[R] Merge and replace data

Eric Berger er|cjberger @end|ng |rom gm@||@com
Tue Sep 5 13:59:30 CEST 2023

As Duncan points out, ifelse() provides a more general approach than
the specific pmax().

Even more generally, you might want to consider the apply() function
(and its relatives sapply(), lapply(), ...)

For example

apply(cbind(x1,x2), MAR=1, max)

In the above statement, x1 and x2 are combined into two columns, MAR=1
says apply the following function to each row,
and max specifies the function to be applied. The summary is that this
statement applies the max to each row in the object
specified by the first argument. It's very general because you can
replace max by an arbitrary function.

Similarly, if you get more into R you might modify the above using
chained operations. The following rewrites the above
by piping cbind(x1,x2) into the apply command. Not a necessary feature
to know but eventually will assist you in writing code that does
not require a lot of intermediate variables.

cbind(x1,x2) |> apply(MAR=1,max)

On Tue, Sep 5, 2023 at 2:45 PM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
> On 05/09/2023 4:55 a.m., roslinazairimah zakaria wrote:
> > Hi all,
> >
> > I have these data
> >
> > x1 <- c(116,0,115,137,127,0,0)
> > x2 <- c(0,159,0,0,0,159,127)
> >
> > I want : xx <- c(116,115,137,127,159, 127)
> >
> > I would like to merge these data into one column. Whenever the data is '0'
> > it will be replaced by the value in the column which is non zero..
> > I tried append and merge but fail to get what I want.
> >
> Others have pointed out pmax(x1, x2).  For the sample data, even x1+x2
> would give the same answer.
> The real question is what you want to do if both x1 and x2 have non-zero
> entries, or negative entries:
>    pmax(x1, x2)
> will pick the larger one.  It might be zero if the other is negative,
> and that doesn't match your description.
>    x1+x2
> will just give the sum, which doesn't match your description if both are
> non-zero.
> But maybe you want the x1 value unless it is zero, even if it is smaller
> than the x2 value:  then you would use
>    ifelse(x1 != 0, x1, x2)
> Similarly
>    ifelse(x2 != 0, x2, x1)
> would prefer the x2 value.
> You should think about what you would do in these other cases as well.
> Duncan Murdoch
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