < 9 < 3) true?

user_15026002 0
Viewed 933

This was a question in my preparation exam:

int val = 0;
int x = 0;
int y = 1;
if (x < val < y)
    printf(" true ");
else
    printf(" false ");

Why is this true? I tried changing x and val and it ignored those changes, as long as y was larger than 0 (so 1, 2, 3...) the statement was true. So for example: if (3 < 9 < 2) will be true.

c
6 Answers

( 2 < 9 < 3 ) is evaluated as ( ( 2 < 9 ) < 3).
In the first step 2 < 9 is evaluated to be true, which is represented as integer value 1 and results in ((1) < 3) for the second step.
That is obviously true.

You probably wanted something like ((x < val) && ( val < y)).

user_77334180

The first is check whether x < value. If true, it return 1 so the next is check whether 1 < y.

user_134240840

First you check x and val. If its value is correct, you convert it to 1 and check it with y.

user_149337910

If you build with -Wall option using GCC compiler you will get a warning like this:

<source>:7:11: warning: comparisons like 'X<=Y<=Z' do not have their mathematical meaning [-Wparentheses]
     if (x < val < y)

Detailed explanation of this warning can be found in GCC documentation. The relevant part is as following:

-Wparentheses
     ...
    Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation. 
    ...
user_721780

NOTE: Comparisons in C follow the left to the right approach. (if all operators are of same priority)

Thus, evaluation of (2 < 9 < 3) will be done in 2 steps:

  1. step 1. evaluation of (2 < 9): it is true, thus, integer value=1.
  2. step 2. evaluation of ( (2 < 9) < 3 ) i.e (1 < 3): it is also true.
user_138987020

I think you wanted (2 < 9) && (9 < 3).

In C, usually operators follow left to the right approch(except operators like =, -=, +=, etc.). Operator <, > follows left to the right approch too. So, 2<9<3 in C means (2 < 9) < 3. 2 < 9 is 1, so, the value of 2 < 9 < 3 is 1.

You can fix it with (2 < 9) && (9 < 3).

user_155434760
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