Serial Monitor printing - Please help me with this beginner sketch

I’m a beginner, I have this problem while playing around with Serial Monitor

If I disable line 27 only, then line 28 printed normally and vice versa.
But when I activate both line 27 & 28, they didn’t work, the Serial Monitor printed NOTHING!

I couldn’t figure why, is that just me or a serial printing problem? Would you help me to make both line work?

// board: Arduino UNO R3
//the schematic is simple: only a potentiometer connect to 5V, GND, and A0

int potPin = A0; 
int read_Value;
const int num = 10;
int index = -1; // -1 because it will be increased by 1 in the first loop
                // it will never be -1 again in any next loop
int array1[num];// so there is 11 variable, index from 0 to 10

void setup() {
  Serial.begin(9600);
  
  for (int i = 0; i<= num; i+=1) {
    array1[i] = 1; 
  }
}

void loop() {
  if ( index >= num) {
    index = 0;
  } else {
    index += 1;
  }
  read_Value = analogRead(potPin);

  Serial.println(read_Value);  // Line 27
  Serial.println(array1[index]); // Line 28
  delay(100);
}

Post your code in your original post in tags

If an array has 10 elements, they are indexed 0 through 9. Write beyond that and you're messing with memory you don't own. Results will be undefined.

As indicated by gfvalvo.

And this comment is incorrect

int array1[num];// so there is 11 variable, index from 0 to 10

There are 10 variables, index from 0 to 9.

The little change you need to make in your sketch is this: for (int i = 0; i < num; i += 1) in order to get print out on your Serial Monitor. The reason has been explained in Post#2.

Also, as you are PRE-incrementing, the index this code needs to change or you’ll be reading one element past the end as well…

 if ( index >= num - 1) {
    index = 0;
  } else {
    index += 1;
  }

TBH you’d be far better POST-incrementing. It’s easier to handle…

// board: Arduino UNO R3
//the schematic is simple: only a potentiometer connect to 5V, GND, and A0

int potPin = A0; 
int read_Value;
const int max_index = 10;
int index = 0;
int array1[max_index];// so there are 10 variables, index from 0 to 9

void setup() {
  Serial.begin(9600);
  
  for (int i = 0; i < max_index; i++) {
    array1[i] = 1; 
  }
}

void loop() {
  read_Value = analogRead(potPin);

  Serial.println(read_Value);
  Serial.println(array1[index]);

  index++;
  if (index >= max_index) index = 0;

  delay(100);
}