const unsigned int VIEWTIME = 200;

const unsigned int NLED      = 7; // <= bitsize(row_t)
const unsigned int NLEDON    = 2;
const unsigned int MAXSHIFT  = NLED-NLEDON;
const unsigned int led[NLED] = {13, 12, 11, 10, 9, 8, 7};

typedef byte row_t;

void setup() {
  for ( int i = 0; i < NLED; i++) {
    pinMode(led[i], OUTPUT);
  }
}

void ledWrite(row_t long leds) {
  for ( int i = 0; i < NLED; i++) {
    digitalWrite(led[i], bitRead(leds, i));
  }
}

row_t nledon(uint8_t n) {
  return (1<<n)-1; // 0..01..1 (n x 1)
}

row_t shiftLeft(row_t leds) {
  return leds <<= 1;
}

row_t shiftRight(row_t leds) {
  return leds >>= 1;
}

void loop() {
  row_t ledon = nledon(NLEDON);  // 0000011 (NLEDON==2 and NLED==7)
  for ( int i = 0; i < MAXSHIFT; i++) {
    ledWrite(ledon);
    ledon = shiftLeft(ledon);    // 0000011 -> 0000110 -> .. -> 1100000
    delay(VIEWTIME);
  }
  for ( int i = 0; i < MAXSHIFT; i++) {
    ledWrite(ledon);
    ledon = shiftRight(ledon);   // 1100000 -> 0110000 -> .. -> 0000011
    delay(VIEWTIME);
  }
}