Guess The Number

Before starting to code this project, please find a partner and play this game together verbally for a few rounds.

  1. Alice thinks of a number
  2. Bob guesses a number
  3. Alice says "higher", "lower", or "correct"
  4. repeat 2 & 3 until Bob guesses correctly
  5. repeat 1-4 with Alice guessing

After you get a feel for the game, it's time to write a software version of the game, where the human thinks of a number between 1 and 100 and the computer tries to guess it.

Think carefully about how the computer will guess a number. Start with it guessing a random number between 1 and 100. Once that's working, high five, take a break, then try to use a more efficient guessing algorithm instead.

"Make it work, make it right, make it fast -- in that order." - Kent Beck


$ node guess.js
Please think of a number between 1 and 100 (inclusive).
I will try to guess it.

Is it... 50? N
Is it higher (H), or lower (L)? H
Is it... 75? N
Is it higher (H), or lower (L)? L
Is it... 63? N
Is it higher (H), or lower (L)? H
Is it... 69? N
Is it higher (H), or lower (L)? L
Is it... 66? Y
Your number was 66!


  • Before writing real code in a text editor, try writing pseudocode or a flowchart on paper, or as comments in your program file. That will help you to understand the essence of the algorithm without getting bogged down in syntax (like semicolons and parentheses).

  • Look back at the programs you've already written. Can you use any functions or logic you've already written?

  • Think of the guesses not just as confirming a possibility, but as eliminating a range of possiblities.

  • We've called the currently-being-asked-about number a "guess", but a better name might be "pivot" or "fulcrum" since it's essentially dividing the set of solutions into two parts. (See the Khan Academy link below for a visualization of this effect.)

  • Remember to keep track of the previous answers, or at least keep track of the current high and low ends of the range.

  • Off-by-one errors will bite you here. Be clear in your own mind (and in the code!) about the distinction between "greater than" and "greater than or equal to" and the like.

  • Sign reversal errors are also a strong possibility. Be clear about the distinction between "my guess is higher than your number" and "your number is higher than my guess" and so forth.

  • When you feel like your game is mostly functional have a classmate play it. See how they break it, and try to make your game more robust.


Pick a number, any number

Given the player starts the game with the command node index.js in the terminal

Then the computer should ask the player if their number is a random number between 1 and 100

And waits for an answer, which it stores as a variable

Let the computer win

Given the computer has guessed a number

When the player responds "yes" (or "y")

Then the game exits with a victory message.

e.g. Your number was XX!

The computer guessed wrong

Given the computer has guessed a number

When the player responds "no" (or "n")

Then the computer asks if the number is higher or lower

Modify your guess range

Given the computer guessed the incorrect number

When the player responds "higher" ("h") or "lower" ("l")

Then the computer modifies the range it guesses within based on the answer


Is your number 41? no
Is your number higher(h) or lower(l)? h

And guesses a new number within the modified range

Given the example above the computer now guesses between 42 and 100

If the number is higher, you'll want to modify the low end of the range, and vice versa if the number is lower.

Make it smarter

  • In the optimal solution, the game will find the correct number in no more than [log2(n)+1] guesses -- so for n=100, that's 7 guesses max. This solution is a good example of a binary search algorithm.

Given The player chooses a number between 1 and 100

When the computer guesses a number

Then the computer should make a smart guess

So That the computer guesses the number in no more than 7 tries

Extend the guess range

Given the game has not been started

When the game is first called with node index.js

Then allow the user to set the high range so it could be any number greater than 1

Cheat Detector

Given a higher/lower response contradicts an earlier response

Then the computer complains

And asks again, or exits. e.g. You said it was lower than 25, so it can't also be higher than 24!

Note: it is possible to set the logic up in a way so that the game won't allow the player to cheat, but we still need to be notified if they try it.

Role Reversal!

Now try and create the reverse game where the computer picks a number, and you guess it. Create a new file called reverse_game.js to hold this program

Given The game has been started with node reverse_game.js

Then the game will ask you to guess a number

And if you guess wrong it will tell you if you guessed too high or too low

And prompt you to guess again

When you guess correctly the computer will congratulate you, and end the game


Play it again!

When the game finishes, instead of exiting, it should ask you if you want to play again. If you say "yes" it should restart the game from the beginning, otherwise it exits.

How many tries?

When the game finishes modify the victory message so it tells the user how many guesses it took for the computer to guess the correct number.

Combine the games

Modify the program in index.js so that the user can choose whether to play the normal game, or the reverse game when the program is started with node index.js