Analog inputs showing value even there is nothing connected to it [duplicate]

Question

This question already has an answer here:

• Getting random values on Arduino Duemilanove 328 1 answer
• Should all unused analog input ports be tied to ground for accurate A2D measurements? 2 answers

After watching something weird on the serial monitor I wrote a simple code to monitor the analog pin input values. There was nothing connected to the analog pins still they where showing decimal representation of some analog values.

the code is

float ana [] = {A0,A1,A2,A3,A4,A5};
float dec [6];  

void setup() 
{
Serial.begin(9600);
}

void loop() 
{ 
  for(int i = 0; i<6 ; i++)
  {
    dec[i] = analogRead(ana[i]);
    Serial.println(dec[i]);
    delay(1000);
  }
}

How is this even possible?

this is the output result

Answer 1

Your input is floating if nothing is connected. It is reading whatever random noise it happens to pick up. That's what it is supposed to do. For it to read 0 it would need to be connected to ground. Google "floating input" for more info.