Reason for this output??

[b56r1]

#include<stdio.h>
main()
{
int a=10;
printf("%d %d %d\n",a=10,a=20,a=30);
}

o/p: 10 10 10.
Here, the latest value is updated and printed as the expression statements separated by commas in funtions are evaluted from right to left.

But, in below program by using global variable or static variable why the updated value is not printed??

#include<stdio.h>
int a=10;
main()
{
printf("%d %d %d\n",a=10,a=20,a=30);
}

o/p: 10 20 30.

For static variable,

#include<stdio.h>
main()
{
static int a=10;
printf("%d %d %d\n",a=10,a=20,a=30);
}

o/p: 10 20 30

[jnawrocki]

I suspect it has something to do with how your compiler handles the different types of variables. Both static and global variables remain intact for the entire run of the program. On the other hand local variables are allocated on the stack and remain intact only during the current execution of the function in which it is defined.

That being said Microsoft Visual returns the amount assigned in the first a= for all occurances of a in the printf:

int a=10;
printf("%d,%d,%d",a=5,a=10,a=15);

Each of the three types of variables returns 5,5,5.

[b56r1]

jnawrocki,

I'm using GCC complier in LINUX operating sys. I know the explanation what u gave,but why its not updating with latest values for global variables ..

[fpsasm]

jnawrocki,

I'm using GCC complier in LINUX operating sys. I know the explanation what u gave,but why its not updating with latest values for global variables ..

I've just compiled all the examples you gave, and the outputs are 10 20 30.

You'd expect the output to be 10 20 30, since a= is an assignment.

By the way, it's advised not to use global variables in C, or C++.

(p.s the code brackets help alot :) )

[nezachem]

printf("%d %d %d\n",a=10,a=20,a=30);

> expression statements separated by commas

This is not a comma expression - this is an argument list. They look very similar, but they are really different creatures (think about this: the comma expression yields just one value). In the argument list case there's no guarantee in which order the arguments are evaluated.

[chrjs]

Its because when gcc compiles the code, it first evaluates the equations then pushes the registers for the function call. In the first example with the local variable it evaluates the expressions a=10,a=20, and a=30 into the same location in the stack, then it pushes that location in the stack 3 times. In the second example with the global variable, it evaluates into the global variable and then moves the global's value into separate registers. After the evaluation, it then pushes the individual registers into the stack for the printf call.

You can compile the progams with the -S flag and look at the .s (assembler) files to see exactly how it works.

[pheininger]

printf("%d %d %d\n",a=10,a=20,a=30);

The language standard indicates this statement has "undefined behavior" because the variable a is changed more than once between two squence points (start of the statement and before the function call but after all the parameters have been evaluated). In this case, there are three assignment operators with variable a as an lvalue. So any result from this statement and the rest of the program is correct behavior according to the language standard. This would include printing anything or nothing from that statement or stopping execution completely.

What life time or scope for the variable a does not change that it has undefined behavior. So yes, changing those things and getting different results is allowed by the language standard.