R isTRUE logic incoherence - r

Question

I am tying to code a new variable based on ifs and elses using isTRUE and am having difficulty. I would like to have a condition such as
if (isTRUE(t$a > t$b)) {
t$c <- 0
} else if (isTRUE(t$a < t$b)) {
t$c <- 1
} else {
t$c <- 2
}
Consider the following data:
t<-as.data.frame(c(1:5))
names(t)<-"a"
t$b<-c(5:1)
Running the above code gives c values as always being 2 i.e. isTRUE(t$a > t$b) and isTRUE(t$a < t$b) are always FALSE.

Answer 1

Read the help for isTRUE:
‘isTRUE(x)’ is an abbreviation of ‘identical(TRUE, x)’, and so is
true if and only if ‘x’ is a length-one logical vector whose only
element is ‘TRUE’ and which has no attributes (not even names).
This is probably not what you want.
I'm guessing that you want a vector, t$c that is 0 if t$a>t$b, 1 if t$a<t$b and 2 otherwise. In R, we can do that in a single vectorised operation:
Easier setup:
> t = data.frame(a=1:5, b=5:1)
> t
a b
1 1 5
2 2 4
3 3 3
4 4 2
5 5 1
Now if c is 0 if a>b, 1 if a
> t$c=2-((t$a>t$b)+(t$a!=t$b))
> t
a b c
1 1 5 1
2 2 4 1
3 3 3 2
4 4 2 0
5 5 1 0
Logical operations (>, != etc) operate along vectors, and evaluate numerically to 1 for TRUE and 0 for FALSE. If you try typing parts of my expression for t$c you should learn how this all works together.
If you don't like that tricksy boolean arithmetic, a couple of nested ifelse functions work:
t$c = ifelse(t$b>t$a, 1, ifelse(t$b==t$a,2,0))
This has the advantage of being a bit more readable - if b>a its 1 otherwise if b=a its 2 otherwise its 0. Note how ifelse works, like lots of R functions, on each element of a vector.

Answer 2

Here's an approach using sign() and indexing a vector of desired values:
t <- data.frame(a=1:5,b=5:1);
t;
## a b
## 1 1 5
## 2 2 4
## 3 3 3
## 4 4 2
## 5 5 1
t$c <- c(1,2,0)[sign(t$a-t$b)+2];
t;
## a b c
## 1 1 5 1
## 2 2 4 1
## 3 3 3 2
## 4 4 2 0
## 5 5 1 0
The advantage here is that you can easily change the desired values later, because they're defined explicitly in the indexed vector as (a<b,a==b,a>b). Spacedman's solution to use logical arithmetic is rather brilliant (+1 from me!), but does not easily lend itself to future changes in values.

Answer 3

use a nested ifelse
t$c=ifelse(t$a<t$b,1,
ifelse(t$a>t$b,0,2)
)
from help of nested ifelse
?ifelse
S4 method for class 'db.obj':
ifelse((test, yes, no))
Arguments
test
A db.obj object, which has only one column. The column can be casted into boolean values.
yes
A normal value or a db.obj object. It is the returned value when test is TRUE.
no
The returned value when test is FALSE.