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I have the following data.frame and I want to perform some calculations on the 2nd column. 

> test  
  code age
1  101  15
2  102  25
3  103  16
4  104  u1
5  105  u1
6  106  u2
7  107  27
8  108  27

As you can see, the 2nd column does not include only numbers. I omitted these cases: 

> new<-subset(test,code<104 | code>106)
> new
  code age
1  101  15
2  102  25
3  103  16
7  107  27
8  108  27

But when I try to do a calculation in a new column this is what I get: 

> new["MY_NEW_COLUMN"] <- NA
> new
  code age MY_NEW_COLUMN
1  101  15            NA
2  102  25            NA
3  103  16            NA
7  107  27            NA
8  108  27            NA
> new$MY_NEW_COLUMN <-new[,2] * 5
Warning message:
In Ops.factor(new[, 2], 5) : * not meaningful for factors

Why does that happen? Any suggestions?

mboon
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1 Answers1

32

new[,2] is a factor, not a numeric vector. Transform it first 

new$MY_NEW_COLUMN <-as.numeric(as.character(new[,2])) * 5
scoa
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  • You should remind people to read the R-FAQ where this and many other bits of information are distributed with every copy of R. – IRTFM Nov 14 '16 at 05:40
  • why not directly as.numeric(new[,2]) ? i want to know the difference – MRTgang May 31 '18 at 06:10
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    see `?as.numeric()`: "If ‘x’ is a ‘factor’, ‘as.numeric’ will return the underlying numeric (integer) representation, which is often meaningless as it may not correspond to the ‘factor’ ‘levels’.". – scoa May 31 '18 at 07:49