Lesson 1.14: Project: guess the number

A whole game with everything from this stage: variables, conditions, loops and functions.

The idea

The computer picks a number from 1 to 100. You try to guess it, and it tells you whether it's higher or lower. When you get it, it tells you how many attempts you needed.

It's built like any program: think first, then write.

The plan, in pseudocode

Pseudocode

Algorithm Guess
    secret ← a random number from 1 to 100
    attempts ← 0
    attempt ← 0
    While attempt ≠ secret Do
        Read attempt
        attempts ← attempts + 1
        If attempt < secret Then
            Write "Higher"
        Else
            If attempt > secret Then
                Write "Lower"
            EndIf
        EndIf
    EndWhile
    Write "Got it in ", attempts, " attempts"
EndAlgorithm

It's almost the loop from 1.5. Only one piece is missing: the random number.

A random number

JavaScript

function random(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

Math.random() gives a decimal between 0 and 1, never reaching 1.
Math.floor() drops the decimals.
Together they give a whole number between min and max.

You don't need to understand all of it: it's one of those functions you copy.

The game

JavaScript

function random(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

const secret = random(1, 100);
let attempts = 0;
let attempt = 0;

while (attempt !== secret) {
  attempt = Number(prompt("Guess the number (1-100):"));
  attempts = attempts + 1;

  if (attempt < secret) {
    alert("Higher");
  } else if (attempt > secret) {
    alert("Lower");
  }
}

alert("You got it in " + attempts + " attempts!");

alert() shows a message in a window. Paste the whole game into the console and play. To play again, reload the page before pasting it.

Every line is something you already know: a function (1.10), variables (1.8), a while and an if (1.9) and Number() to convert what prompt() returns.

Try it

Exercise

If you press Cancel, the game keeps asking forever. Make it stop.

Hints:

When you cancel, prompt() returns null.
break leaves a loop at once.
The final message should only show if you guessed it.

Show the solution

JavaScript

while (attempt !== secret) {
  const answer = prompt("Guess the number (1-100):");

  if (answer === null) {
    break;
  }

  attempt = Number(answer);
  attempts = attempts + 1;

  if (attempt < secret) {
    alert("Higher");
  } else if (attempt > secret) {
    alert("Lower");
  }
}

if (attempt === secret) {
  alert("You got it in " + attempts + " attempts!");
}

The answer is kept before converting it: Number(null) is 0, and you could no longer tell that the player cancelled.

Summary

First the plan in pseudocode; then the code.
A whole program is small pieces added up: variables, conditions, loops and functions.
Math.random() gives chance; alert() shows a message; break leaves a loop.