CODEWITHSUNDEEP
c++
Edgar Acuna
Edgar Acuna

Reputation: 21

: warning: ISO C++ forbids variable length array ‘s1’ [-Wvla]

I am building an R package that contains a c++ program. The checking runs fine, but I am getting this message : warning: ISO C++ forbids variable length array ‘s1’ [-Wvla]

The CRAN's maintainer says that the error is in this part of the code is shown below. I am thinking that the argument "nrows" is redundant , but I wonder if there is another way to solve the problem

   double entCI(double input[], int cMatrix[], double partition, 
    int nrows, int  begin, int end)
   {
   double s1[nrows], s2[nrows], entropy;
   int cs1[nrows], cs2[nrows];
   int s1Count=0, s2Count=0, sCount=0;
   while(input[begin]<partition)
   {
    cs1[s1Count]=cMatrix[begin];
    s1[s1Count++]=input[begin++];
     }
    while(begin<end)
    {
    cs2[s2Count]=cMatrix[begin];
    s2[s2Count++]=input[begin++];
    }
    sCount=s1Count+s2Count;
    entropy=(s1Count/double(sCount))*ent(s1,cs1,s1Count)
           +(s2Count/double(sCount))*ent(s2,cs2,s2Count);
    return entropy;
    }

Upvotes: 2

Views: 2236

Answers (3)

Serge Ballesta
Serge Ballesta

Reputation: 149155

Variable Length Arrays is for a long time a feature from gcc. It has been accepted in C99 but not in C++11 (nor in any following C++ version I know).

An easy and clean solution would be to compile that function as C because it does not use any specific C++ feature, simply array manipulation. In fact, this function is plain C that happens to be accepted by g++ but is not correct C++ hence the warning.

My advice is :

  • put the function in a .c file and compile it in C99 mode

  • declare it as extern "C" double entCI(double input[], int cMatrix[], double partition, int nrows, int begin, int end) in other C++ module, or better write the include file declaring it as

    #ifdef C++
extern "C" {
#endif
double entCI(double input[], int cMatrix[], double partition, 
    int nrows, int  begin, int end)
#ifdef C++
}
#endif

Upvotes: 0

eerorika
eerorika

Reputation: 238431

Indeed, the error is on these lines:

double s1[nrows], s2[nrows], entropy;
int cs1[nrows], cs2[nrows];

They declare arrays, whose size depend on the nrows argument. The value of nrows is determined at runtime and therefore the arrays must be variable length. Such array variables are not allowed by the c++ standard as told to you by the warning.

I am thinking that the argument "nrows" is redundant

I don't see how that is. It's used in the function.

but I wonder if there is another way to solve the problem

There are ways to solve the problem. If the size of the array needs to be determined at runtime, it must be allocated dynamically. The simplest and safest way to do that is to use std::vector.

Upvotes: 3

Simia_Dei
Simia_Dei

Reputation: 76

Generally you should use dynamic memory allocation to create array out of variable: double* s1 = new double[nrows]; Then, remember to delete that array.

Other solution is to use std::vector instead of plain array.

Upvotes: 0

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