How does 'if((x) || (y=z))' work?

Question

I don't quite understand how the if statement in this case works. It evaluates the x != 0 statement and when that is not true anymore, it assigns z to y and then breaks the if statement?

int main()
{
    int x, y, z, i;
    x = 3;
    y = 2;
    z = 3;

    for (i = 0; i < 10; i++) {
        if ((x) || (y = z)) {
            x--;
            z--;
        } else {
            break;
        }
    }

    printf("%d %d %d", x, y, z);
}

im not fluid in C but i believe it assigns `z` to `y` and then checks if `y` is true. — Nils Lindemann, Mar 08 '17 at 19:20
If `x` is non-zero, it doesn't do the `y = z` part, and `if` body executes. When `x` *is* zero, it does the `y = z` and if `z` was non-zero, the `if` body executes. The `break` only happens when both `x` and `z` are zero. — Dmitri, Mar 08 '17 at 19:22
Yes, I know. This code is from a book, and I'm trying to understand it. — Mike B., Mar 08 '17 at 19:23
If your book _really_ has `void main()` time to throw out that book. — Chad, Mar 08 '17 at 19:24
I think this example only exists to throw off the javascript kids. — Filburt, Mar 08 '17 at 19:32
`||` is a conditional `or` it will execute the statement on the right only if the first condition is false. — noel zubin, Mar 08 '17 at 19:32
@TonyTannous `main` is the one C function that does not *have to* explicitly return a value. Please read [What should main() return in C and C++?](http://stackoverflow.com/questions/204476/what-should-main-return-in-c-and-c) — Weather Vane, Mar 08 '17 at 19:33
@WeatherVane, Chad this is why I love being on StackOverflow. I learn everyday things I didn't know. Thanks guys! — Tony Tannous, Mar 08 '17 at 19:36
@TonyTannous me too: Weather Vane, aged 105 3/4 .... if `main` returns at all, which in embedded it won't if there is nowhere to return *to*. — Weather Vane, Mar 08 '17 at 19:40
@SouravGhosh: To be fair, debuggers are not necessarily that great at showing the inner workings of single expressions. — Matthieu M., Mar 09 '17 at 08:23
@MatthieuM. Maybe you're right but that is not a reason to stop me or anybody from trying at least. :) For further confusions, we are anyways here. — Sourav Ghosh, Mar 09 '17 at 08:39
@SouravGhosh: I agree, it's always better to try to debug first, but in this particular case I'm afraid it won't be very helpful. — Matthieu M., Mar 09 '17 at 08:52
@NathanOliver C99 5.1.2.2.1 "It shall be defined with a return type of int ... or in some other implementation-defined manner." The book may be for some implementation that defines main to return void? — Vality, Mar 10 '17 at 06:20
@NathanOliver In addition "In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined." It is also possible the book regards programming for some embedded freestanding environment. — Vality, Mar 10 '17 at 06:28

adentinger · Answer 1 · 2017-03-13T16:01:03.213

Let's decompose that into smaller bits.

if (x) is the same as if (x != 0). If x != 0, then you know the condition is true, so you don't do the other portion of the if.
If part 1. was false, then y = z assigns z into y and returns the final value of y.
From point 2., we can understand that if (y = z) is equivalent to y = z; if (y != 0)

Thus, from points 1. and 3., we can understand that :

if ((x) || (y = z)) {
    doSomething();
}
else {
    doSomethingElse();
}

Is the same as :

if (x != 0) {
    doSomething();
}
else {
    y = z;
    if (y != 0) {
        doSomething();
    }
    else {
        doSomethingElse();
    }
}

It's true it's not particularly readable code though.

MD XF · Answer 2 · 2017-03-08T19:42:42.217

18

No. if ((x) || (y = z)) { in C-English is basically:

if x is nonzero, evaluate the following code.
if x is zero, set y to z.
if y is nonzero, evaluate the following code.
otherwise, break out of the loop.

If x is zero or y is zero, it breaks out of the loop.

edited Mar 08 '17 at 19:42

answered Mar 08 '17 at 19:25

MD XF

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`(y = z)` is only executed when `x == 0`. – Weather Vane Mar 08 '17 at 19:28
@WeatherVane Correct. Clarified. – MD XF Mar 08 '17 at 19:29
1

Your answer is even better than mine! ♥ the English translation. I'll remember that for future explanations. – adentinger Mar 08 '17 at 19:30
1

There is a slight error here. The statement should read: if `x` **IS** 0, set `y` to `z` and check if `y` isn't 0 – user1952500 Mar 08 '17 at 19:34
@user1952500 Thanks. Corrected. – MD XF Mar 08 '17 at 19:36
Your last line should read "if `x` is zero **and** `y` is zero ... ". – Shaggy Mar 09 '17 at 07:03
I find “the following code” a bit misleading. Does it mean the following part of the expression? Or the body of the if-statement? (The latter is true, of course.) – Melebius Mar 09 '17 at 08:53

Adrian Bobrowski · Answer 3 · 2017-03-08T22:18:26.670

7

int main()
{
    int x = 3;
    int y = 2;
    int z = 3;
    unsigned int i;

    for (i = 0; i < 10; i++)
        if (x != 0) {
            x = x-1;
            z = z-1;
        } 
        else {
            y = z;

            if (y != 0) {
                x = x-1;
                z = z-1;
            }
            else {
                break;
            }
        }
    }
    printf("%d %d %d", x, y, z);
}

edited Mar 08 '17 at 22:18

answered Mar 08 '17 at 19:23

Adrian Bobrowski

2,681
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Ah now I see :). Ok, you should elaborate for OP. – Tony Tannous Mar 08 '17 at 19:29
2

Please do not preach for `void main()`. – Sourav Ghosh Mar 08 '17 at 20:28

score 3 · Answer 4 · answered Mar 08 '17 at 19:24

In C, there is short-circuiting, so the statement y=z will not be evaluated until x becomes zero.

When x == 0, since z also decrements the same way, z == 0. Hence y will also be zero at that time due to the assignment. The statement y=z also returns y at this point which will be evaluated as a condition, and since that is also 0, the else break will be hit.

Hence I believe the answer should be 0 0 0.

score 1 · Answer 5 · edited Aug 01 '18 at 17:51

1

When you use assignment in an if statement, the result of the assignment is returned. so when you write :

if (x = y)

It will be always true unless the value of y is 0, so 0 is returned as the result of assigning and the if statement is not executed.(anything except 0 is considered as true.)

So when you write :

if ( x || (x = y))

The if statement doesn't execute only if x is 0 & y is 0.

edited Aug 01 '18 at 17:51

BattleTested_закалённый в бою

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answered Mar 15 '17 at 17:12

Fatemeh Karimi

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score 0 · Answer 6 · answered Apr 05 '17 at 10:17

Here

if ((x) || (y = z))

there are two condition one condition is if ((x)) and another condition is if ((y = z)) if one of them is true then if portion is execute otherwise else condition work

only and only when both condition are false then else execute.

How does 'if((x) || (y=z))' work?

6 Answers6