# Generating compound interest with integers

David Herron

Greenhorn

Posts: 12

posted 5 years ago

I am trying to change this code to use only integers to calculate the compound interest.

This is what I have so far,

And here is the output I get,

Why do I get the output after year 2? I assume it has something to do with the remainder.

I also have to format this output with the decimal point, etc.. which I think I will be ok with after I get through this part.

Thanks!

This is what I have so far,

And here is the output I get,

Why do I get the output after year 2? I assume it has something to do with the remainder.

I also have to format this output with the decimal point, etc.. which I think I will be ok with after I get through this part.

Thanks!

Campbell Ritchie

Sheriff

Posts: 50171

79

posted 5 years ago

Are you really taking powers of 105 or similar? Then you get 100000 × 105 × 105 which is 1102500000. Now next time you multiply by 105, you get an overflow error. Meanwhile the divisor is continuing to increase, and when the divisor is larger than the dividend, your integer division gives 0.

I am not sure what to recommend. You could try using

I am not sure what to recommend. You could try using

`long`arithmetic, but that will eventually run out of space. The best way to calculate money is with the BigDecimal class.
David Herron

Greenhorn

Posts: 12

posted 5 years ago

I realize these numbers are huge and I figured that was possibly the issue. I guess I am taking the problem too literally.

After the book shows this program as an example, it goes on to say that this (using double) is not the best method for achieving accurate results, then it states "

Then I am given this problem, so that how I got here!

Thanks! I will go the BigDecimal route!

After the book shows this program as an example, it goes on to say that this (using double) is not the best method for achieving accurate results, then it states "

*you've been warned!*". It never gives a solution apart from a quick reference to the existence of BigDecimal but not how to use it.Then I am given this problem, so that how I got here!

Thanks! I will go the BigDecimal route!

Campbell Ritchie

Sheriff

Posts: 50171

79

posted 5 years ago

So far, so good. You can get 1 from 0.00625 if you repeatedly multiply by 2 or 5, so the division will eventually come out exactly. But what if the division isn't exact?

Another thing is, BigDecimal is an immutable class. You will see the values of number1 and number2 do not change in the arithmetic.

Its equals() method includes the scale as well as the value, so 1.0 and 1.00 are not the same, but new BigDecimal("1.0").compareTo(new BigDecimal("1.000") will return 0, so you can use the compareTo method.

Don't use the constructor which takes a

Read the BigDecimal and RoundingMode and MathContext documentation.

Note the %f tag defaults to 6 decimal places.java BigDecimalDemo

No command-line arguments: enter two numbers eg 1.23 45.678:

39856398.767624534 0.00625

39856398.767625 + 0.006250 = 39856398.773875

39856398.767625 - 0.006250 = 39856398.761375

39856398.767625 × 0.006250 = 249102.492298

39856398.767625 ÷ 0.006250 = 6377023802.819925

39856398.767625 % 0.006250 = 0.005125

So far, so good. You can get 1 from 0.00625 if you repeatedly multiply by 2 or 5, so the division will eventually come out exactly. But what if the division isn't exact?

We now need a version 2.Apart from the imports, the change is on lines 27-28. You supply a rounding mode and a precision. In this case 4 appears to mean 4 significant figures, not four places after the decimal point. If you try to find the divide() method, you find it is overloaded about 6 times to allow for that problem. Any overloading can be used, but they may give slightly different results.java BigDecimalDemo 9.0 0.7

Exception in thread "main" java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

at java.math.BigDecimal.divide(BigDecimal.java:1616)

at BigDecimalDemo.ShowOperations(BigDecimalDemo.java:25)

at BigDecimalDemo.main(BigDecimalDemo.java:41)

Another thing is, BigDecimal is an immutable class. You will see the values of number1 and number2 do not change in the arithmetic.

Its equals() method includes the scale as well as the value, so 1.0 and 1.00 are not the same, but new BigDecimal("1.0").compareTo(new BigDecimal("1.000") will return 0, so you can use the compareTo method.

Don't use the constructor which takes a

`double`, but use one of the other constructors. You can see if you try System.out.println(new BigDecimal(1.23)); and with "1.23", and see the differences.

Read the BigDecimal and RoundingMode and MathContext documentation.

Campbell Ritchie

Sheriff

Posts: 50171

79

David Herron

Greenhorn

Posts: 12

posted 5 years ago

Thank you for this. More than thorough. I now have this code to reference. Upon further reading, I realized the assignment is not asking us to implement BigDecimal. But since this is the proper way to do things i will be going over it as well as the RoundingMode and MathContext documentation.

Thank you again!

Thank you again!

Campbell Ritchie

Sheriff

Posts: 50171

79

posted 4 years ago

So, try this:. . . as an alternative to line 27 above. That seems to give 4 places after the decimal point.Last year, I wrote: . . . the divide() method . . . is overloaded about 6 times to allow for that problem. Any overloading can be used, but they may give slightly different results. . . .

Campbell Ritchie

Sheriff

Posts: 50171

79

posted 2 years ago

In the first post in this thread, the image has vanished. I think it should be a printout like thisThat is another reason for not using screenshots: text posted doesn't mysteriously vanish.

It is sorta covered in the JavaRanch Style Guide. |