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Patricio Moracho
Asked: 2020-05-25 06:51:05 +0800 CST 2020-05-25 06:51:05 +0800 CST

How does control flow work in a language that is entirely vector?

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In every language there are clauses to control the flow of execution, in R in particular I am talking about the if/else, the whileand the repeat. These are not very different from those that we can find in any other language, they evaluate a certain condition, depending on one Vedadero/Falsoof it, it will be the direction they take. But in R , there is a big little difference.

Being a purely vector language, there are no "scalar" data, although there are different types of data, this can only exist in a "container" (the most elementary is the vector), when we do it a = 1in any language, we are assigning a space to store a single integer data, in R , it is the same, but with a subtle difference, an array of type integer is created, with a single element.

Now, with the evaluation of the conditions the same thing happens, it does not return a TRUE/FALSEscalar, but rather a boolean vector, however the flow control, like the rest of the languages, is clearly "scalar", a single TRUE/FALSEwill determine the flow to follow So: How is this situation of needing a single piece of data for evaluation compatible in the language, when in reality the language does not have it?

r

1 Answers

  1. Best Answer

    We are going to use ifas an example, but the operation is the same in any of the other clauses. In the first place, it never hurts to consult the documentation, and what we can notice is that if/else, he whileand he repeatshare the same help and what it says about the condiciónto be evaluated is very significant:

    cond : A length-one logical vector that is not NA. Conditions of length greater than one are currently accepted with a warning, but only the first element is used. An error is signaled instead when the environment variable _R_CHECK_LENGTH_1_CONDITION_ is set to true. Other types are coerced to logical if possible, ignoring any class.

    Summarizing:

    • The condition is expected to return a one-element vector and not beNA
    • Eventually more than one element is accepted, but only the first is evaluated, and aWarning
    • Eventually it can be configured that instead of a WarningaError

    Let's see, with this, obviously there are no problems:

    > condicion <- c(TRUE)
> if (condicion) print ("True")
[1] "True"

    Now, what happens with a vector other element?

    > condicion <- c(TRUE,FALSE)
> if (condicion) print ("True")
[1] "True"
Warning message:
In if (condicion) print("True") :
  the condition has length > 1 and only the first element will be used

    Exactly what is mentioned in the documentation, is allowed but only the first element of the vector is evaluated, and a Warning. Finally: Is there a way around this Warning? Yes, several, but it will depend on what we are looking for, let's see a little:

    If we want to check only the first element

    > # Si nos sirve solo evaluar el primer caso
> if (condicion[1]) print ("True")
[1] "True"

> # Lo mismo pero usamos el doble && (and) ya que el mismo
> # solo retorna el and lógico del primer elemento consigo mismo
> if (condicion && condicion) print ("True")
[1] "True"

> # Con el || (Or) es igual
> if (condicion || condicion) print ("True")
[1] "True"

    If either element is TRUE

    > # any, como su nombre lo indica, evalua que cualquier 
> # elemento sea TRUE
> if (any(condicion)) print ("True")
[1] "True"

    If all elements must be TRUE

    > # all, solo será TRUE si todos lo son
> condicion <- c(TRUE,FALSE)
> if (all(condicion)) print ("True")
> 
> condicion <- c(TRUE,TRUE)
> if (all(condicion)) print ("True")
[1] "True"
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