How does Ruby do calculations?

Ruby
1

Could anybody please give me an answer to the following?

puts 98 + (59872 / (13*8)) * -51

PEMDAS

Parentheses, Exponents, Multiplication, Division, Addition, Subtraction

Multiplication and Division,Addition and Subtraction done in the order
they
occur in the problem.

The answer from Ruby for the above calculation is -29227

If you calculate the same problem in Microsoft Excel and a Scientific
calculator
the answer is -29262

How is this possible and why is there a difference in the answer?

Thanks
Stefan

2

Subject: How does Ruby do calculations?
Date: lun 12 ago 13 01:46:10 +0200

Quoting Stefan du Plooy ([email protected]):

occur in the problem.

The answer from Ruby for the above calculation is -29227

If you calculate the same problem in Microsoft Excel and a Scientific
calculator
the answer is -29262

How is this possible and why is there a difference in the answer?

If you just add a .0 in the right place…

puts 98 + (59872 / (13*8.0)) * -51

-29262.307692307695

In your line, all numbers were integers, so integer arithmetics was
used:

98 + (59872 / 104) * -51
98 + 575 * -51
98 + (-29325)

-29227

Carlo

3

How is this possible and why is there a difference in the answer?

You’re doing integer math, which can be weird, since it truncates the
result to integer values. In your example, the truncation error is
multiplied, yielding a large difference in value. Try this:

98 + (59872 / (13.0*8)) * -51

Just making one of the numbers a float will trigger floating point
evaluation. If you want even more precision, use the ‘bigdecimal’
library.

kaspar

4

Thank you, your answer totally cleared up the misunderstanding.

Kind regards
Stefan

If you just add a .0 in the right place…

puts 98 + (59872 / (13*8.0)) * -51

-29262.307692307695

In your line, all numbers were integers, so integer arithmetics was
used:

98 + (59872 / 104) * -51
98 + 575 * -51
98 + (-29325)

-29227

Carlo

5

On Tue, Aug 13, 2013 at 8:25 AM, Kaspar S.
[email protected]wrote:

evaluation. If you want even more precision, use the ‘bigdecimal’ library.

kaspar

You have to make sure they’re a float at each calculation e.g:
1/2 + 1/2.0 # => 0.5

The first 1/2 is still doing integer division, so it should be:
1/2.0 + 1/2.0 # => 1.0

Also, if you’re going to use floats, make sure you’re familiar with
their
imprecision:
7.01 - 7.00 # => 0.009999999999999787

-Josh

6

PEMDAS

Parentheses, Exponents, Multiplication, Division, Addition, Subtraction

Multiplication and Division,Addition and Subtraction done in the order
they
occur in the problem.

7

Excellent tutorial and very well explained!