C increment and decrement output problems

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First printf output is 11, 11. Second one is 10, 20 why is it 20 shouldn't it be 21?

C++

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int a=10,b=++a;
    printf("%d,%d\n",a,b);
    b=a+--a;
    printf("%d,%d",a,b);
}

What I have tried:

I really don't understand how it works.

Posted 23-Apr-19 9:37am

Member 14325983

Updated 23-Apr-19 20:36pm

KarstenK

v2

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3 solutions

Solution 3

Quote:
I really don't understand how it works.

This is a gray zone.
A C compiler is free to rewrite your code the way it sees fit inside a line of code.
So different compiler will get different result and all are correct, it is unpredictable, and most of the time, unexpected.
The rule of safety is to never use more than once a variable that get changed in a line of code.

Posted 23-Apr-19 11:42am

Patrice T

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k5054 · Accepted Answer · 2019-04-23T09:54:00

This has to do with sequence points. I believe that both 20 and 21 are valid answers, since the compiler is free to choose when to evaluate --a, so it can either decrement a before or after the addition. The only thing you know for sure is that following the assignment to b, a will have the value 20.
You should avoid expressions that access and use the increment/decrement operators on the same variable at the same time in different locations. This includes passing the variable to functions. For example

C

int a=10;
printf("%d, %d\n", a, --a)

might print "10, 9", or it might print "9, 9"

Gerry Schmitz · Accepted Answer · 2019-04-23T09:58:00

Solution 2

It'll do the "prefix" operator (--) before anything else.

when a is 11, then

b=a+(--a)
= 10 + 10

a is updated "inline"; there is no intermediate storage.

Posted 23-Apr-19 9:58am

Gerry Schmitz

Comments

k5054 23-Apr-19 16:15pm

This is not correct. The C standard does not specify the order in which operands are evaluated. Given the following program:

#include <stdio.h>
int main()
{
    int a = 10;
    int b = a + --a;
    printf("%d, %d\n", a, b);
    return 0;
}

Compiling with gcc I get the result 9, 18;, but when I compile with clang I get 9, 19. Additionally, clang tells me: foo.c:6:17: warning: unsequenced modification and access to 'a' [-Wunsequenced], and using -Wall gcc tells me foo.c:6:17: warning: operation on ‘a’ may be undefined [-Wsequence-point]

[no name] 23-Apr-19 16:37pm

I'm addressing his situation; which in this case, appears to be deterministic.

All you've pointed out is that there are "differences in compilers"; which boils down to, "know your tools".

The "standard" stresses "sequencing"; nothing else.

k5054 23-Apr-19 16:58pm

Its not deterministic, though, is it? The same compiler could evaluate the operands in different order at different optimization levels. Worse, if you're hoping to create portable code, which might be compiled on other systems, perhaps with compilers you don't have access to, not knowing that the order of evaluation is up to the compiler, is going to cause bugs.

[no name] 23-Apr-19 17:51pm

It is definitely deterministic if I use brackets; and that's all we're dealing with: sequencing, whether by default or forced.

k5054 23-Apr-19 18:02pm

Still wrong, even b = a + (--a)
evaluates differently in gcc and clang. I think a true language lawyer would point out that this is undefined behavior, and so the implementation is free to do what it wants. Making hairy demons fly out of your nose, or launching nuclear missiles at Mount Doom in Mordor are both possible options. Neither are likely, but not prohibited by the standard.

[no name] 23-Apr-19 18:14pm

If I find a C compiler that adhered to "C# sequencing", it would still not satisfy you because of your experiences with Z.

You have no clue to what OP is using but are convinced your "reality" applies to everyone else.

OP was looking for a reasonable explanation; not FUD.

C increment and decrement output problems

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C increment and decrement output problems

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