Shiny application to learn p-value

app.r

##
p_stat_t <- function(v, df = 1, 
                     alternative = "two.sided"){
    if (!alternative %in% c("two.sided", "less", "greater")) {
        stop("alternative must be either \"two.sided\", \"less\", or \"greater\".")
    }
    if(alternative == "two.sided"){
        p0 <- 2*pt(abs(v), df, lower.tail = FALSE)
    }else if(alternative == "less"){
        p0 <- pt(v, df)
    }else if(alternative == "greater"){
        p0 <- pt(v, df, lower.tail = FALSE)
    }
    return(p0)
}

q_stat_t <- function(p, df = 1, 
                     alternative = "two.sided"){
    if (!alternative %in% c("two.sided", "less", "greater")) {
        stop("alternative must be either \"two.sided\", \"less\", or \"greater\".")
    }
    if(alternative == "two.sided"){
        q0 <- qt(p/2, df)#*sign(v)
    }else if(alternative == "less"){
        q0 <- qt(p, df)
    }else if(alternative == "greater"){
        q0 <- qt(p, df, lower.tail = FALSE)
    }
    return(q0)
}
##

require(shiny)

ui <- fluidPage(

    # Application title
    titlePanel("Learn to p-value"),

    sidebarLayout(
        sidebarPanel(
            sliderInput("alpha",
                        "alpha (false positive rate):",
                        min = 0.01,
                        max = 0.5,
                        value = 0.1)
            ),
    mainPanel(plotOutput("distPlot"))
    )
)

# Define server logic required to draw a histogram
server <- function(input, output) {

    output$distPlot <- renderPlot({
        stat <- 1.2
        curve(p_stat_t(x, df=9), -4, 4,
              xlab="t-stat",ylab="alpha")
        legend("topright",
               legend = c("positive","negative"),
               col=c("firebrick","steelblue"),
               pch=c(4,1), lwd=2, lty=0,
               box.lwd = 0)
        alpha <- input$alpha
        q <- q_stat_t(alpha, df=9)
        sig <- p_stat_t(stat, df=9)<=alpha
        points(x=stat, y=0,
               pch=ifelse(sig,4,1),
               col=ifelse(sig,"firebrick","steelblue"),lwd=2)
        abline(h=alpha,lty=2)
        abline(v=q,lty=2)
        abline(v=-q,lty=2)
    })
}

# Run the application 
shinyApp(ui = ui, server = server)