MIDI keyboard with Teensyduino

midi_keyboard.ino

/*
 * MIT License
 * Copyright (c) 2018 Sid Mani
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

const int row[9] = {26,0,1,2,3,10,11,12,13}; //positive in
const int col[6] = {4,5,6,7,8,9}; //out
int active[9][6] = {
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0},
{0,0,0,0,0,0}};
const int offset = 36;

const int channel = 1;
void setup()
{
  for (int i = 0; i<9; i++)
  {
  pinMode(row[i], OUTPUT);
  }
  for (int i = 0; i<6; i++)
  {
  pinMode(col[i], INPUT);
  }
}
void loop()
{
 for (int r = 0; r<9; r++)
  {
    digitalWrite(row[r], HIGH);
    for (int c = 0; c<6; c++)
     {

       int curr = digitalRead(col[c]);
       if (active[r][c] != curr)
       {
         if (curr == 1)
         {
           usbMIDI.sendNoteOn(offset + r * 6 + c, 99, channel);
         }
         else
         {
             usbMIDI.sendNoteOff(offset + r * 6 + c, 99, channel);
         }
         active[r][c] = curr;
       }
     }
     digitalWrite(row[r], LOW);
  }
}