Percentage chance of saying something?

I want to write a program that:

• 80% of the time will say sendMessage("hi");
• 5% of the time will say sendMessage("bye");
• and 15% of the time will say sendMessage("Test");

Does it have to do something with Math.random()? like

if (Math.random() * 100 < 80) {
  sendMessage("hi");
}
else if (Math.random() * 100 < 5) {
  sendMessage("bye");
}

Yes, Math.random() is an excellent way to accomplish this. What you want to do is compute a single random number, and then make decisions based on that:

var d = Math.random();
if (d < 0.5)
    // 50% chance of being here
else if (d < 0.7)
    // 20% chance of being here
else
    // 30% chance of being here

That way you don't miss any possibilities.

For cases like this it is usually best to generate one random number and select the case based on that single number, like so:

int foo = Math.random() * 100;
if (foo < 80) // 0-79
    sendMessage("hi");
else if (foo < 85) // 80-84
    sendMessage("bye");
else // 85-99
    sendMessage("test");

I made a percentage chance function by creating a pool and using the fisher yates shuffle algorithm for a completely random chance. The snippet below tests the chance randomness 20 times.

var arrayShuffle = function(array) {
   for ( var i = 0, length = array.length, swap = 0, temp = ''; i < length; i++ ) {
      swap        = Math.floor(Math.random() * (i + 1));
      temp        = array[swap];
      array[swap] = array[i];
      array[i]    = temp;
   }
   return array;
};

var percentageChance = function(values, chances) {
   for ( var i = 0, pool = []; i < chances.length; i++ ) {
      for ( var i2 = 0; i2 < chances[i]; i2++ ) {
         pool.push(i);
      }
   }
   return values[arrayShuffle(pool)['0']];
};

for ( var i = 0; i < 20; i++ ) {
   console.log(percentageChance(['hi', 'test', 'bye'], [80, 15, 5]));
}

I do this all the time with my discord bots

const a = Math.floor(Math.random() * 11);
	if (a >= 8) { // 20% chance
		/*
		  CODE HERE
		*/
	} else { // 80% chance
		/*
		  CODE HERE
		*/
	}

You can change 11 to 101 if you want.

The reason why it has an extra one is so it does 1 - 10 instead of 1 - 9 (or 1 - 100 instead of 1 - 99)

Generate a 20% chance to get "Yupiii!" in the console.log

const testMyChance = () => {

  const chance = [1, 0, 0, 0, 0].sort(() => Math.random() - 0.5)[0]

  if(chance) console.log("Yupiii!")
  else console.log("Oh my Duck!")
}

testMyChance()

Java

/**
 * Zero or less returns 'false', 100 or greater returns 'true'. Else return probability with required percentage.
 * @param percentage for example 100%, 50%, 0%.
 * @return true or false with required probability.
 */
private static boolean probably(int percentage) {
    double zeroToOne = Math.random(); // greater than or equal to 0.0 and less than 1.0
    double multiple = zeroToOne * 100; // greater than or equal to 0.0 and less than 100.0
    return multiple < percentage;
}

JavaScript

function probably(percentage) {
  return Math.random() * 100 < percentage;
}

Here is a very simple approximate solution to the problem. Sort an array of true/false values randomly and then pick the first item.

This should give a 1 in 3 chance of being true..

var a = [true, false, false]
a.sort(function(){ return Math.random() >= 0.5 ? 1 : -1 })[0]