R中的IF / THEN / ELSE

Question

everyone.

Hopefully an easy syntax question. I'm trying to create a new variable in a table in R which would say "1" if my patient was in the age range I was looking at, or "0" for no. The age range I'm interested is between 2-155. The code is running without any errors, but it is not working. When I look in my table, the new variable will say 1 even though the age4 is 158 Here is what I have:

table$newvar <- if (table$age4>=2 && table$age4 <=155) {table$newvar=1} else {table$newvar=0}

Any help is appreciated! Thanks in advance!

Answer 1

Two changes should be made:

1. Use the vectorized ifelse() function to generate the new column data.
2. Use the vectorized & logical-AND operator when combining the results of the comparisons.

---

table <- data.frame(age4=seq(1,200,10));
table$newvar <- ifelse(table$age4>=2 & table$age4<=155,1,0);
table;
##    age4 newvar
## 1     1      0
## 2    11      1
## 3    21      1
## 4    31      1
## 5    41      1
## 6    51      1
## 7    61      1
## 8    71      1
## 9    81      1
## 10   91      1
## 11  101      1
## 12  111      1
## 13  121      1
## 14  131      1
## 15  141      1
## 16  151      1
## 17  161      0
## 18  171      0
## 19  181      0
## 20  191      0

---

The reason your code is not working is because the if statement and the && operator are not vectorized. The && operator only examines the first element of each operand vector, and only returns a one-element vector representing the result of the logical-AND on those two input values. The if statement always expects a one-element vector for its conditional, and executes the if-branch if that element is true, or the else-branch if false.

If you use a multiple-element vector as the conditional in the if statement, you get a warning:

if (c(T,F)) 1 else 0;
## [1] 1
## Warning message:
## In if (c(T, F)) 1 else 0 :
##   the condition has length > 1 and only the first element will be used

But for some odd reason, you don't get a warning if you use a multiple-element vector as an operand to && (or || ):

c(T,F) && c(T,F);
## [1] TRUE

That's why your code appeared to succeed (by which I mean it didn't print any warning message), but it didn't actually do what was intended.

Answer 2

When used in arithmetic TRUE and FALSE become 1 and 0 so:

transform(table, newvar = (age4 >= 2) * (age4 <= 155) )

These also work:

transform(table, newvar = as.numeric( (age4 >= 2) & (age4 <= 155) ) )

transform(table, newvar = ( (age4 >= 2) & (age4 <= 155) ) + 0 )

transform(table, newvar = ifelse( (age4 >= 2) & (age4 <= 155), 1, 0) )

transform(table, newvar = (age4 %in% 2:155) + 0) # assuming no fractional ages