programming question! Options

[pebkac]

Ok, so I'm taking Desrosiers (yeah, he sucks) for CS 1412 and I'm working on a programming assignment right now (in C#).

What I basically have to do is create a class that stores a fraction. One of the things it's supposed to do is take a floating point number and convert that to a fraction. What my class basically does is round the floating point number to 5 decimal places, then multiplies it by 100,000. It then uses the resulting integer as the numerator and 100,000 as the denominator and simplifies that.

That works fine for the most part, but what I would also like to be able to do is have it recognize .33333 as 1/3, .66667 as 2/3, 1.33333 as 4/3, etc.

I can take the modulus of the floating point number and see if the remainder is zero, but I think that way would be really prone to rounding errors.

I don't necessarily have to implement that (he said that the method can have limitations as long as we document them), but it would be nice if I could do that. Does anyone have any advice?

[Spectatrix]

If it's got a repeating digit (e.g. 1/3 = 0.333, 2/3 = 0.666, 1/6 = 0.1666), etc:

Call the floating point number x.
Multiply it by 10 to shift the decimal point over one. Call this y.

y-x will get rid of the repeating part of the floating point number. Thus you'll have 9x = some rational fraction. Then you can plug this into your remaining stuff. Rational fraction * 100000 as your numerator, 900000 as your denominator. Simplify.

Got that general algorithm from here: http://www.webmath.com/dec2fract.html Plug in a decimal number to see a sample work-through.

Edit: This is ONLY for irrational numbers with a single repeating digit. If it's a repeating sequence (e.g. 1/11 = 0.090909...) then you'd need to multiply by a power of 10 equal to the number of digits that are repeated. 100x for two digits (like 1/11), 1000x for three digits, etc. Might be more complicated than you want to implement.

This post has been edited by Spectatrix: Feb 1 2008, 03:06 PM

[Jim]

If it's got a repeating digit (e.g. 1/3 = 0.333, 2/3 = 0.666, 1/6 = 0.1666), etc:

Call the floating point number x.
Multiply it by 10 to shift the decimal point over one. Call this y.

y-x will get rid of the repeating part of the floating point number. Thus you'll have 9x = some rational fraction. Then you can plug this into your remaining stuff. Rational fraction * 10000 as your numerator, 90000 as your denominator. Simplify.

Got that general algorithm from here: http://www.webmath.com/dec2fract.html Plug in a decimal number to see a sample work-through.

That program is borked. Put in an irrational number and see what happens. My example was .4657101685

It appears that there is a certain threshold the program can take (which is understandable). However, the programmers should have seen this and then had the output be something different saying that the limit has been reached instead of giving me -(2/-4) as the answer.

[2wolUTT]

That program is borked. Put in an irrational number and see what happens. My example was .4657101685

It appears that there is a certain threshold the program can take (which is understandable). However, the programmers should have seen this and then had the output be something different saying that the limit has been reached instead of giving me -(2/-4) as the answer.

That's what I said.