Pointer for arrays in C++

Question

I've some problems to use pointers in C++. I implemented three methods with different sized arrays but the same calculations. So I decided to extract the calculations and place them in a new method which expects an array. But that doesn't work and I don't know how to modify my program.

void method1() {
  float a[3][3];
  calculate(a, 3);
}

void method2() {
  float a[4][4];
  calculate(a, 4);
}

void method3() {
  float a[5][5];
  calculate(a, 5);
}

void calculate(float *param[], int n) {
  // Code
}

I know that I can use global variables or vectors etc. but I need the logic in this structure.

Here's the compiler error:

Test.cpp: In function 'void method1()':
Test.cpp:7:16: error: cannot convert 'float (*)[3]' to 'float**' for argument '1' to 'void calculate(float**, int)'
Test.cpp: In function 'void method2()':
Test.cpp:12:16: error: cannot convert 'float (*)[4]' to 'float**' for argument '1' to 'void calculate(float**, int)'
Test.cpp: In function 'void method3()':
Test.cpp:17:16: error: cannot convert 'float (*)[5]' to 'float**' for argument '1' to 'void calculate(float**, int)'

Thanks in advance!

[ARRAYS ARE NOT POINTERS](http://stackoverflow.com/questions/1641957/is-array-name-a-pointer-in-c) — Mooing Duck, Mar 23 '13 at 00:35

score 5 · Accepted Answer · answered Mar 23 '13 at 00:38

template <int n>
void calculate(float (&param)[n][n]) {
  std::cout << param[x][y];
}

void method1() {
  float a[3][3];
  calculate(a);
}

or, if you want to support dynamic sizes (unlikely), you'll have to do a wierd trick and lie to the compiler pretending that it's a 1d array, and that gets tricky fast.

int index(int x, int y, int n) {return y*n+x;}

void calculate(float* param, int n) {
    std::cout << param[index(x, y, n)];
}

void method1() {
  float a[3][3];
  calculate(a[0], 3);
}

Great! I took the template version and it works fine. Thanks a lot! — altralaser, Mar 23 '13 at 02:12

score 1 · Answer 2 · answered Mar 23 '13 at 00:53

If you wish to pass the array without using a template, you may do something like:

static const int size = 3;

void calculate(float (&param)[size][size]) {
}

int main() {
  float  a[size][size];

  calculate(a);
  return 0;
}

size can also be a #define, but i prefer static const.

score 1 · Answer 3 · answered Mar 23 '13 at 03:43

You can use a pointer to pointer...

void calculate(float *param[], int n)
{
    // Code
}

void method1() {

    float **a = new float*[3];

    a[0] = new float[3];
    a[1] = new float[3];
    a[2] = new float[3];

    a[0][0] = 1.0f;
    a[0][1] = 2.0f;
    a[0][2] = 3.0f;

    a[1][0] = 4.0f;
    a[1][1] = 5.0f;
    a[1][2] = 6.0f;

    a[2][0] = 7.0f;
    a[2][1] = 8.0f;
    a[2][2] = 9.0f;

    calculate(a, 3);
}


void method2() {
    float **a = new float*[4];

    a[0] = new float[4];
    a[1] = new float[4];
    a[2] = new float[4];
    a[3] = new float[4];

    calculate(a, 4);
}

void method3() {
    float **a = new float*[5];

    a[0] = new float[5];
    a[1] = new float[5];
    a[2] = new float[5];
    a[3] = new float[5];
    a[4] = new float[5];

    calculate(a, 5);
}

Otherwise you have to specify the column size of the array in the function signature...

void calculate(float param[][3], int n) {
  // Code
}

void method1() {
  float a[3][3];
  calculate(a, 3);
}
/*
void method2() {
  float a[4][4];
  calculate(a, 4);
}

void method3() {
  float a[5][5];
  calculate(a, 5);
}*/

score 0 · Answer 4 · answered Mar 23 '13 at 00:41

0

If you want to handle only square arrays, then you may use template:

template <int N>
calculate(float param[N][N]) {
// code
}

answered Mar 23 '13 at 00:41

Alex1985

658
3
7

Pointer for arrays in C++

4 Answers4