[R] customize the step value

[Rui Barradas]

Hello,

Às 14:07 de 29/10/21, Duncan Murdoch escreveu:
> On 29/10/2021 4:34 a.m., PIKAL Petr wrote:
>> Hi
>>
>> One has to be careful when using fractions in seq step.
>>
>> Although it works for 0.5
>>> (seq(0,10, .5) - round(seq(0,10,.5),2))==0
>>   [1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE 
>> TRUE
>> TRUE
>> [16] TRUE TRUE TRUE TRUE TRUE TRUE
>>
>> in case of 0.3 (or others) it does not always result in expected 
>> values (see
>> FAQ 7.31 for explanation)
>>
>>> (seq(0,10, .3) - round(seq(0,10,.3),2))==0
>>   [1]  TRUE  TRUE  TRUE FALSE  TRUE  TRUE FALSE  TRUE  TRUE FALSE  
>> TRUE  TRUE
>> [13] FALSE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE  TRUE  TRUE  TRUE 
>> FALSE
>> [25] FALSE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE  TRUE
> 
> 
> Petr is right, it's unsafe to use fractional values for the step.  0.5 
> works because it has a power of 2 in the denominator and so does the 
> start value, but it's easy to make mistakes when you rely on that (e.g. 
> changing the step size from 0.5 to 0.3 would break things).
> 
> A better idea is to modify a sequence of integers.  For example, to get 
> 1.5 to 3.5 by 0.5, you can do (3:7)*0.5, and for 0 to 3 by 0.3, use 
> (0:10)*0.3.

But even this is not safe.
Perhaps the most frequent sequence causing problems (R-Help questions) 
is the one from 0 to 1 by 0.1 and why it always returns FALSE when 
tested against certain values such as 0.6.
And it also fails to create it with (0:10)*0.1.


x <- seq(0, 1, by = 0.1)
any(x == 0.6)
#[1] FALSE

y <- (0:10)*0.1
any(y == 0.6)
#[1] FALSE

all(x == y)
#[1] TRUE


Off-topic: FAQ 7.31 addresses this.

Hope this helps,

Rui Barradas

> 
> Duncan Murdoch
> 
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