CODEWITHSUNDEEP
c++int
codedude
codedude

Reputation: 6549

C++ Weird Variable Issues

I'm using the following code to calculate and display the final score for a math game in C++. 

int score = (correctNumber / 3) * 100;
cout << score;

The variable "correctNumber" is always a value between 0 and 3. However, unless "correctNumber" = 3, then the variable "score" always equals "0". When "correctNumber" equals 3 then "score" equals 100.

Say "correctNumber" was equal to 2. Shouldn't "score" be 67 then? Is this some issue with int variable type being unable to calculate decimal points?

Upvotes: 1

Views: 204

Answers (4)

imreal
imreal

Reputation: 10378

The problem is that (correctNumber / 3) is an integer, so you can't get 0.666 or any fraction to multiply by 100, which what I believe is you want.

You could try to force it to be a float like this:

int score = ((float)correctNumber / 3) * 100;

This, however, will give you 66 instead of 67, cause it doesn't round up. You could use C99 round() for that.

int score = round(((float)correctNumber / 3) * 100);

UPDATE:

You can also use + 0.5 to round, like this:

int score = (correctNumber / 3.0f) * 100.0f + 0.5f;

Upvotes: 1

Qaz
Qaz

Reputation: 61970

I'm assuming correctNumber is an int, based on what you described. What's happening is integer truncation. When you divide an int by an int, the result always rounds down:

1/3 = 0.3333 = 0 as an integer
2/3 = 0.6667 = 0 as an integer
3/3 = 1.0000 = 1 as an integer

The easy way to remedy this here is to multiply it first:

int score = correctNumber * 100 / 3;

However, this still leaves you with 66 for 2, not 67. A clear and simple way of dealing with that (and many other rounding situations, though the rounding style is unconfigurable) is std::round, included since C++11:

int score = std::round(currentNumber * 100. / 3);

In the example, the dot in 100. makes 100 a double (it's the same thing as 100.0), so the result of the operation will be the floating-point value you want, not a pre-truncated value passed in as a floating-point value. That means you'll end up with 66.66666... going into std::round instead of 66.

Upvotes: 4

milleniumbug
milleniumbug

Reputation: 15834

Your guess is correct. int can't store real numbers.

But you can multiply first, and then divide, like

score = correctNumber * 100 / 3;

score will have 0, 33, 66, 100, depending on values of correctNumber

Upvotes: 2

OnaBai
OnaBai

Reputation: 40887

You are doing math as integer so 1 / 3 is 0.

Try:

int score = (100 * correctNumber) / 3

and if you want to round:

int score = (100 * correctNumber + 1) / 3

Upvotes: 7

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