Please tell me answer for this code?

Question

I have this code troubling me for a while.The expression at Line 7 is troubling me. Is it giving 0 or -1. If its 0 then answer is 2 else answer is 4.

/* How to find value of c in Line 7 expression */

#include<stdio.h>

int main()
    {
    int a,b,c=1;
    a=b=c;                         // a,b,c have equal value
    c=b+=a=-c;                     // what will be the output of this expression?
    c=-c;
    c=(++c)*2;                    
    printf(“%d”,c);
    return 0;
    }

`here compiler gives error` and `.....then answer is 2 else answer is 4`. how? — Sourav Ghosh, Dec 30 '14 at 09:45
Stop writing incomprehensible code. Just do one thing at a time and you do not go far wrong — Ed Heal, Dec 30 '14 at 09:45
Also if you keep writing more code like this, you'll have no friends. — Maroun, Dec 30 '14 at 09:46
Compile with all warnings and debug info (`gcc -Wall -Wextra -g`). If the compiler gives no warnings or errors, **use the debugger** (`gdb`) to run the code step by step — Basile Starynkevitch, Dec 30 '14 at 09:46

score 2 · Answer 1 · answered Dec 30 '14 at 09:46

2

In C the pre-increment (decrement) and the post-increment (decrement) operators requires an L-value expression as operand. Providing an R-value or a const qualified variable results in compilation error.

An lvalue is a value that can be assigned to.

answered Dec 30 '14 at 09:46

Sadique

22,572
7
65
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but c is lvalue and its given in code – Vaibhav Gautam Dec 30 '14 at 09:52
`c` is an lvalue the expression evaluated within the braces `++(-c)` i.e `-c` is not - it evaluates to some constant value. – Sadique Dec 30 '14 at 09:53
So how can I write this to avoid lvalue error – Vaibhav Gautam Dec 30 '14 at 09:56
You can write this `c=++(-c)*2` as this `c=-c; c=(++c)*2` - but it will still lead to undefined behavior. – Sadique Dec 30 '14 at 09:58
Assuming there's no lvalue error what could be the output according to you – Vaibhav Gautam Dec 30 '14 at 09:58
writting c=++(-c)*2 as this c=-c; c=(++c)*2 means same? – Vaibhav Gautam Dec 30 '14 at 10:00
`writting c=++(-c)*2 as this c=-c; c=(++c)*2 means same? ` - Only the compilation error would go away but the operation is still undefined - read here - http://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points – Sadique Dec 30 '14 at 10:01
According to you what could be output of: c=b+=a=-c; – Vaibhav Gautam Dec 30 '14 at 10:04

score 2 · Answer 2 · answered Dec 30 '14 at 09:46

2

/* what is value of c */

That's easy to answer: Your question does not make any sense.

Since your code won't compile, c won't have a "value".

answered Dec 30 '14 at 09:46

Hans Klünder

2,176
12
8

Yeah I know its giving error. Thats why I want to know two things from it: 1. why lvalue error 2. output of c in line 7(the c=b+=a=-c;) – Vaibhav Gautam Dec 30 '14 at 09:53
1

So, your question should be: Why does it fail to compile? And not: What's the value of c? – Hans Klünder Dec 30 '14 at 09:54

Please tell me answer for this code?

2 Answers2