Expression must be a modifiable lvalue

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?

4 Answers

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)) 

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.

You test k = M instead of k == M.
Maybe it is what you want to do, in this case, write if (match == 0 && (k = M))

Remember that a single = is always an assignment in C or C++.

Your test should be if ( match == 0 && k == M )you made a typo on the k == M test.

If you really mean k=M (i.e. a side-effecting assignment inside a test) you should for readability reasons code if (match == 0 && (k=m) != 0) but most coding rules advise not writing that.

BTW, your mistake suggests to ask for all warnings (e.g. -Wall option to g++), and to upgrade to recent compilers. The next GCC 4.8 will give you:

 % g++-trunk -Wall -c ederman.cc ederman.cc: In function ‘void foo()’: ederman.cc:9:30: error: lvalue required as left operand of assignment if ( match == 0 && k = M ) ^ 

and Clang 3.1 also tells you ederman.cc:9:30: error: expression is not assignable

So use recent versions of free compilers and enable all the warnings when using them.