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  lt  C  k            match --       if   match    0  amp  amp  k   M                  std  cout  lt  lt    equals   lt  lt  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

User · Answer

Remember that a single   is always an assignment in C or C      Your test should be if   match    0  amp  amp  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  amp  amp   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  amp  amp  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

User · Answer

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

User · Answer

The assignment operator has lower precedence than  amp  amp   so your condition is equivalent to   if   match    0  amp  amp  k    m    But the left-hand side of this is an rvalue  namely the boolean resulting from the evaluation of the sub shy expression match    0  amp  amp  k  so you cannot assign to it   By contrast  comparison has higher precedence  so match    0  amp  amp  k    m is equivalent to   if   match    0   amp  amp   k    m

User · Answer

You test k   M instead of k    M  Maybe it is what you want to do  in this case  write if  match    0  amp  amp   k   M

[c++] Expression must be a modifiable lvalue

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