[R] if else statement for rain data to define zero for dry and one to wet

William Dunlap wdunlap at tibco.com
Sat Jun 6 23:48:13 CEST 2015


Your f1() has an unneeded for loop in it.
   f1a <- function(mat) mat > 0.1, 1, 0)
would do the same thing in a bit less time.

However, I think that a simple
   mat > 0.1
would be preferable.  The resulting TRUEs and FALSEs
are easier to interpret than the 1s and 0s that f1a()
produces and arithmetic functions treat them TRUE
as 1 and FALSE as 0 internally.  E.g., mean(mat>0.1)
gives the proportion of wet(tish) days.



Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Sat, Jun 6, 2015 at 1:55 PM, Dennis Murphy <djmuser at gmail.com> wrote:

> I'm sorry, but I have to take issue with this particular use case of
> ifelse(). When the goal is to generate a logical vector, ifelse() is
> very inefficient. It's better to apply a logical condition directly to
> the object in question and multiply the result by 1 to make it
> numeric/integer rather than logical.
>
> To illustrate this, consider the following toy example. The function
> f1 replicates the suggestion to apply ifelse() columnwise (with the
> additional overhead of preallocating storage for the result), whereas
> the function f2 applies the logical condition on the matrix itself
> using vectorization, with the recognition that a matrix is an atomic
> vector with a dim attribute.
>
> set.seed(5290)
>
> # 1000 x 1000 matrix
> m <- matrix(sample(c(0, 0.05, 0.2), 1e6, replace = TRUE), ncol = 1000)
>
> f1 <- function(mat)
>   {
>      newmat <- matrix(NA, ncol = ncol(mat), nrow = nrow(mat))
>      for(i in seq_len(ncol(mat)))
>          newmat[, i] <- ifelse(mat[, i] > 0.1, 1, 0)
>      newmat
>   }
>
> f2 <- function(mat) 1 * (mat > 0.1)
>
>
> On my system, I got
>
> > system.time(m1 <- f1(m))
>    user  system elapsed
>    0.14    0.00    0.14
>
> > system.time(m2 <- f2(m))
>    user  system elapsed
>    0.01    0.00    0.01
>
> > identical(m1, m2)
> [1] TRUE
>
> The all too common practice of using  ifelse(condition, 1, 0) on an
> atomic vector is easily replaced by 1 * (condition), where the result
> of condition is a logical atomic object coerced to numeric.
>
> To reduce memory, one should better define f2 as
>
> f2 <- function(mat) 1L * (mat > 0.1)
>
> but doing so in this example no longer creates identical objects since
>
> > typeof(m1)
> [1] "double"
>
> Thus, f1 is not only inefficient in terms of execution time, it's also
> inefficient in terms of storage.
>
> Given several recent warnings in this forum about the inefficiency of
> ifelse() and the dozens of times I've seen the idiom implemented in f1
> as a solution over the last several years (to which I have likely
> contributed in my distant past as an R-helper), I felt compelled to
> say something about this practice, which BTW extends not just to 0/1
> return values but to
> 0/x return values, where x is a nonzero real number.
>
> Dennis
>
>
> On Sat, Jun 6, 2015 at 12:50 AM, Jim Lemon <drjimlemon at gmail.com> wrote:
> > Hi rosalinazairimah,
> > I think the problem is that you are using "if" instead of "ifelse". Try
> this:
> >
> > wet_dry<-function(x,thresh=0.1) {
> >  for(column in 1:dim(x)[2]) x[,column]<-ifelse(x[,column]>=thresh,1,0)
> >  return(x)
> > }
> > wet_dry(dt)
> >
> > and see what you get.
> >
> > Also, why can I read your message perfectly while everybody else can't?
> >
> > Jim
> >
> >>> -----Original Message-----
> >>> From: roslinaump at gmail.com
> >>> Sent: Fri, 5 Jun 2015 16:49:08 +0800
> >>> To: r-help at r-project.org
> >>> Subject: [R] if else statement for rain data to define zero for dry and
> >>> one to wet
> >>>
> >>> Dear r-users,
> >>>
> >>> I have a set of rain data:
> >>>
> >>> X1950 X1951 X1952 X1953 X1954 X1955 X1956 X1957 X1958 X1959 X1960 X1961
> >>> X1962
> >>>
> >>> 1   0.0   0.0  14.3   0.0  13.5  13.2   4.0     0   3.3     0     0
>  0.0
> >>>
> >>>
> >>> 2   0.0   0.0  21.9   0.0  10.9   6.6   2.1     0   0.0     0     0
>  0.0
> >>>
> >>>
> >>> 3  25.3   6.7  18.6   0.8   2.3   0.0   8.0     0   0.0     0     0
> 11.0
> >>>
> >>>
> >>> 4  12.7   3.4  37.2   0.9   8.4   0.0   5.8     0   0.0     0     0
>  5.5
> >>>
> >>>
> >>> 5   0.0   0.0  58.3   3.6  21.1   4.2   3.0     0   0.0     0     0
> 15.9
> >>>
> >>>
> >>> I would like to go through each column and define each cell with value
> >>> greater than 0.1 mm will be 1 and else zero. Hence I would like to
> attach
> >>> the rain data and the category side by side:
> >>>
> >>>
> >>> 1950   state
> >>>
> >>> 1 0.0    0
> >>>
> >>> 2 0.0    0
> >>>
> >>> 3 25.3   1
> >>>
> >>> 4 12.7   1
> >>>
> >>> 5 0.0    0
> >>>
> >>>
> >>> ...
> >>>
> >>>
> >>> This is my code:
> >>>
> >>>
> >>> wet_dry  <- function(dt)
> >>>
> >>> { cl   <- length(dt)
> >>>
> >>>   tresh  <- 0.1
> >>>
> >>>
> >>>   for (i in 1:cl)
> >>>
> >>>   {  xi <- dt[,i]
> >>>
> >>>      if (xi < tresh ) 0 else 1
> >>>
> >>>   }
> >>>
> >>> dd <- cbind(dt,xi)
> >>>
> >>> dd
> >>>
> >>> }
> >>>
> >>>
> >>> wet_dry(dt)
> >>>
> >>>
> >>> Results:
> >>>
> >>>> wet_dry(dt)
> >>>
> >>>    X1950 X1951 X1952 X1953 X1954 X1955 X1956 X1957 X1958 X1959 X1960
> >>> X1961
> >>> X1962 X1963 X1964 X1965 X1966 X1967 X1968 X1969 X1970 X1971 X1972 X1973
> >>> X1974 X1975 X1976 X1977
> >>>
> >>> 1    0.0   0.0  14.3   0.0  13.5  13.2   4.0   0.0   3.3   0.0   0.0
> >>> 0.0
> >>>   4.2   0.0   2.2   0.0   4.4   5.1     0   7.2   0.0   0.0   0.0   5.1
> >>> 0   0.0     0   0.3
> >>>
> >>> 2    0.0   0.0  21.9   0.0  10.9   6.6   2.1   0.0   0.0   0.0   0.0
> >>> 0.0
> >>>   8.4   0.0   4.0   0.0   4.9   0.7     0   0.0   0.0   0.0   0.0   5.4
> >>> 0   3.3     0   0.3
> >>>
> >>> 3   25.3   6.7  18.6   0.8   2.3   0.0   8.0   0.0   0.0   0.0   0.0
> >>> 11.0
> >>>   4.2   0.0   2.0   0.0  14.2  17.1     0   0.0   0.0   0.0   0.0   2.1
> >>> 0   1.7     0   4.4
> >>>
> >>> 4   12.7   3.4  37.2   0.9   8.4   0.0   5.8   0.0   0.0   0.0   0.0
> >>> 5.5
> >>>   0.0   0.0   5.4   0.0   6.4  14.9     0  10.1   2.9 143.4   0.0   6.1
> >>> 0   0.0     0  33.5
> >>>
> >>>
> >>> It does not work and give me the original data.  Why is that?
> >>>
> >>>
> >>> Thank you so much for your help.
> >>>
> >>>       [[alternative HTML version deleted]]
> >>>
> >>> ______________________________________________
> >>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >>> https://stat.ethz.ch/mailman/listinfo/r-help
> >>> PLEASE do read the posting guide
> >>> http://www.R-project.org/posting-guide.html
> >>> and provide commented, minimal, self-contained, reproducible code.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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