while infinite loop?

Question

I came across this question in this forum

#include <iostream>

using namespace std;

int main(int argc, char** argv) {

    int x=0;
    while (x<3) {
        x = x++;
        cout << x << endl;
    }

    return 0;
}

given the code above, why is the while loop infinite? Using gcc 4.4 under mac os, the while loop does terminate :) so the question does not apply for all architectures. The output I get tough is
1
2
3

I don't see 0, and I guess the reason is related to the double assignment?

under mac osx x64 (gcc 4.4) the loop does terminate. Apparently, using some other compilers and/or platform it may be infinite programmers.stackexchange.com/questions/25836/… — zzzbbx
– zzzbbx, Commented Mar 16, 2011 at 22:46

BlackBear · Accepted Answer · 2011-03-16 21:36:45Z

14

x = x++;

is undefined behavior

answered Mar 16, 2011 at 21:36

BlackBear

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12 Comments

John Over a year ago

Which means that the loop is neither infinite nor finite?

BlackBear Over a year ago

@John: this means we cannot say why the program behaves this or that way

Bo Persson Over a year ago

@John: Which means that the program is invalid, and anyting can happen.

Anomie Over a year ago

And note that really means anything. It could be that x winds up at the old value, or the new value, or some completely different value, or the program could crash, or the program could start nethack, or demons could come flying out of your nose.

John Over a year ago

@voodoomsr: Nope: stackoverflow.com/questions/98340/…

|

mjsr · Accepted Answer · 2011-03-16 21:40:24Z

0

you never see zero because the increment is before the cout.

answered Mar 16, 2011 at 21:40

mjsr

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2 Comments

Anomie Over a year ago

It is undefined behavior because the value of x is set twice between sequence points. The compiler is free to do the assignment and increment in any order, or really to do anything else that might make sense to it.

mjsr Over a year ago

i think different but im going to read further in the c++ manual that i have. Long time ago i learn that the POSTIncrement like this x=a++; means x=a; a=a+1; i dont get whay in this situation where a=x is differen

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