CODEWITHSUNDEEP
objective-c
NCFUSN
NCFUSN

Reputation: 1644

Objective-C: Strange calculation result

I am learning Objective-C and have completed a simple program and got an unexpected result. This program is just a multiplication table test... User inputs the number of iterations(test questions), then inputs answers. That after program displays the number of right and wrong answers, percentage and accepted/failed result.

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
NSLog(@"Welcome to multiplication table test");
int rightAnswers; //the sum of the right answers
int wrongAnswers; //the sum of wrong answers
int combinations; //the number of combinations@


NSLog(@"Please, input the number of test combinations");
scanf("%d",&combinations);

for(int i=0; i<combinations; ++i)
{
    int firstInt=rand()%8+1;
    int secondInt=rand()%8+1;
    int result=firstInt*secondInt;
    int answer;


    NSLog(@"%d*%d=",firstInt,secondInt);
    scanf("%d",&answer);
    if(answer==result)
    {
        NSLog(@"Ok");
        rightAnswers++;
    }
    else
    {
        NSLog(@"Error");
        wrongAnswers++;
    }
}
int percent=(100/combinations)*rightAnswers;
NSLog(@"Combinations passed: %d",combinations);
NSLog(@"Answered right: %d times",rightAnswers);
NSLog(@"Answered wrong: %d times",wrongAnswers);
NSLog(@"Completed %d percent",percent);
if(percent>=70)NSLog(@"accepted");
else
    NSLog(@"failed");
[pool drain];
return 0;

}

Problem (strange result)

When I input 3 iterations and answer 'em right, i am not getting of 100% right. Getting only 99%. The same count I tried on my iPhone calculator.

100 / 3 = 33.3333333... percentage for one right answer (program displays 33%. The digits after mantissa getting cut off)

33.3333333... * 3=100%

Can someone explain me where I went wrong? Thanx.

Upvotes: 0

Views: 987

Answers (4)

ughoavgfhw
ughoavgfhw

Reputation: 39905

This is a result of integer division. When you perform division between two integer types, the result is automatically rounded towards 0 to form an integer. So, integer division of (100 / 3) gives a result of 33, not 33.33.... When you multiply that by 3, you get 99. To fix this, you can force floating point division by changing 100 to 100.0. The .0 tells the compiler that it should use a floating point type instead of an integer, forcing floating point division. As a result, rounding will not occur after the division. However, 33.33... cannot be represented exactly by binary numbers. Because of this, you may still see incorrect results at times. Since you store the result as an integer, rounding down will still occur after the multiplication, which will make it more obvious. If you want to use an integer type, you should use the round function on the result:

int percent = round((100.0 / combinations) * rightAnswers);

This will cause the number to be rounded to the closest integer before converting it to an integer type. Alternately, you could use a floating point storage type and specify a certain number of decimal places to display:

float percent = (100.0 / combinations) * rightAnswers;
NSLog(@"Completed %.1f percent",percent); // Display result with 1 decimal place

Finally, since floating point math will still cause rounding for numbers that can't be represented in binary, I would suggest multiplying by rightAnswers before dividing by combinations. This will increase the chances that the result is representable. For example, 100/3=33.33... is not representable and will be rounded. If you multiply by 3 first, you get 300/3=100, which is representable and will not be rounded.

Upvotes: 3

Jesus Ramos
Jesus Ramos

Reputation: 23268

You might want to implement some sort of rounding because 33.333....*3 = 99.99999%. 3/10 is an infinite decimal therefore you need some sort of rounding to occur (maybe at the 3rd decimal place) so that the answer comes out correct. I would say if (num*1000 % 10 >= 5) num += .01 or something along those lines multiply by 100 moves decimal 3 times and then mod returns the 3rd digit (could be zero). You also might only want to round at the end once you sum everything up to avoid errors. EDIT: Didn't realize you were using integers numbers at the end threw me off, you might want to use double or float (floats are slightly inaccurate past 2 or 3 digits which is OK with what you want).

Upvotes: 2

Eiko
Eiko

Reputation: 25632

100/3 is 33. Integer mathematics here.

Upvotes: 0

Chuck
Chuck

Reputation: 237030

Ints are integers. They can't represent an arbitrary real number like 1/3. Even floating-point numbers, which can represent reals, won't have enough precision to represent an infinitely repeating decimal like 100/3. You'll either need to use an arbitrary-precision library, use a library that includes rationals as a data type, or just store as much precision as you need and round from there (e.g. make your integer unit hundredths-of-a-percent instead of a single percentage point).

Upvotes: 3

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