So okay, our Shiny app is only going to be useful if it can take in input values, do
some R calculations and then spit out the result of those calculations or results.
And so the first thing we need to be able to do is actually take in some input
values.
And so this next code demonstration, all we're going to do is create a slider
which has to occur in the user interface and
then the values for those sliders that the user selects is going to be passed
into the server function which is then just going to send it back as is to
the user interface function to redisplay it, okay?
So, this is just another building exercise.
We're not going to do anything with the input other than spit it back,
right back out at the user interface to display it as is.
So, let's try.
Okay, now let's see if we can get our
Shiny app to actually take in values from the user, via a slider.
Put them to the server, and then the server will return the values.
Okay, so I have my ui.R shinyUI function here.
And in my sidebar layout, on the sidebar panel I'm going to have a slider.
So I'm going to give a user prompt that says move the slider, and
then slider input.
This function right here is going to create the slider and display it, and
then I'm going to name, I have it named slider2, okay.
So I've named the slider slider2, and then the command says Slide Me!
Okay, it's going to go from 0 to 100 and
it's going to have the starting value at 0.
Okay, now let's talk about the main panel in a second.
Okay, so I take that slider value, I input it, it's given a value called slider 2,
the input from the slider displayed to the user.
Okay, now my shiny server is a function that takes as its argument
a function that takes input and output, okay?
And then you have to have these curly braces,
because you need to have, it's defining a function.
So a function has to have the curly braces, and then down here, you have to
remember to close with the curly braces and the shiny server parenthesis, okay?
And then, what it's going to do is it's going to take the input, okay,
input is what's input from the ui.R, shiny UI function.
And it says take an input, and let's get rid of that real quick just so
it's consistent with what's in our mark down code, from the slides.
And it's going to say take in slider 2.
And then what I want to do is just render it.
In other words, display the text and I'm going to name that output, text1.
Okay, so then when we go back to UI.r in the mainPanel, it says,
here's the slider value, and textOutput is text1.
So that same text that we labeled from the output from the server function,
it's going to get displayed in the name panels.
Let's run it and just see how it looks.
Okay, move the slider part.
Here's the slider.
As I move it, the slider value is displayed over here.
So, what's happening?
Right, is the slider is receiving a value from the user,
that value is given the label slider2, okay?
In the server.R function it's grabbing that from the slot
input $slider2, okay?
And then it's spitting it back out
as Output $text1 and then the ui.R is displaying it.
Now there's nothing in particular, I want to emphasize, there's nothing in
particular about having the slider in the side bar panel and the text
in the main panel and that there's nothing particular about this order.
It's not sort of running the slider and then displaying it
because this R code happens to occur later on in the text than this R code, right?
We could have had the slider output in the main panel and the slider.
I'm sorry, the slider in the main panel, in the output, in the sidebar.
So that's an important thing to think about with Shiny and reactive expressions.
It's not kind of, you don't want to think about it running linearly,
like a regular R program.
Of course, at some level it is because, it is in our program but,
the way which the server is going it's sort of running reactively and
kind of constantly going back and forth.
Okay, so you just want to kind of change your mindset about that a little bit
then how it is in normal R programming.
Okay, and now notice another thing is if I were to
label this text output$text and re-run it, okay.
It doesn't display anything because the label text doesn't mean anything to ui.R.
It's thinking it's text1 when it says textOutput, okay.
So let's reload it and then now it'll work.
See, it's displaying it again.
Same thing with the slider.
If I were to call this slider instead of slider2 or call it slider1, okay.
It doesn't display anything because the render text from the UI data or
the server function is looking for slider2 and
the UI has never put out anything called slider2, anything labeled slider2.
Okay, so if I were to change this to slider1, reload it, it now works.
So remember that your labels have to match up.
Brian Caffo, PhDProfessor, Biostatistics
Jeff Leek, PhDAssociate Professor, Biostatistics
Roger D. Peng, PhDAssociate Professor, Biostatistics
