How does Ruby do calculations?

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

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

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

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

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

PEMDAS

Parentheses, Exponents, Multiplication, Division, Addition, Subtraction

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

Excellent tutorial and very well explained!

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