How to make a program that guesses what number I'm thinking of?

Hello everyone!

I am new to these forums. I come here today because I need help with a C++ programming assignment for school, and in the long term I also hope to find some mentors and friends here. I hope to revisit these forums many times to come both to get help and offer help to others as I become a proficient programmer.

So! Here is the assignment! I need to make a C++ program that will try to guess what number I am thinking of. It's sort of like that old magic trick where the magician asks "is this your card?" :)

The program will prompt the user to think of a number between 1 and 100. The program will then print out a number and ask the user to either confirm the number as the correct number (the number the user thought of), or in case the number is wrong, the user will tell the program that the number is either lower or higher than the displayed number. The program will continue guessing until the correct number is found, at which point the user will tell the program that the number is correct and the program will print out message celebrating its success at guessing the number. One important requirement is that the program may not use any more than 7 guesses to find the number.

(By the way, do you say "program will prompt" or "computer will prompt"? Is it "user will tell the program" or "user will tell the computer"? Is it in any context important to make this kind of distinction between computer and program?)

Here is the skeleton code I got so far:

Code:

---

#include <iostream>

using namespace std;

int main()
{
    cout << "Think of a number between 1 and 100. Press ENTER when ready." << endl;

    cout << "Is it " << " ?" << endl;
    cout << "Press 1 and ENTER if the number is correct." << endl;
    cout << "Press 2 and ENTER if your number is lower." << endl;
    cout << "Press 3 and ENTER if your number is higher." << endl;

    cout << "Found it!" << endl;

    return 0;
}

---

I know it's not much right now. :) But at least I got some text on the screen. It's a start! It also outlines how the program is supposed to work. I also know how to store the user returned value and store it in a variable. But I don't know how to build the logic or the algorithm for this.

Where do I go from here? I know those three prompts will have to iterate until the correct number is found. So do I use a while loop in here?

The last printout will have to display at the end of the program, and only after the correct number is found. So do I use an else condition here?

I read on the web that an algorithm called binary search is perfect for this situation. I also read that C++ has some built in functionality for this in the standard library. But my problem is that I have no idea how to implement this. I am completely green to both C++ and programming in general! I have read some tutorials, watched some tutorials, and I understand some of the fundamental concepts of programming. But I have no idea how to do something like binary search.

The course I'm in is a beginner's course, and I don't understand how they expect us to know this kind of stuff. We haven't even learned yet how to do inclusions, library imports or how header files work and all that stuff. The course book is mediocre at best. So please Internet, can you help me? You don't have to give me a complete solution, but maybe you can give me some pointers (no pun intended) so I can finish this on my own?

The basic idea behind a binary search is that you have a start value and an end value. The 'guess' is the 'middle' between the start and end. If the guess is the right number than great. If guess is less than the required number then the start number becomes the guess (+ 1 ?) and the process is repeated. If guess is greater then the required number then the end number becomes the guess (- 1 ?) and the process is repeated. This process loops until the guess is the required number. For a maximum number of 100, the maximum number of guesses required is indeed 7 - each time through the loop you half the available pool of numbers.

Try coding this up and repost your code and we'll provide further guidance.

Cheers! :)

How do I make a program pause and wait for me to press a key before it continues?

Right after the first printout I want it to pause, and then continue when I have pressed Enter. How do I do that?

Code:

---

system("pause");

---

Quote:

---

Originally Posted by 2kaud

Code:

---

system("pause");

---

---

"Error! System was not declared in this scope."

I hve put it in the main, right after the first printout line.

Do I have to store the return value from cin before I use it in a test?

Code:

---

cin >> a;

if(a<b)
{
        cout << "Hello!";
}

---

Can I use the returned value in a test directly?

Code:

---

if(cin>><b)
{
        cout << "Hello!";
}

---

I know it looks stupid and incomplete, with all those less than and greater than signs. But hey maybe I'm just using the wrong syntax, I don't know.

P.s. No edit buton in here?

What compiler are you using?

This works for VS2015

Code:

---

#include <iostream>
using namespace std;

int main()
{
        cout << "Hello" << endl;
        system("pause");
}

---

However, depending upon the compiler you may also need to include cstdlib

Code:

---

#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
        cout << "Hello" << endl;
        system("pause");
}

---

Quote:

---

Originally Posted by cozySam

Do I have to store the return value from cin before I use it in a test?

Code:

---

cin >> a;

if(a<b)
{
        cout << "Hello!";
}

---

Can I use the returned value in a test directly?

Code:

---

if(cin>><b)
{
        cout << "Hello!";
}

---

I know it looks stupid and incomplete, with all those less than and greater than signs. But hey maybe I'm just using the wrong syntax, I don't know.

P.s. No edit buton in here?

---

Yes, you have to store the value read using cin into a variable before it can be used.

Code:

---

int num;
cin >> num;

---

This reads an integer value into the variable num.

Code:

---

#include <iostream>

using namespace std;

int main()
{
    cout << "Think of a number between 1 and 100. Press ENTER when ready." << endl;

    int answer, currentNumber=50;

    cout << "Is it " << currentNumber << "?" << endl;
    cout << "Press 1 and ENTER if the number is correct." << endl;
    cout << "Press 2 and ENTER if your number is lower." << endl;
    cout << "Press 3 and ENTER if your number is higher." << endl;

    cin >> answer;

    if(answer==1)
    {
        cout << "Found it!" << endl;
    }else
    {
        if(answer==2)
        {
            currentNumber/2;
        }else
        {
            if(answer==3)
            {
                currentNumber;
            }
            }
        }
    }

    return 0;
}

---

Something like that! :p I don't know. I don't know if I should loop it and then do the if else, or if else and then loop it. I think I'll just give it up and shelf it until I have actually learned to program.

I know those if else, if else look stupid, and I hate it. I also know I can use switch instead, but I am still not confident enough to use those. They are too scary! :p

I think I will stick to the "hello world" for now and build from there. Maybe my teacher should teach me these things first, before giving me assignment like this one. It's a distance learning course too, and he has not answered on my last question in 2 days now (they have to answer within 4 hours of any given work day).

I'm not looking for someone to solve this for me really. I would like more than anything to solve it on my own. But I think expectations are set too high with this assignment (it's only the second one in the course). It's just so frustrating when you have something that looks so simple and you can't solve it. I'm sure you know that feeling. :)

Code:

---

if(you give me some more clues and pointers)
{
    i might give this second try
}else
{
    i will leave it for some later time
}

---

What's with this brace style anyway? It looks a bit stupid does it not? Having the opening brace start on a new line makes my editor confused, i.e. it doesn't always recognize what brace belongs to what other brace, especially when nesting several if and else. But this is how my teacher likes it, so I am trying to adapt to it, but I'm having hard time with it (as does my editor).

Can someone work through the "binary search" alg. with me here mentally? What's going on? How does it work? I know intuitively how it works, but I want to see the math in this.

So the range is 1 to 100. You can be the user and I will be the computer (it's a flipped version of the guess the number game, i.e. flipped roles). You think of number 37. You ask me to guess the number.

I say 50.
You say lower.
I take 50 and divide by 2.
I say 25.
You say higher.
I do what ??? What do I do with the 25?
I say 50 again?

I get stuck here mentally. I know I should say 37 here (according to the teacher), but I don't see the math. How do I arrive there?

Do I have to store and carry those previous numbers, like 50, 25, etc?

s = 1, e = 100
g = (1 + 100) / 2 = 50 (integer division!)
I say 50
You say lower
s = 1, e = g - 1 = 50 - 1 = 49
g = (1 + 49) / 2 = 25
I say 25
You say higher
s = g + 1 = 25 + 1 = 26, e = 49
g = (26 + 49) / 2 = 37
I say 37
You say guessed it!

The guesses etc go in a loop until the correct number is guessed.

Quote:

---

Having the opening brace start on a new line makes my editor confused

---

Where the braces are located is immaterial to the operation of the program and is just writing style.
Consider

Code:

---

if (a == b) {c = d; e = f;} else {z = y; y = x;}

---

This is valid but is terrible to read!

Now consider one style

Code:

---

if (a == b) {
    c = d;
    e = f;
} else {
    z = y;
    y = x;
}

---

Another style could be

Code:

---

if (a == b)
{
    c = d;
    e = f;
}
else
{
    z = y;
    y = x;
}

---

or even

Code:

---

if (a ==b)
{
    c = d;
    e = f;
} else
{
    z = y;
    y = x;
}

---

A lot of style is personal preference (or in-house coding style if working for a company) for code readability.

Note that if there is only one statement in the then or else part then braces are not needed. Consider

Code:

---

if (a == b)
  c = d;
else
  e = f;

---

Quote:

---

I don't know if I should loop it and then do the if else, or if else and then loop it

---

Which loops have you covered? There are 3 - do, for and while.

Quote:

---

Maybe my teacher should teach me these things first, before giving me assignment like this one. It's a distance learning course too,

---

You might find this site useful
http://www.learncpp.com/

What book(s) are you using from which to study c++?

Quote:

---

I also know I can use switch instead, but I am still not confident enough to use those. They are too scary!

---

If you that that's scary, wait until you start to cover some of the advanced c++ stuff like template meta-programming! :D

Quote:

---

Originally Posted by 2kaud

What compiler are you using?

---

I am using Code Blocks with MinGW (GCC) compiler.

Quote:

---

Originally Posted by 2kaud

This works for VS2015

Code:

---

#include <iostream>
using namespace std;

int main()
{
        cout << "Hello" << endl;
        system("pause");
}

---

However, depending upon the compiler you may also need to include cstdlib

Code:

---

#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
        cout << "Hello" << endl;
        system("pause");
}

---

---

The second example worked for me. So it needed that inclusion.

Quote:

---

Originally Posted by 2kaud

Which loops have you covered? There are 3 - do, for and while.

---

I have covered while loops, and I have seen for loops in other examples with other languages. But I have not worked with it yet with C++ in this course.

I almost have it now. But I keep going around the loop even after finding the correct number. How do I prevent this?

Code:

---

int main()
{
    // Create an array of 100 elements
    int arr[100];

    // Set values to elements, ranging from 1 to 100
    for (int i = 0; i < 100; i++)
    {
        arr[i] = i + 1;
    }

    cout << "Think of a number between 1 and 100. Press ENTER when ready." << endl;
    cin.get();

    int key;
    int first = arr[0];
    int last = (sizeof(arr)/sizeof(int)-1);

    while (first <= last)
    {
        int mid = (first + last)/2;

        cout << "Is it " << mid << "?" << endl;
        cout << "Press 1 and ENTER if the number is correct." << endl;
        cout << "Press 2 and ENTER if your number is lower." << endl;
        cout << "Press 3 and ENTER if your number is higher." << endl;

        cin >> key;

        switch(key)
        {
            case 1:
            cout << "Found it!" << endl;

            case 2:
            last = mid +1;
            break;

            case 3:
            first = mid + 1;
            break;

            default:
            return -1;
            break;
        }
    }

    return 0;
}

---

When the response is 1 then you need to break out of the loop. So there must be some condition for the while loop to stop when the number has been found.

Incidentally, you don't need the array as first is 1 and last is 100 at the beginning.

Quote:

---

Originally Posted by 2kaud

When the response is 1 then you need to break out of the loop. So there must be some condition for the while loop to stop when the number has been found.

Incidentally, you don't need the array as first is 1 and last is 100 at the beginning.

---

I need to set the first to be greater than last? That should stop it.

But I used a return instead. This is what I have now:

Code:

---

int main()
{
    // Create an array of 100 elements
    int arr[100];

    // Set values to elements, ranging from 1 to 100
    for (int i = 0; i < 100; i++)
    {
        arr[i] = i + 1;
    }

    cout << "Think of a number between 1 and 100. Press ENTER when ready." << endl;
    cin.get();

    int key;
    int first = arr[0];
    int last = (sizeof(arr)/sizeof(int)-1);

    while (first <= last)
    {
        int mid = (first + last)/2;

        cout << "Is it " << mid << "?" << endl;
        cout << "Press 1 and ENTER if the number is correct." << endl;
        cout << "Press 2 and ENTER if your number is lower." << endl;
        cout << "Press 3 and ENTER if your number is higher." << endl;

        cin >> key;

        switch(key)
        {
            case 1:
            cout << "Found it!" << endl;
            return 0;

            case 2:
            last = mid - 1;
            break;

            case 3:
            first = mid + 1;
            break;

            default:
            return -1;
            break;
        }
    }

    return 0;
}

---

I have turned it in this way. I did add a counter as the last thing before turning it in. It failed to work properly anyway. I needed to print out the number of tries the computer had made once it found the number. I did something stupid with j++ and got 1 for 3 tries (where key was 37).

Note that I changed last = mid + 1 in case 2.

I'm curious though, how would you do this? Why do I not need an array? Feel free to post your own solution or change mine.

Quote:

---

Originally Posted by cozySam

I'm curious though, how would you do this? Why do I not need an array? Feel free to post your own solution or change mine.

---

You should ask yourself why you do need it. You know the upper and lower bounds are 0 and 100. Why not just set first and last directly. You don't use the array or any of the values other than the first one.

Consider

Code:

---

#include <iostream>
using namespace std;

int main()
{
        cout << "Think of a number between 1 and 100. Press ENTER when ready." << endl;
        cin.get();

        int key;
        int first = 1;
        int last = 100;
        int cnt = 0;

        do {
                int mid = (first + last) / 2;
                ++cnt;

                cout << "Is it " << mid << " ?" << endl;
                cout << "Press 1 and ENTER if the number is correct." << endl;
                cout << "Press 2 and ENTER if your number is lower." << endl;
                cout << "Press 3 and ENTER if your number is higher." << endl;
                cin >> key;

                switch (key) {
                        case 1:
                                cout << "Found it in " << cnt << " tries" << endl;
                                break;

                        case 2:
                                last = mid - 1;
                                break;

                        case 3:
                                first = mid + 1;
                                break;

                        default:
                                cout << "Invalid entry." << endl;
                                --cnt;
                                break;
                }

        } while (key != 1);

        return 0;
}

---

Note that this code doesn't check for incorrect responses - like always entering 2 etc. I'll leave that checking to you as an exercise! :d