Invalid lvalue in assignment while trying to swap pointers

[ra123kesh]

while executing this code i was getting a error Invalid lvalue in assignment. Can any one tell how to correct this.

dataItem* d=(dataItam*)malloc(10*sizeof(dataItem));
dataItem* temp;
temp=(d+6);
(d+6)=(d+8);//error line
(d+8)=temp;//error line

Please some on help on this

[Luuk]

dataItem* d=(dataItam*)malloc(10*sizeof(dataItem));

you have a typing error?
and you should use CODE-tags when posting code....

[donbock]

The C Standard uses the term lvalue to refer to something that can be on the left side of the assignment operator (=). You want to pay special attention to what's on the left side of the equals sign when the error message refers to "invalid lvalue". Therefore, I am immediately suspicious of (d+6) and (d+8) on lines 4 and 5.

d is a pointer. The term pointer arithmetic is used for exprssions that add a pointer and an integer. (d+6) is equivalent to &d[6]. Notice that the expression evaluates to the address of an array element. By the way, one of the biggest differences between pointer arithmetic and array notion is that you are allowed to subtract integers from a pointer but you are not allowed to have a negative array index.

Indeed, the expressions on the left side of the equals sign in lines 4 and 5 are not valid lvalues. There are two ways to fix this: dereference the pointer arithmetic expressions (*(d+6)) or use array notation.

However, fixing the invalid lvalue errors will provoke additional errors. You want to swap the contents of two array elements, not their addresses.

[weaknessforcats]

This code:

temp=(d+6);
  (d+6)=(d+8);//error line
 (d+8)=temp;//error line
 

The problem here is d+6 is an expression which is either true or false. An expression cannot be an LVAL. You need an actual variable on the left of the assignment operator. d[6] would be that variable.

The code should be:

temp=(d+6);
  d[6]= *(d+8);  //assign actual variable
  d[8] = *temp;  //dereference temp to get the dataItem
                 //then assign the dataItem t d[8]