Operator Precedence - Expression Evaluation

Question

For the following code snippet I get the output as 1. I want to know how it came?

void main()
{
int x=10,y=20,z=5,i;
i=x<y<z;
printf("%d",i);
}

score 7 · Accepted Answer · answered Mar 28 '11 at 13:50

7

i=x<y<z;, gets interpreted as i=(x<y)<z, which in turn gets interpreted as i=1<z, which evaluates to 1.

answered Mar 28 '11 at 13:50

yan

20,644
3
38
48

Seems to be a rather common mistake, at least GCC (probably other compilers, too) prints out a warning, if warnings are enabled: `warning: comparisons like 'X<=Y<=Z' do not have their mathematical meaning` – Saytonurn Mar 28 '11 at 17:06

score 2 · Answer 2 · answered Mar 28 '11 at 13:51

2

10 is less than 20, resulting in 1, and 1 is less than 5, resulting in 1. C doesn't chain relational operators as some other languages do.

answered Mar 28 '11 at 13:51

Ignacio Vazquez-Abrams

776,304
153
1,341
1,358

score 1 · Answer 3 · edited Mar 28 '11 at 14:02

1

It operates as follows: Since < is a logical expression, x<y i.e 10<20 is true i.e 1. So it becomes 1<z i.e 1<5 which is again true i.e. 1 which is assigned to i. So i is 1.

edited Mar 28 '11 at 14:02

sth

222,467
53
283
367

answered Mar 28 '11 at 13:52

Chaithra

1,130
3
14
22

score 0 · Answer 4 · edited Apr 13 '17 at 12:45

C++ doesn't support multi-part comparisons like that.

x < y < z

is interpreted as

(x < y) < z

or that is, determine if x < y, then see if that boolean is less than z.

There's some discussion on why that is over at the software engineering StackExchange.

When you find yourself trying to do this, instead you need to write it as two separate comparisons joined by a boolean:

(x < y) && (y < z)

score 0 · Answer 5 · answered Mar 28 '11 at 13:50

0

This is because your code evaluates as:

void main()
{
    int x=10,y=20,z=5,i;
    i=((x<y)<z); //(x<y) = true = 1, (1 < 5) = true
    printf("%d",i);
}

answered Mar 28 '11 at 13:50

Eugene Burtsev

1,465
4
24
45

score 0 · Answer 6 · answered Mar 28 '11 at 13:50

0

what output did you want?

In C,

i = 2 < 3; //i == 1.
i = 4 < 3; //i == 0.

If condition evaluates to false, value returned is 0, and 1 otherwise.
Also, x < y < z will be evaluated as ((x < y) < z).

answered Mar 28 '11 at 13:50

N 1.1

12,418
6
43
61

score 0 · Answer 7 · answered Mar 28 '11 at 13:52

0

x<y // 1 as (10 < 20) will return 1
result of(x<y)<z // 1 as (1<5) will return 1

answered Mar 28 '11 at 13:52

Ashish Kasma

3,594
7
26
29

Operator Precedence - Expression Evaluation

7 Answers7

Linked