I think I just learned this myself:

Don't put the $ extension in the bracket :

 df1$cola[is.na(df1$cola)]<-
>
> df2$cola
>

Instead substitute using brackets within the brackets:
 df1["cola"]is.na(df1["cola"])]<-
>
> df2["cola"]



> then the "cola" s can be substituted.
>
Maybe this will help


On Wed, Jul 11, 2012 at 10:11 AM, paulalou <pls28@medschl.cam.ac.uk> wrote:

> Hi,
>
> I have two dataframes:
>
> The first, df1, contains some missing data:
>
>    cola colb colc cold cole
> 1    NA    5    9   NA   17
> 2    NA    6   NA   14   NA
> 3     3    NA   11   15   19
> 4     4    8   12   NA   20
>
> The second, df2, contains the following:
>
>   cola colb colc cold cole
> 1  1.4  0.8 0.02  1.6  0.6
>
> I'm wanting all missing data in df1$cola to be replaced by the value of
> df2$cola. Then the missing data in df1$colb to be replaced with the
> corresponding value in df2$colb etc.
>
> I can get this to work column by column with single input lines but as my
> original dataset is a lot larger I'm wanting a create a loop but can't work
> out how.
>
> The single line command is:
>
>  df1$cola[is.na(df1$cola)]<-df2$cola
>
> I've tried a replace function within a loop but get error messages:
>
> list<-colnames(df1)
>
> for (i in list) {
>  r<-replace(df1$i,df1$i[is.na(df1$i)],df2$i)
>  }
>
>
> with error messages of:
>
> Warning messages:
> 1: In is.na(mymat$snp) :
>   is.na() applied to non-(list or vector) of type 'NULL'
>
> Can anyone help me with this?
>
> Thanks
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/Help-with-loop-tp4636140.html
> Sent from the R help mailing list archive at Nabble.com.
>
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>



-- 
Charles Stangor
Professor and Associate Chair

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