CODEWITHSUNDEEP
c++arraysinitializer-list
Dalek
Dalek

Reputation: 4318

How can I assign an array from an initializer list?

I have a limited knowledge about c++. I tried to compile a c++ library and when I run the make file for the following header file

mcmc_dhs.h

#include <algorithm>
#include <map>

// intrinsic shape and (reduced) shear just add?
//#define WLNOISE

// use shear instead of reduced shear for model
//#define NOREDSHEAR

/// parameters for the M200-concentration relation
const number mcreal[2] = {9.59,-0.102}; // Dolag et al. (2004)
//const number mcreal[2] = {5.26,-0.100}; // Neto et al. (2007) [Millenium Run]

/// critical density at z=0 (h100=1) in [Msun/Mpc^3]
const number rhocrit = exp(log(rhoCrit)+3.*log(Mpc)-log(Msun)); 

/// two extra halo parameters: r200 (and concentration: 2)
#define PARAMS 1

/// define region (square; twice value here) about halo that considers sources for model
#define REGION 10.0*arcmin

class mcmc_dhs : public mcmc
{
 public:

  mcmc_dhs() : 
  data(), cosmohandler(0.3,0.7,0.21,0.8,0.04),
    lenseff(), intrvar()
    {
      boundaries = 
    {{0,512},{0,512},{0.01,5.},{100.,3000.},{0.1,50}};
    }
  ~mcmc_dhs() {}

  /// size of grid for looking up sources
  static const int Ngrid = 200;

It returns the following error message:

mcmc_dhs.h:55:67: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11 [enabled by default]
      boundaries = {{0,512},{0,512},{0.01,5.},{100.,3000.},{0.1,50}};
                                                                   ^
mcmc_dhs.h:55:17: error: assigning to an array from an initializer list
      boundaries = {{0,512},{0,512},{0.01,5.},{100.,3000.},{0.1,50}};
                 ^
In file included from ../modules/matrix.h:15:0,
                 from ../modules/probdensity.h:4,
                 from ../modules/mcmc.h:4,
                 from mcmc_dhs.h:4,

Upvotes: 23

Views: 89748

Answers (3)

Miles C.
Miles C.

Reputation: 111

When you said

boundaries = 
{{0,512},{0,512},{0.01,5.},{100.,3000.},{0.1,50}};

it was incorrect, because C++ does not let you reassign array values. There is an easy workaround, but it is somewhat tedious. All you have to do is assign the values one by one.

For example:

boundaries[0][0] = 0;
boundaries[0][1] = 512;
boundaries[1][0] = 0;
boundaries[1][1] = 512;

and so on. I had this same problem in an Arduino program.

Upvotes: 2

user13833197
user13833197

Reputation: 9

Array are essentially just pointers. C++ (being a symbol based programming language) has its own interpretations for arrays. Meaning:

int* a[3]; you've declared the array, but currently the values assigned to each element is some junk values that were already stored at the memory location allotted to your array.

a={1,2,3}; won't work because: C++ treat the array name 'a' as a pointer pointing at the address location of the 1st element in array. 'a' is basically interpreted by C++ as '&a[0]' which is the address of the element a[0]

So, you have 2 ways to go about assigning values

  1. using array indexing (your only option if you don't know what pointers are)

    int a[3]; for(int i=0;i<3;++i) // using for loop to assign every element a value { cin>>a[i]; }

2 treating it as a pointer and using pointer operation

    int a[3];
    for(int i=0;i<3;++i) // using for loop to assign every element a value
    {
    cin>>*(a+i); // store value to whatever it points at starting at (a+0) upto (a+2)
    }

Note: can't use ++a pointer operation as ++ changes the position of a pointer whereas a+i will not change the location of pointer 'a', and anyways using ++ will give a compiler error.

Recommend reading Stephen Davis C++ for dummies book.

Upvotes: 0

vsoftco
vsoftco

Reputation: 56557

You cannot assign directly to an array after its declaration. Basically your code is the same as

int main()
{
    double arr[2][2];
    arr = { {1, 2}, {3, 4.5} }; // error
}

You have to either assign the value at declaration

double arr[2][2] = { {1, 2}, {3, 4.5} };

or use a loop (or std::copy) to assign elements. Since your array seems to be a member variable, you can also initialize it in the constructor initialization list:

 mcmc_dhs() : data(), cosmohandler(0.3,0.7,0.21,0.8,0.04), 
              lenseff(), intrvar(), 
              boundaries{{0,512},{0,512},{0.01,5.},{100.,3000.},{0.1,50}}
 { 
    // rest of ctor implementation
 }

Upvotes: 27

Related Questions