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void loop() { 
  int x;
  int y[100];
  for(int i = 0; i<100; i++){ 
    y[i] = x;

You are reading data without checking if there is any data there. That's like watching the TV without checking if it is turned on. Of course you get weird data.

SoftwareSerial RX(0,1); //(rx pin, tx pin)

Pins 0 and 1 are used by HardwareSerial. Why are you using them for SoftwareSerial?

SoftwareSerial RX(0,1); //(rx pin, tx pin)


So you initialized HardwareSerial, which now controls pins 0 and 1.

Then you try to initialize SoftwareSerial on those same pins! Would you mind explaining why?

Whatever your explanation is, it won't work.