Why is the output different?

[vanangamudi]

Look at the follwing code both are same(atleast to some extent) but the output seems to be different. Why???

#include<stdio.h>
//#include<iostream>
//#include<conio.h>

void main()
{
    //clrscr();

    int c, a = 7;
    int b = a++ + ++a + a++ + a++ - ++a;
    printf("\n%d",b);
    printf("\n%d",a);
    a = 7;
    c = a++ + ++a + a++ + a++ - ++a;
    a = 7;
    printf("\n%d",c);
    printf("\n%d",a);
    printf("\n%d %d %d %d %d", a++, ++a , a++ , a++ , ++a);

}

23
12
23
7
11 12  9 8  12

#include<stdio.h>
//#include<iostream>
//#include<conio.h>

void main()
{
    //clrscr();

    int c, a = 7;
    int b = a++ + ++a + a++ + a++ - ++a;
    printf("\n%d",b);
    printf("\n%d",a);
    a = 7;
    c = a++ + ++a + a++ + a++ - ++a;
    a = 7;
    printf("\n%d",c);
    printf("\n%d",a);
    printf("\n%d", a++);
    printf(" %d", ++a);
    printf(" %d", a++);
    printf(" %d", a++);
    printf(" %d", ++a);

}

23
12
23
7
7 9 9 10 12

can anyone explain why???

[Simian Man]

Welcome to the wonderful world of undefined behavior! The compiler is allowed to evaluate arguments in any order it chooses. So just don't write code that depends on the output of argument evaluation.

[NovaAesa]

Recompile your code with warnings turned on, and the compiler will give you some hints. As Simian Man said, there's lots of undefined behaviour in your code. For example, your first program:

ps@pris:~$ vim test.c
ps@pris:~$ gcc -Wall test.c 
test.c:5:6: warning: return type of ‘main’ is not ‘int’
test.c: In function ‘main’:
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
ps@pris:~$

[NovaAesa]

Recompile your code with warnings turned on, and the compiler will give you some hints. As Simian Man said, there's lots of undefined behaviour in your code. For example, your first program:

ps@pris:~$ vim test.c
ps@pris:~$ gcc -Wall test.c 
test.c:5:6: warning: return type of ‘main’ is not ‘int’
test.c: In function ‘main’:
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:10:37: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:14:28: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
test.c:18:55: warning: operation on ‘a’ may be undefined
ps@pris:~$

EDIT: I'll elaborate. When you do something such as:

foo++;

the value of the expression is simply "foo + 1", however foo then gets incremented. When does it get incremented though? It gets incremented *some* time before the next sequence point. Exactly when it gets incremented before the next sequence point is undefined. Have a read about it here http://en.wikipedia.org/wiki/Sequence_point

[hakermania]

Normally, as I know it,

int a=0;
cout << a++ <<endl;

will show

0

but value of 'a' will be 1 after this.

Conversely, if you do

int a=0;
cout << ++a << endl;

then it will show 1 and the value of 'a' will be 1 afterwards...

Screenshot:

[trent.josephsen]

EDIT: I'll elaborate. When you do something such as:

foo++;

the value of the expression is simply "foo + 1", however foo then gets incremented. When does it get incremented though? It gets incremented *some* time before the next sequence point. Exactly when it gets incremented before the next sequence point is undefined. Have a read about it here http://en.wikipedia.org/wiki/Sequence_point

I'm sure you mean that the value of the expression

++foo;

is (foo + 1). The value of foo++ is still just foo.

[NovaAesa]

I'm sure you mean that the value of the expression

++foo;

is (foo + 1). The value of foo++ is still just foo.

You're absolutely right, my mistake! I should really reread my posts more thoroughly before hitting submit.