How there is Lvalue required error

Question

There is a piece of code which is producing error of "Lvalue required". The code is as,

     #include<stdio.h>
     #include<conio.h>
     #define max 10
     int main()
     {
      printf("%d",max++);
      return 0;
     }

It was evident that Lvalue error will come in above code so i changed the code to

     int a;
     printf("%d",a=max++);

I thought now the value of constant is assigned to a proper variable but still the error appeared. Then i checked for

     printf("%d",a=max+1);

it works without any error.What is the problem with the second piece of code ?

However, `(a=max)++` would compile. – aschepler Sep 09 '13 at 13:31 — aschepler, Sep 09 '13 at 13:31

Bathsheba · Answer 1 · 2013-09-09T12:16:42.290

5

max is a literal so max++ will fail. (Just as 10++ will fail).

However, max + 1 is valid (just as 10 + 1 is).

Remember that #defines are resolved by the preprocessor which happens before compilation takes place.

To explain your compiler's return error:

Loosely speaking, an lValue is the thing on the left hand side of an assignment; i.e. in

a = b;

a is the lValue. The statement 10 = b; is clearly meaningless (you can't assign a value to 10): more formally it is invalid since 10 is not an lValue.

edited Sep 09 '13 at 12:16

answered Sep 09 '13 at 11:56

Bathsheba

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you mean max is a keyword? – mrigendra Sep 09 '13 at 11:56
@mrigendra, no you defined it to be the literal 10. – Bathsheba Sep 09 '13 at 11:57
1

@mrigendra: it will fail for every word you give it. See my final comment in my (amended) answer. – Bathsheba Sep 09 '13 at 11:58

lulyon · Answer 2 · 2013-09-09T12:02:37.490

2

max is replace by 10 after c preprocessing, which is what #define supposed to do.

10 is a rvalue, or literal. No assignment related operation should do to a literal. But ++ operation involves assignment.

For example, you can do 10++; for the literal 10.

edited Sep 09 '13 at 12:02

answered Sep 09 '13 at 11:57

lulyon

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P0W · Accepted Answer · 2013-09-09T12:06:34.853

2

max will be replaced by 10 after preprocessing, so

max++ => 10++      //Error  ++ requires lvalue
a=max++ => a=10++  //Error  same as above

a=max+1 => a=10+1 //Works

edited Sep 09 '13 at 12:06

answered Sep 09 '13 at 11:57

P0W

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2

++ is an assignment. The same way `10 = 10 + 1` is illegal, `10++` is illegal. – Klas Lindbäck Sep 09 '13 at 12:02
in the second does it means a=10=10+1 ,thats why its an Lvalue error – mrigendra Sep 09 '13 at 12:03
@mrigendra if you think `a=max++` changes to `a=10=0+1` before compilation, that is not the case, however it changes `a=10++` before compilation and "lvalue" is reference to an object that has storage, an identifiable address, "lvalue required error" says that the compiler needs a storage on which it can perform the operation. – P0W Sep 09 '13 at 12:17

score 2 · Answer 4 · answered Sep 09 '13 at 11:59

You need to read about compiler macro definitions.

What they actually do is if you type:

#define SOMETHING 3

is changing every occurrence of something with the further value before compiling the code.

Similar if you use such macro:

#define SOMETHING(x) (x + x)

It will change the occurrence of SOMETHING(value) to value + value.

The LValue is basically the operand that can be used in the left site of assign operator, in your case it can be a "a" variable.

The value++ is translated to the operation value = value + 1 and wouldn't cause any problems if the variable had been used. You, however had used "max" which is not a variable of any type but its a macro defined from your preprocessor as constant variable. The preprocessor will swap each max to 10 so you end up with the expression 10++ which will evaluate to 10 = 10 + 1 which is just wrong.

Note: you should use defines with capital leather to easier distinguish between variables and preprocessor definitions.

score 0 · Answer 5 · answered Sep 09 '13 at 13:28

Remember that the ++ operator has a side effect of updating the value of its operand; for that to work, the operand must be an lvalue, which is an expression that refers to a memory location such that the memory may be read or modified; unfortunately, no memory is set aside for integer constants like 10 , hence the error message.

How there is Lvalue required error

5 Answers5