Unexpected NULL while concatenating 2 strings using cat()

Question

t1 <- c('064359208644', '108595042227', '115705579765', '139254098290', '163646243244')
t2 <- c('064359208644', '139254098290', '163646243244')

missing_elem <- as.character(setdiff(t1, t2))
missing_elem

if(length(missing_elem) != 0){
  ss <- cat("Missing ids:", missing_elem)
}else{
  ss <- "There are no missing ids"
}

ss

When I print the output of ss here it shows NULL but if t1 and t2 are the same and length(missing_elem) is 0 then ss is properly set to "There are no missing ids" and shows accordingly. Any idea why this might happen?

C. Braun · Accepted Answer · 2018-05-08 17:40:06Z

2

The function cat does not return concatenated strings, but rather prints to standard output and returns NULL (which is why the value of ss is NULL). You likely want to use paste:

if(length(missing_elem) != 0){
  ss <- paste("Missing ids:", paste(missing_elem, collapse = ', '))
}else{
  ss <- "There are no missing ids"
}

> ss
[1] "Missing ids: 108595042227, 115705579765"

answered May 8, 2018 at 17:40

C. Braun

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1 Comment

Digvijay Sawant Over a year ago

AAhhh!! Good catch. Thank you.

C-x C-c · Accepted Answer · 2018-05-08 17:38:07Z

2

If you want to just get a character back, just use paste instead of cat.

answered May 8, 2018 at 17:38

C-x C-c

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1 Comment

Frank Over a year ago

Fwiw, in my own code I do cat("Missing IDs:", paste("\t", missing_elem), sep="\n") for this. You could add a quote from ?cat, like "Value: None (invisible NULL)."

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