Handling Invalid Input

Part of the Logic & Flow section of Coddy's Lua journey — lesson 47 of 54.

Challenge

Easy

Complete your contact list application by adding proper handling for invalid menu commands. When the user enters a command that doesn't match any of the available options, the program should inform them and continue running.

Building on the previous challenge, add an else condition to handle any command that isn't add, list, search, delete, or quit. This improves the user experience by providing clear feedback when an unrecognized command is entered.

Requirements:

Keep all code from the previous challenge (setup, menu display, main loop, add, list, search, and delete functionality)
Add an else condition to handle any command that doesn't match the valid options
When an invalid command is entered, print: Invalid command. Please try again.
The program should continue running after displaying the invalid command message
All existing commands (add, list, search, delete, quit) should still work exactly as before

Example interaction:

Welcome to Contact List Manager!

Available Commands:
add - Add a new contact
list - Display all contacts
search - Search for a contact
delete - Delete a contact
quit - Exit the program

Enter command:
add
Enter name:
Alice
Enter phone number:
555-1234
Contact added successfully!

Enter command:
show
Invalid command. Please try again.

Enter command:
remove
Invalid command. Please try again.

Enter command:
list
Contact List:
1. Alice - 555-1234

Enter command:
help
Invalid command. Please try again.

Enter command:
quit
Goodbye!

Try it yourself

-- Contact List Manager with Delete Functionality
local contacts = {}

print("Welcome to Contact List Manager!")
print()
print("Available Commands:")
print("add - Add a new contact")
print("list - Display all contacts")
print("search - Search for a contact")
print("delete - Delete a contact")
print("quit - Exit the program")
print()

while true do
    print("Enter command:")
    local command = io.read()
    
    if command == "add" then
        print("Enter name:")
        local name = io.read()
        print("Enter phone number:")
        local phone = io.read()
        
        local contact = {name = name, phone = phone}
        table.insert(contacts, contact)
        print("Contact added successfully!")
        print()
        
    elseif command == "list" then
        if #contacts == 0 then
            print("No contacts available.")
        else
            print("Contact List:")
            for i, contact in ipairs(contacts) do
                print(i .. ". " .. contact.name .. " - " .. contact.phone)
            end
        end
        print()
        
    elseif command == "search" then
        print("Enter name to search:")
        local searchName = io.read()
        local found = false
        
        for i, contact in ipairs(contacts) do
            if contact.name == searchName then
                print("Found: " .. contact.name .. " - " .. contact.phone)
                found = true
                break
            end
        end
        
        if not found then
            print("Contact not found.")
        end
        print()
        
    elseif command == "delete" then
        print("Enter name to delete:")
        local deleteName = io.read()
        local found = false
        
        for i, contact in ipairs(contacts) do
            if contact.name == deleteName then
                table.remove(contacts, i)
                print("Contact deleted successfully!")
                found = true
                break
            end
        end
        
        if not found then
            print("Contact not found.")
        end
        print()
        
    elseif command == "quit" then
        print("Goodbye!")
        break
    end
end