R - Unexpected output when running a function on a dataframe

Question

I'm writing a function that computes a new column based on several fields in my data frame. Part of the process is to do a pre-check on the range of one the input fields before actual processing. Weird thing is that my pre-check always returns the same value, that is, the outcome of the pre-check for the first row of the dataframe.

See example below with a simplified version of my function, which only includes the check. Even though the variable is properly passed to the function, and is different every time, the outcome of the pre-check stays the same. What am I missing?

> my_function = function(x){
+   print(paste(x, x < 32 && x > 0))
+   if (x < 32 && x > 0) {
+     # do_something ...
+   }
+ }
> 
> df = data.frame("A" = c(10,20,30,40), "B" = c(0,10,20,30), "C" = c(40,30,20,10))
> my_function(df$A)
[1] "10 TRUE" "20 TRUE" "30 TRUE" "40 TRUE"
> my_function(df$B)
[1] "0 FALSE"  "10 FALSE" "20 FALSE" "30 FALSE"
> my_function(df$C)
[1] "40 FALSE" "30 FALSE" "20 FALSE" "10 FALSE"

Answer 1

You need vectorised ifelse with a single & (instead of &&) if you want to test a condition on every element of a vector.

From ?ifelse

‘ifelse’ returns a value with the same shape as ‘test’ which is filled with elements selected from either ‘yes’ or ‘no’ depending on whether the element of ‘test’ is ‘TRUE’ or ‘FALSE’.

From ?`&&`

‘&’ and ‘&&’ indicate logical AND and ‘|’ and ‘||’ indicate logical OR. The shorter form performs elementwise comparisons in much the same way as arithmetic operators. The longer form evaluates left to right examining only the first element of each vector. Evaluation proceeds only until the result is determined. The longer form is appropriate for programming control-flow and typically preferred in ‘if’ clauses.

The short form & performs an element-wise comparison, while && evaluates only the first element of the vector.

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Here is an example based on your df

f1 <- function(x) if (x < 32 && x > 0) x + 100 else x - 100;
f2 <- function(x) ifelse(x < 32 & x > 0, x + 100, x - 100);

f1(df$A)
#[1] 110 120 130 140

f2(df$A)
#[1] 110 120 130 -60