Subset Dataframe

Hi all!

I am super new to R and computer science in general and tried to write my first shiny app using the Titanic dataset. I would like to let the user choose which class and which sex should be displayed on a histogram. The ui was rather simple but I am struggling with the server function. I would be really grateful if somebody could help me and explain how to include the if statements. I will include my code here:

library(shiny)
library(shinythemes)
library(dplyr)
library(ggplot2)
library(readr)

Titanic <- as.data.frame(Titanic)

ui <- fluidPage(

titlePanel("Survival Titanic"),

sidebarLayout(
sidebarPanel(
selectInput(inputId = "Passenger_Class",
label = "Enter passenger category:",
choices = c("1st", "2nd", "3rd", "Crew", "All") ),

  selectInput(inputId = "Sex", 
              label = "Enter gender of interest", 
              choices = c("Male", "Female", "Both")  ),
  
  submitButton(text = "Create Plot")),

mainPanel(plotOutput(outputId = "histogram"))))

server <- function(input, output) {

selectData = reactive({

ifelse (input$PassengerClass =="All", Titanic_subset <- Titanic,
Titanic_subset <- Titanic %>% filter(Class == input$PassengerClass))

if (input$Sex == "Male") {Titanic_subset <- Titanic_subset %>% filter(Sex == "Male")}
if (input$Sex == "Female") {Titanic_subset <- Titanic_subset %>% filter(Sex == "Female")}
if (input$Sex == "Both") {Titanic_subset <- Titanic_subset}

output$histogram <- renderPlot({
ggplot(Titanic, aes( x= Class, y= Freq, fill = Survived) ) +
geom_bar(stat = "identity", position = position_dodge() ) +
ylab ("Number of Passengers")})

})}

Run

shinyApp(ui = ui, server = server)

The following code seems to do what you want. Some of the changes are:
I moved ggplot() outside of reactive()
reactive() returns a function, so I used selectData() as the data argument in ggplot.
Passenger_Class was misspelled in server()
I used if() instead of ifelse to filter for the Passenger_Class
I explicitly returned Titanic_subset at the end of reactive.

library(shiny)
#library(shinythemes)
library(dplyr)
library(ggplot2)
library(readr)

Titanic <- as.data.frame(Titanic)

ui <- fluidPage(
  
  titlePanel("Survival Titanic"),
  
  sidebarLayout(
    sidebarPanel(
      selectInput(inputId = "Passenger_Class",
                  label = "Enter passenger category:",
                  choices = c("1st", "2nd", "3rd", "Crew", "All") ),
      
      selectInput(inputId = "Sex", 
                  label = "Enter gender of interest", 
                  choices = c("Male", "Female", "Both")  ),
      
      submitButton(text = "Create Plot")),
    
    mainPanel(plotOutput(outputId = "histogram"))))

server <- function(input, output) {
  
  selectData = reactive({
    
    if (input$Passenger_Class =="All") {
      Titanic_subset <-  Titanic
      } else {
        Titanic_subset <- Titanic %>% filter(Class == input$Passenger_Class)
      }
    
    if (input$Sex == "Male") {Titanic_subset <- Titanic_subset %>% filter(Sex == "Male")}
    if (input$Sex == "Female") {Titanic_subset <- Titanic_subset %>% filter(Sex == "Female")}
    if (input$Sex == "Both") {Titanic_subset <- Titanic_subset}
    Titanic_subset
  })  
    output$histogram <- renderPlot({
      ggplot(selectData(), aes( x= Class, y= Freq, fill = Survived) ) +
        geom_bar(stat = "identity", position = position_dodge() ) +
        ylab ("Number of Passengers")})
    
  }

shinyApp(ui = ui, server = server)

Thank you so much for your quick help and explanation!