Protected Calls with pcall()

Part of the Logic & Flow section of Coddy's Lua journey — lesson 33 of 54.

While error() stops your program when something goes wrong, sometimes you want to handle errors without crashing. Lua's pcall() function lets you safely execute code that might fail.

The name pcall() stands for "protected call." It wraps a function call in a protective layer that catches any errors. Instead of your program crashing, pcall() returns information about what happened.

Here's the basic syntax:

local status, result = pcall(functionName, arg1, arg2)

The pcall() function returns two values. The first is a boolean status: true if the function succeeded, or false if an error occurred. The second value depends on the status—if successful, it's the function's return value; if there was an error, it's the error message.

Here's a practical example:

function divide(a, b)
    if b == 0 then
        error("Cannot divide by zero")
    end
    return a / b
end

local success, result = pcall(divide, 10, 0)

if success then
    print("Result: " .. result)
else
    print("Error caught: " .. result)
end

In this example, the division by zero triggers an error, but instead of crashing, pcall() catches it. The program continues running and can handle the error gracefully by checking the status value.

Challenge

Easy

Write a function safeCalculate that takes operation, a, and b and returns the result of the calculation or an error message.

The function should use pcall() to safely perform the requested mathematical operation. Create an inner function that performs the calculation based on the operation string, then wrap it with pcall() to catch any errors.

Logic:

Create an inner function that checks the operation type and performs the calculation
For "add", return a + b
For "subtract", return a - b
For "multiply", return a * b
For "divide", check if b is 0 and call error("Division by zero") if true, otherwise return a / b
For any other operation, call error("Invalid operation")
Use pcall() to execute the inner function
If successful, return "Result: " followed by the result
If an error occurs, return "Error: " followed by the error message

Parameters:

operation (string): The operation to perform ("add", "subtract", "multiply", or "divide")
a (number): The first operand
b (number): The second operand

Returns: A string containing either "Result: " followed by the calculation result, or "Error: " followed by the error message (string)

Cheat sheet

The pcall() function (protected call) allows you to safely execute code that might fail without crashing your program.

Basic syntax:

local status, result = pcall(functionName, arg1, arg2)

pcall() returns two values:

status: true if the function succeeded, false if an error occurred
result: The function's return value if successful, or the error message if an error occurred

Example:

function divide(a, b)
    if b == 0 then
        error("Cannot divide by zero")
    end
    return a / b
end

local success, result = pcall(divide, 10, 0)

if success then
    print("Result: " .. result)
else
    print("Error caught: " .. result)
end

This catches the error instead of crashing, allowing the program to continue and handle the error gracefully.

Try it yourself

function safeCalculate(operation, a, b)
    -- Write code here
end

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Logic & Flow

1Advanced Table Iteration

Iterating with pairs()Iterating with ipairs()pairs() vs. ipairs()Recap - Character Sheet

4Introduction to Metatables

What is a Metatable?setmetatable & getmetatable The __index Metamethod The __newindex Metamethod The __tostring Metamethod Arithmetic Metamethods Part 1 Arithmetic Metamethods Part 2 Recap - Read-Only Table

7Advanced String Manipulation

string.gsub() for Substitution Intro to String Patterns string.find()string.match()Iterating with string.gmatch()Recap - Log File Parser

2More Table Library Functions

table.concat()table construction & unpack()table.sort()Custom Sorting with Functions Recap - High Score Board

5Project: Text Adventure Engine

Project Setup: The Room Linking Rooms

Handling Invalid Moves

Displaying Exits

8Project: Contact List

Project Setup The Main Loop

Adding a Contact

Listing All Contacts

Searching for a Contact

Deleting a Contact

Handling Invalid Input

3Advanced Function Concepts

Returning Multiple Values Variadic Functions (...)Functions First-Class Values Anonymous Functions What is a Closure?Recap - Simple Event Handler

6Error Handling

The error() FunctionProtected Calls with pcall()Recap - Safe Division

9Coroutines for Beginners

What is a Coroutine?coroutine create & resume Pausing with coroutine.yield()resume & yield Checking Coroutine Status Recap - Number Generator Recap - Vector Math