Why is answer Zero for division?

Question

Within my Activity I am attempting to divide two values then multiply them by 100 in order to give a percentage score.

My issue is that the percentage score is always zero, even though this is impossible with the values I am using.

What am I doing wrong?

Declaring the 2 variables at start of activity:

int score = 0;
int totalQuestions=0;

Onclick logic showing how they are calculated:

public void onClick(View v) {

        if (checkForMatch((Button) v)) {
            //increment no of questions answered (for % score)
            totalQuestions++;
            //increment score by 1
            score++;

        } else {
            //increment no of questions answered (for % score)
            totalQuestions++;

        }

    }


public void writeToDatabase() {
        // create instance of databasehelper class
        DatabaseHelper db = new DatabaseHelper(this);

        int divide = (score/ totalQuestions );

        int percentageScore = (divide * 100);

        Log.d("Pertrace", "per score "+ percentageScore);
        Log.d("divide", "divide "+ divide);


        // Adding the new Session to the database
        db.addScore(new Session(sessionID, "Stroop", SignInActivity
                .getUserName(), averageMedLevel, medMax, averageAttLevel,
                attMax, percentageScore, myDate, "false", fileNameRaw, fileNameEEGPower, fileNameMeditation, fileNameAttention));

        // single score, used for passing to next activity
        single = db.getScore(sessionID);

    }

Note: from my Trace logs i can see that is it the int divide that is zero, why would this be the case considering that score and totalQuestions are always greater than zero? E.g. 20 and 25.

@NiekHaarman I think that's only half the problem, since 20/25 is already 0. — Sean Owen, Aug 26 '14 at 17:36
possible duplicate of [Division of integers in Java](http://stackoverflow.com/questions/7220681/division-of-integers-in-java) — Wooble, Aug 26 '14 at 17:37

ata · Answer 1 · 2014-08-26T17:38:56.533

2

You are saving them in int. Save values in float or double.

Also, when division occurs, the intermediate result is saved in one of the variable that is used in division. If that is an int, it will be truncated before being saved in double. So do something like double divide = (double)score * totalQuestions

edited Aug 26 '14 at 17:38

answered Aug 26 '14 at 17:33

ata

8,853
8
42
68

That's not all of the problem. Even if the variable it's stored in were `float` or `double` the result of the division was already 0. – Sean Owen Aug 26 '14 at 17:37

Rahul Tripathi · Accepted Answer · 2014-08-26T18:13:53.567

2

The reason is this line

int divide = (score/ totalQuestions);

You are dividing the numbers and storing in an int.

You need to store in a double

double divide = (double)score / totalQuestions;

If you want the result as int

double divide = (double)score / totalQuestions;
int percentageScore = (int) Math.ceil(divide * 100);

edited Aug 26 '14 at 18:13

answered Aug 26 '14 at 17:36

Rahul Tripathi

168,305
31
280
331

That still won't work, since the division will have already been carried out as integer division. You get 0.0. – Sean Owen Aug 26 '14 at 17:36
@SeanOwen:- Thanks. Removed the brackets! – Rahul Tripathi Aug 26 '14 at 17:38
Your current solution always returns 100 – Sean Owen Aug 26 '14 at 17:44
@SeanOwen:- I didnt get that. What I missed? – Rahul Tripathi Aug 26 '14 at 17:46
There are still unnecessary brackets in the next-to-last line, and a space before the division operator in two places would be nice. – David Conrad Aug 26 '14 at 18:11

score 1 · Answer 3 · answered Aug 26 '14 at 17:43

1

You are performing integer division. First cast score to a double (so you get floating point math), then I would use Math.round() to round the result of the multiplication. For example,

int score = 3;
int totalQuestions = 4;
double divide = ((double) score / totalQuestions);
int percentageScore = (int) Math.round(divide * 100);
System.out.println(percentageScore);

Output is the expected

answered Aug 26 '14 at 17:43

Elliott Frisch

198,278
20
158
249

Thank you, how could I add implement an algorithm so that now the total questions answered is also considered, I.e. if u answer 75/100 correct that is a better score than 3/4 as it is more reliable due to a larger sample size? – user3968848 Aug 26 '14 at 20:15
@user3968848 Mathematically that's nonsense. Statistically it might be true. – Elliott Frisch Aug 26 '14 at 20:18
Sorry I forgot to add the fact that it should be a higher score as the game is timed and the user was able to answer 70 questions correct within the time limit as opposed to 7 – user3968848 Aug 26 '14 at 20:24

score 0 · Answer 4 · answered Aug 26 '14 at 17:35

0

The operands need to be float or double, and so does the variable you put it in:

double divide = (double) score/ totalQuestions;

answered Aug 26 '14 at 17:35

Sean Owen

66,182
23
141
173

thanks, how can I then round the double answer up to the nearest int? Note: the final answer must be an int! – user3968848 Aug 26 '14 at 17:39
@user3968848:- Check my answer. I have updated that! – Rahul Tripathi Aug 26 '14 at 17:41
After multiplying by 100? Use `Math.ceil()` – Sean Owen Aug 26 '14 at 17:42

Why is answer Zero for division?

4 Answers4