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官方文档是这么写的，但还是不太明白是什么意思？哪位给翻译一下，或是给举下应用的例子，谢谢！
req {shiny} R Documentation
Check for required values
Description
Ensure that values are available ("truthy"–see Details) before proceeding with a calculation or action. If any of the given values is not truthy, the operation is stopped by raising a "silent" exception (not logged by Shiny, nor displayed in the Shiny app's UI).
Usage
req(...)
Arguments
...
Values to check for truthiness.
Details
The req function was designed to be used in one of two ways. The first is to call it like a statement (ignoring its return value) before attempting operations using the required values:
rv <- reactiveValues(state = FALSE)
r <- reactive({
req(input$a, input$b, rv$state)
# Code that uses input$a, input$b, and/or rv$state...
})
In this example, if r() is called and any of input$a, input$b, and rv$state are NULL, FALSE, "", etc., then the req call will trigger an error that propagates all the way up to whatever render block or observer is executing.
The second is to use it to wrap an expression that must be truthy:
output$plot <- renderPlot({
if (req(input$plotType) == "histogram") {
hist(dataset())
} else if (input$plotType == "scatter") {
qplot(dataset(), aes(x = x, y = y))
}
})
In this example, req(input$plotType) first checks that input$plotType is truthy, and if so, returns it. This is a convenient way to check for a value "inline" with its first use.
Truthy and falsy values
The terms "truthy" and "falsy" generally indicate whether a value, when coerced to a logical, is TRUE or FALSE. We use the term a little loosely here; our usage tries to match the intuitive notions of "Is this value missing or available?", or "Has the user provided an answer?", or in the case of action buttons, "Has the button been clicked?".
For example, a textInput that has not been filled out by the user has a value of "", so that is considered a falsy value.
To be precise, req considers a value truthy unless it is one of:
FALSE
NULL
""
An empty atomic vector
An atomic vector that contains only missing values
A logical vector that contains all FALSE or missing values
An object of class "try-error"
A value that represents an unclicked actionButton
Note in particular that the value 0 is considered truthy, even though as.logical(0) is FALSE.
If the built-in rules for truthiness do not match your requirements, you can always work around them. Since FALSE is falsy, you can simply provide the results of your own checks to req:
req(input$a != 0)
Value
The first value that was passed in.
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