float[] sineWave = new float[width];

for (int i = 0; i < width; i++) {
  // Fill the array with values from sin()
  float r = map(i, 0, width, 0, TWO_PI);
  sineWave[i] = abs(sin(r));
}

for (int i = 0; i < sineWave.length; i++) {
  // Set the stroke values to numbers read from the array
  stroke(sineWave[i] * 255);
  line(i, 0, i, height);
}