Expression must be a modifiable lvalue

Question

I have this following code:

int M = 3;  int C = 5;  int match = 3; for ( int k =0; k < C; k ++ ) {     match --;      if ( match == 0 && k = M )     {          std::cout << " equals" << std::endl;     } } 

But it gives out an error saying:

Error: expression must be a modifiable value

on that "if" line. I am not trying to modify "match" or "k" value here, but why this error? if I only write it like:

if ( match == 0 ) 

it is ok. Could someone explain it to me?

Answer 1

The assignment operator has lower precedence than &&, so your condition is equivalent to:

if ((match == 0 && k) = m) 

But the left-hand side of this is an rvalue, namely the boolean resulting from the evaluation of the sub­expression match == 0 && k, so you cannot assign to it.

By contrast, comparison has higher precedence, so match == 0 && k == m is equivalent to:

if ((match == 0) && (k == m)) 

Answer 2

In C, you will also experience the same error if you declare a:

char array[size]; 

and than try to assign a value without specifying an index position:

array = '\0';  

By doing:

array[index] = '0\'; 

You're specifying the accessible/modifiable address previously declared.