## Rational Numbers

### January 25, 2011

A rational number, or fraction, is the ratio of two integers; for instance, 4/7 is the ratio of 4 (the numerator) divided by 7 (the denominator). No rational number may have a denominator of zero. Rational numbers respond to all the same operations as integers, including addition, subtraction, multiplication and division, and they may also be compared by the less-than operator.

Your task is to write functions that perform arithmetic on rational numbers. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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[…] today’s Programming Praxis exercise, our goal is to implement common operations on rational numbers. […]

I believe your multiplication function is wrong. 1/3 * -1/7 should produce -1/21, not -3/7, shouldn’t it?

My Haskell solution (see http://bonsaicode.wordpress.com/2011/01/25/programming-praxis-rational-numbers/ for a version with comments):

Fixed. I must have been asleep when I did that.

Knuth, volume 3, Seminumerical Algorithms, has some simple algorithms for rational arithmetic that computes GCDs on the smallest possible numbers. It complicates matters a bit, but it makes a big difference in some problems; you can find the code in _num.scm in the Gambit source code. (Search for (define-prim (##ratnum.= x y) …)

For example, with your code using Gambit’s bignum operations I get for

(define one (cons 1 1))

(define number-of-loops 1000)

(time

(do ((i 0 (+ i 1))

(r one (plus one (divide one r))))

((= i 1000) (display r)(newline))))

(time

(do ((i 0 (+ i 1))

(r 1 (+ 1 (/ 1 r))))

((= i 1000) (display r)(newline))))

gsi rational

(113796925398360272257523782552224175572745930353730513145086634176691092536145985470146129334641866902783673042322088625863396052888690096969577173696370562180400527049497109023054114771394568040040412172632376 . 70330367711422815821835254877183549770181269836358732742604905087154537118196933579742249494562611733487750449241765991088186363265450223647106012053374121273867339111198139373125598767690091902245245323403501)

(time (do ((i 0 (+ i 1)) (r one (plus one (divide one r)))) ((= i 1000) (display r) (newline))))

217 ms real time

210 ms cpu time (210 user, 0 system)

390 collections accounting for 58 ms real time (50 user, 0 system)

108458544 bytes allocated

61 minor faults

no major faults

113796925398360272257523782552224175572745930353730513145086634176691092536145985470146129334641866902783673042322088625863396052888690096969577173696370562180400527049497109023054114771394568040040412172632376/70330367711422815821835254877183549770181269836358732742604905087154537118196933579742249494562611733487750449241765991088186363265450223647106012053374121273867339111198139373125598767690091902245245323403501

(time (do ((i 0 (+ i 1)) (r 1 (+ 1 (/ 1 r)))) ((= i 1000) (display r) (newline))))

1 ms real time

10 ms cpu time (0 user, 10 system)

1 collection accounting for 0 ms real time (0 user, 0 system)

411264 bytes allocated

1 minor fault

no major faults

Seems reminiscent of SICP :-)

Since I implemented fractions as pairs, I included two procedures to get the

numerator and denominator. I also included pretty printing of fractions. My

Python solution:

Oops! That was an earlier version. The constructor should be:

A Ruby example

Thanks for the refresher in how to find the GCD, it’s been a long time since I had to do that….

In J, rational numbers are built-in, so this is really a demonstration of J’s extended integer and rational number arithmetic.

In J, a rational number is written as ‘NrD’, where ‘N’ and ‘D’ are integers.

If D is one, the ‘r1′ will be omitted. If D is zero, infinity is represented as “_”.

Negative numbers are indicated with a ‘_’ prefix, eg: ‘_2′ is negative 2.

Now that we now how to read and make rational numbers, let’s do some rational arithmetic in J (a very common thing to do in carpentry).

#include

typedef struct rational

{

int n;

int d;

} rational;

rational normalize(rational r)

{

int a = gcd(r.n, r.d);

r.n/=a;

r.d/=a;

return r;

}

rational add(rational r1, rational r2)

{

rational sum;

sum.d=(r1.d)*(r2.d);

sum.n=(r1.n)*(r2.d)+(r2.n)*(r1.d);

return normalize(sum);

}

rational subtract(rational r1, rational r2)

{

rational sum;

sum.d=(r1.d)*(r2.d);

sum.n=(r1.n)*(r2.d)-(r2.n)*(r1.d);

return normalize(sum);

}

rational multiply(rational r1, rational r2)

{

rational sum;

sum.d=(r1.d)*(r2.d);

sum.n=(r1.n)*(r2.n);

return normalize(sum);

}

rational divide(rational r1, rational r2)

{

rational sum;

sum.d=(r1.d)*(r2.n);

sum.n=(r1.n)*(r2.d);

return normalize(sum);

}

int l_t(rational r1, rational r2)

{

int lcm=(r1.*r2.d)/gcd(r1.d*r2.d);

if(r1.n*(lcm/r1.d)b){

a=a+b;

b=a-b;

a=a-b;

}

if(a==0) return b;

return gcd(b%a, a);

}

void print_rational(rational r)

{

printf(“%d/%d”, r.n, r.d);

}

void read_rational(rational *r)

{

scanf(“%d/%d”, &r->n, &r->d);

}

int main()

{

rational r1, r2;

read_rational(&r1);

read_rational(&r2);

print_rational(divide(r1, r2));

return 0;

}