Please explain output of given program.

Question

the code is given below: it gives output true2.

    #include<stdio.h>
    int main()
    {
    int a=10;
    if(a==a--)
    printf("true 1\t");
    a=10;
    if(a==--a)
    printf("true2 \t");
    return 0;
    }

The output *can't* be explained because you have [*undefined behavior*](https://en.wikipedia.org/wiki/Undefined_behavior). Read about [evaluation order and sequencing](http://en.cppreference.com/w/c/language/eval_order). — Some programmer dude, Sep 30 '16 at 16:44
This is a pointless junk, not a code. (Sorry). What is it expected to show? — Eugene Sh., Sep 30 '16 at 16:44
for ease of readability and understanding, 1) consistently indent the code. Indent after every opening brace '{'. Unindent before every closing brace '}'. 2) separate code blocks (for, if , else, while, do...while, switch, case, default) via a blank line — user3629249, Sep 30 '16 at 16:47
when compiling, always enable all the warnings, then fix those warnings. (for `gcc`, at a minimum use: `-Wall -Wextra -pedantic` I also use: `-Wconversion -std=gnu99` ). Compiling with the warnings enabled will produce two instances of: `warning: operation on 'a' may be undefined [-Wsequence-point]` One each for line 5 and line 8 — user3629249, Sep 30 '16 at 16:52

score 2 · Answer 1 · edited Sep 14 '17 at 04:06

The comparison done in both of the if statements result in undefined behaviour. So, anything could happen. Because a is read and modified without an intervening sequence point. The comparison operator == doesn't introduce a sequence point. You probably need to learn about undefined behaviour and sequence points, etc to understand the problem better.

Modern compilers may also help you. For example, Clang issues:

warning: unsequenced modification and access to 'a' [-Wunsequenced]
    if(a==a--)
       ~   ^
warning: unsequenced modification and access to 'a' [-Wunsequenced]
    if(a==--a)
       ~  ^

for the two if statements (GCC also produces similar warnings with gcc -Wall -Wextra).

score 0 · Answer 2 · edited May 23 '17 at 12:15

0

In general, it is not a good practice to do a-- (or --a) inside a condition, because it is not clear to read. In order to understand the difference between a-- and --a please see the answer at: Incrementing in C++ - When to use x++ or ++x?

edited May 23 '17 at 12:15

Community

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answered Sep 30 '16 at 16:46

Idan

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2
8

The question is about c – Eugene Sh. Sep 30 '16 at 16:47
I know, but it is exactly the same in C and C++ – Idan Sep 30 '16 at 16:48
That depends. Sometimes, pre- or postdecrement do increase also readability. But the issue here is not the difference between pre- and postdecrement, it is "unsequenced modification and access", as in the answer by @usr. – drRobertz Sep 30 '16 at 18:05

Please explain output of given program.

2 Answers2