Initializing an array within a class

Question

I'm making a game of snake on the adafruit Neopixel shield and have run in to a problem. How do you fill out an array (in this case a 5*8 array) with values once the class has been called. This is my code so far

H file

#include <Arduino.h>
#include <Adafruit_NeoMatrix.h>
#include <streaming.h>

class Snake : public Adafruit_NeoMatrix 
{
    public:
    Snake();
    void drawSnake();

    private:
    int snakeBoard[5][8];
};

CPP file

#include "Snake.h"

Snake::Snake() : Adafruit_NeoMatrix(8, 5, D6,
  NEO_MATRIX_TOP        + NEO_MATRIX_LEFT +
  NEO_MATRIX_ROWS       + NEO_MATRIX_PROGRESSIVE,
  NEO_GRB               + NEO_KHZ800)
{
  begin(); // start the matrix
  show();

  int snakeBoard[5][8] = {{1,1,0,0,0,0,0,0},
                        {0,0,1,0,0,0,0,0},
                        {0,0,1,0,1,1,-1,0},
                        {0,0,1,1,1,0,0,0},
                        {0,0,0,0,0,0,0,0}};
}

void Snake::drawSnake()
{
  for(int y = 0; y < 5; y ++)
  {
    for(int x = 0; x < 8; x ++)
    {
      Serial << "X: " << x << " Y: " << y << " Value" << snakeBoard[x][y] << endl;
      delay(100);
    }
  }
  drawPixel(5,5,Color(0,0,255));
  show();
}

When I run the program random garbage is printed in the serial monitor.

And in your constructor you declare a new array named snakeBoard which is deleted automatically once you leave the constructor. This array covers up the array declared in the class definition — chrisl
– chrisl, Commented Apr 2, 2018 at 14:05

Maximilian Gerhardt · Accepted Answer · 2018-04-02 14:38:15Z

As already mentioned by @chrisl, you are creating a new local variable which overshadows your class member.

An easy way to get what you want is to memcpy your initial values into the array:

Snake::Snake() : Adafruit_NeoMatrix(8, 5, D6,
  NEO_MATRIX_TOP        + NEO_MATRIX_LEFT +
  NEO_MATRIX_ROWS       + NEO_MATRIX_PROGRESSIVE,
  NEO_GRB               + NEO_KHZ800)
{
  begin(); // start the matrix
  show();

  int initialSnakeBoard[5][8] {{1,1,0,0,0,0,0,0},
                        {0,0,1,0,0,0,0,0},
                        {0,0,1,0,1,1,-1,0},
                        {0,0,1,1,1,0,0,0},
                        {0,0,0,0,0,0,0,0}};
  memcpy(snakeBoard, initialSnakeBoard, sizeof(initialSnakeBoard));
}

You can achieve the same thing by utilizing C++11 constructor lists to initialize your members (see here):

Snake::Snake() : Adafruit_NeoMatrix(8, 5, D6,
  NEO_MATRIX_TOP        + NEO_MATRIX_LEFT +
  NEO_MATRIX_ROWS       + NEO_MATRIX_PROGRESSIVE,
  NEO_GRB               + NEO_KHZ800), 
  snakeBoard {{1,1,0,0,0,0,0,0},
                        {0,0,1,0,0,0,0,0},
                        {0,0,1,0,1,1,-1,0},
                        {0,0,1,1,1,0,0,0},
                        {0,0,0,0,0,0,0,0}}
{
  begin(); // start the matrix
  show();
}

You also have an error in your drawSnake routine: You confused the order of the indexes. With snakeBoard[5][8] you declare 2-dimensional integer array with 5 rows containing 8 elements each. Then, snakeBoard[0] will be a pointer to the first row (which has 8 elements). So the first index is y and must only range from 0 to 4 inclusive. The function must be:

void Snake::drawSnake() {
    for(int y = 0; y < 5; y ++)
    {
        for(int x = 0; x < 8; x ++)
        {
            Serial.println("X: " + String(x) + " Y: " + String(y) + " Value " + String(snakeBoard[y][x]));
            delay(100);
        }
    }
}

This then prints all the expected array elements to the the Serial port, as tested on my Arduino Nano.

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