Expression must be a modifiable lvalue
C++If StatementC++ Problem Overview
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?
C++ Solutions
Solution 1 - C++
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))
Solution 2 - C++
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.
Solution 3 - C++
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))
Solution 4 - C++
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.