There were several interesting points about `ifelse`. The usual behaviour seems to be that all three inputs are evaluated, and the entries of `yes` corresponding to `TRUE` in `test` are combined with the entries of `no` corresponding to `FALSE` in `test`. Moreover, `yes` & `no` seem to be recycled as necessary in case `test` is longer. On top of that, there seems to be some sugar that suppresses evaluations in case `all(test)` and/or `all(!test)`, and the return type can be `logical` even if `yes` & `no` are not. I agreed with the other responses already, but my experiments further confirmed that `ifelse` is not interchangeable with `if(....) .... else ....`.
The documentation confirms most of this, but 'same length and attributes (including dimensions and �"class"�) as �test�' looks wrong. The output seems to be `logical` or something related to the classes of `yes` & `no`. Regards, Jorgen Harmse. > ifelse(FALSE, {cat("Evaluating the vector for 'if'.\n"); 1:3}, > {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'else'. [1] 0 > ifelse(rep(FALSE,5L), {cat("Evaluating the vector for 'if'.\n"); 1:3}, > {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'else'. [1] 0 1 2 3 4 > ifelse(rep(TRUE,3L), {cat("Evaluating the vector for 'if'.\n"); 1:3}, > {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'if'. [1] 1 2 3 > ifelse(c(TRUE,TRUE,FALSE), {cat("Evaluating the vector for 'if'.\n"); 1:3}, > {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'if'. Evaluating the vector for 'else'. [1] 1 2 2 > ifelse(c(TRUE,TRUE,FALSE,TRUE), {cat("Evaluating the vector for 'if'.\n"); > 1:3}, {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'if'. Evaluating the vector for 'else'. [1] 1 2 2 1 > args(ifelse) function (test, yes, no) NULL > ifelse(c(TRUE,TRUE,FALSE,TRUE,TRUE,FALSE,TRUE), {cat("Evaluating the vector > for 'if'.\n"); 1:3}, {cat("Evaluating the vector for 'else'.\n"); 0:4}) Evaluating the vector for 'if'. Evaluating the vector for 'else'. [1] 1 2 2 1 2 0 1 > ifelse(logical(0L), {cat("Evaluating the vector for 'if'.\n"); 1:3}, > {cat("Evaluating the vector for 'else'.\n"); 0:4}) logical(0) > ifelse(TRUE, integer(0L), numeric(0L)) [1] NA > class(ifelse(TRUE, integer(0L), numeric(0L))) [1] "integer" > ifelse(integer(0L)) # test is an empty vector of integers and yes & no are > missing. logical(0) [[alternative HTML version deleted]]
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