C++ expected constant expression
Solution 1
float x[size][2];
That doesn't work because declared arrays can't have runtime sizes. Try a vector:
std::vector< std::array<float, 2> > x(size);
Or use new
// identity<float[2]>::type *px = new float[size][2];
float (*px)[2] = new float[size][2];
// ... use and then delete
delete[] px;
If you don't have C++11 available, you can use boost::array
instead of std::array
.
If you don't have boost available, make your own array type you can stick into vector
template<typename T, size_t N>
struct array {
T data[N];
T &operator[](ptrdiff_t i) { return data[i]; }
T const &operator[](ptrdiff_t i) const { return data[i]; }
};
For easing the syntax of new
, you can use an identity
template which effectively is an in-place typedef (also available in boost
)
template<typename T>
struct identity {
typedef T type;
};
If you want, you can also use a vector of std::pair<float, float>
std::vector< std::pair<float, float> > x(size);
// syntax: x[i].first, x[i].second
Solution 2
The array will be allocated at compile time, and since size
is not a constant, the compiler cannot accurately determine its value.
Solution 3
You cannot have variable length arrays (as they are called in C99) in C++. You need to use dynamically allocated arrays (if the size varies) or a static integral constant expression for size.
Solution 4
Because it expected a constant expression!
Array dimensions in C (ignoring C99's VLAs) and C++ must be quantities known at compile-time. That doesn't mean just labelled with const
: they have to be hard-coded into the program.
Use dynamic allocation or std::vector
(which is a wrapper around dynamic array allocation) to determine array sizes at run-time.
Solution 5
The line float x[size][2]
won't work, because arrays have to be allocated at compile time (with a few compiler-specific exceptions). If you want to be able to easily change the size of the array x
at compile time, you can do this:
#define SIZE 100
float x[SIZE][2];
If you really want to allocate the array based on information you only have at runtime, you need to allocate the array dynamically with malloc
or new
.
n0ob
Updated on May 03, 2020Comments
-
n0ob about 4 years
#include <iostream> #include <fstream> #include <cmath> #include <math.h> #include <iomanip> using std::ifstream; using namespace std; int main (void) { int count=0; float sum=0; float maximum=-1000000; float sumOfX; float sumOfY; int size; int negativeY=0; int positiveX=0; int negativeX=0; ifstream points; //the points to be imported from file //points.open( "data.dat"); //points>>size; //cout<<size<<endl; size=100; float x[size][2]; while (count<size) { points>>(x[count][0]); //cout<<"x= "<<(x[count][0])<<" ";//read in x value points>>(x[count][1]); //cout<<"y= "<<(x[count][1])<<endl;//read in y value count++; }
This program is giving me expected constant expression error on the line where I declare float x[size][2]. Why?