ok i am not going to spend time trying to understand your code, but i am guessing why you are stuck: you are checking on the condition of proximity but you want to generate random number only when the proximity condition changes. so you need to have a function that evaluates this.

I think this bit of code does what you're trying to achieve - move the mouse over the rectangle to get a new random value otherwise it shows the default value of -1.

1. Boolean flagCollided;
2. int nValue;
3. void setup() {
4.   size(640, 480);
5.   background(128);
6.   rectMode(CENTER);
7.   flagCollided=false;
8.   nValue=-1;
9. }
10. void draw() {
11.   background(128);
12.   fill(255);
13.   rect(width/2, height/2, 50, 50);
14.   if ( (abs(mouseX-width/2)<25) && (abs(mouseY-height/2)<25)) {
15.     if (!flagCollided) {
16.       nValue=int(random(100));
17.       flagCollided=true;
18.     }
19.   }
20.   else {
21.     //default value
22.     flagCollided=false;
23.     nValue=-1;
24.   }
25.   fill(0);
26.   text(nValue, mouseX, mouseY);
27. }

In plain English, the business part is from the line that starts if( (abs(mouseX.....)

If the mouse is over the rect and the collision flag isn't already set -> set a new random value and set the collision flag to true.

If the mouse isn't over the rect set the collision flag to false and use the default value.

Ok, I`ve tried with a boolean function and the second option, but still is not working.

Can you post the code, I don't quite understand the question.