# Math.round() question

Jonathan Oblea

Greenhorn

Posts: 20

posted 12 years ago

Hello All,

I have read the round() method from K&B book and i'm a little confused

on the statement below:

"If the number after the decimal point is less than 0.5 then

Math.round() is equal to Math.floor(). If the number after the

decimal point is greater than equal 0.5 then Math.round() is

equal to Math.ceil()".

Maybe I misunderstood the statement above. Can someone please give me

guidance on this topic?

Example:

1.) Math.round(-5.4) returns -5

Math.floor(-5.4) returns -6

Note: The number after the decimal is less than 0.5 which means

Math.round() should be equal to Math.floor() but it the

result doesn't match?

2.) Math.round(-5.6) returns -6

Math.ceil (-5.6) returns -5

Note: The number after the decimal is greater than equal to 0.5

which means Math.round() should be equal to Math.ceil() but

the result doesn't match?

Thank you in advance,

Jonathan

I have read the round() method from K&B book and i'm a little confused

on the statement below:

"If the number after the decimal point is less than 0.5 then

Math.round() is equal to Math.floor(). If the number after the

decimal point is greater than equal 0.5 then Math.round() is

equal to Math.ceil()".

Maybe I misunderstood the statement above. Can someone please give me

guidance on this topic?

Example:

1.) Math.round(-5.4) returns -5

Math.floor(-5.4) returns -6

Note: The number after the decimal is less than 0.5 which means

Math.round() should be equal to Math.floor() but it the

result doesn't match?

2.) Math.round(-5.6) returns -6

Math.ceil (-5.6) returns -5

Note: The number after the decimal is greater than equal to 0.5

which means Math.round() should be equal to Math.ceil() but

the result doesn't match?

Thank you in advance,

Jonathan

Dirk Schreckmann

Sheriff

Posts: 7023

posted 12 years ago

Jonathan O.,

Welcome back, to JavaRanch!

We ain't got many rules 'round these parts, but we do got one. Please change your display name to comply with The JavaRanch Naming Policy. We'd like it if your last name contained more than just one letter.

Thanks Pardner! Hope to see you 'round the Ranch!

Welcome back, to JavaRanch!

We ain't got many rules 'round these parts, but we do got one. Please change your display name to comply with The JavaRanch Naming Policy. We'd like it if your last name contained more than just one letter.

Thanks Pardner! Hope to see you 'round the Ranch!

Dirk Schreckmann

Sheriff

Posts: 7023

posted 12 years ago

It looks like Kathy and Bert weren't thinking about negative numbers for that explanation of how Math.round() works.

I prefer how the Math class documentation describes the behavior of Math.round().

[ January 07, 2004: Message edited by: Dirk Schreckmann ]

I prefer how the Math class documentation describes the behavior of Math.round().

If it's not already reported, and it doesn't look as if it has been, you might do us all a favor and email Kathy and/or Bert about this error.Returns the closest int to the argument. The result is rounded to an integer by adding 1/2, taking the floor of the result, and casting the result to type int. In other words, the result is equal to the value of the expression:

(int)Math.floor(a + 0.5f)

[ January 07, 2004: Message edited by: Dirk Schreckmann ]

Dmitry Melnik

Ranch Hand

Posts: 328

posted 12 years ago

public static long round(double a)

Returns the closest long to the argument. The result is rounded to an integer by adding 1/2, taking the floor of the result, and casting the result to type long. In other words, the result is equal to the value of the expression:

(long)Math.floor(a + 0.5d)

Which means that the statement from the book you have cited is correct for positive numbers only (and it's not correct for negatives)

Returns the closest long to the argument. The result is rounded to an integer by adding 1/2, taking the floor of the result, and casting the result to type long. In other words, the result is equal to the value of the expression:

(long)Math.floor(a + 0.5d)

Which means that the statement from the book you have cited is correct for positive numbers only (and it's not correct for negatives)

Sriram Chintapalli

Ranch Hand

Posts: 59

posted 12 years ago

Hi Jonathan,

Kathy and Bates did not include negative floats or doubles for explanation of the Math.round() method I had to try and figure it out, perhaps they thought it was too obvious? but they did point out the all interesting -0.5 behaviour:

int i=Math.round(-10.5f); //gives -10

int j=Math.round(10.5f); //gives 11

both are ceil()'ed.

This is my first post so coudnt resist pointing out the 0.5 behavior

Kathy and Bates did not include negative floats or doubles for explanation of the Math.round() method I had to try and figure it out, perhaps they thought it was too obvious? but they did point out the all interesting -0.5 behaviour:

int i=Math.round(-10.5f); //gives -10

int j=Math.round(10.5f); //gives 11

both are ceil()'ed.

This is my first post so coudnt resist pointing out the 0.5 behavior

Dan Gun

Greenhorn

Posts: 26

Jonathan Oblea

Greenhorn

Posts: 20

posted 12 years ago

Hello All, Thank you for your replies.

Can I assume for Math.round() the following?

For positive numbers the following can be applied:

1. less than 0.5 = floor()

2. greater than equal to 0.5 = ceil()

For negative numbers the following can be applied:

1. greater than 0.5 = floor()

2. less than equal to 0.5 = ceil()

Thanks again,

Jonathan

Can I assume for Math.round() the following?

For positive numbers the following can be applied:

1. less than 0.5 = floor()

2. greater than equal to 0.5 = ceil()

For negative numbers the following can be applied:

1. greater than 0.5 = floor()

2. less than equal to 0.5 = ceil()

Thanks again,

Jonathan

Sudhir Meduri

Greenhorn

Posts: 22

posted 12 years ago

I think after looking the following result

Math.round(5.4) :5

Math.round(5.5) :6

Math.round(5.9) :6

Math.round(-5.4) :-5

Math.round(-5.5) :-5

Math.round(-5.9) :-6

I will simply follow the thumb rule

Add +0.5 (no matter pos or neg ) and floor it.

Pls correct me if I am wrong.

Math.round(5.4) :5

Math.round(5.5) :6

Math.round(5.9) :6

Math.round(-5.4) :-5

Math.round(-5.5) :-5

Math.round(-5.9) :-6

I will simply follow the thumb rule

Add +0.5 (no matter pos or neg ) and floor it.

Pls correct me if I am wrong.

Thanks,<br />Sudhir <br />SCJP1.4

Sudhir Meduri

Greenhorn

Posts: 22

posted 12 years ago

This makes the former math teacher in me cringe. because, since we're talking about negative numbers, "greater than 0.5" is confusing. -0.4 is greater than -0.5.

I think what you mean is "larger magnitude" than -0.5

This is why it's probably always better to use the "add 0.5 and take the floor" rule. it's less confusing.

For negative numbers the following can be applied:

1. greater than 0.5 = floor()

2. less than equal to 0.5 = ceil()

This makes the former math teacher in me cringe. because, since we're talking about negative numbers, "greater than 0.5" is confusing. -0.4 is greater than -0.5.

I think what you mean is "larger magnitude" than -0.5

This is why it's probably always better to use the "add 0.5 and take the floor" rule. it's less confusing.

There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors