Do-try-catch execution flow in Swift

Introduction

We use do-try-catch to safely run code that might fail. It helps us handle errors without crashing the program.

When reading a file that might not exist.

When converting user input to a number that might be invalid.

When calling a function that can throw an error.

When working with network requests that might fail.

Syntax

Swift

do {
    try someFunctionThatThrows()
    // code if no error
} catch {
    // code to handle error
}

do starts a block where errors can be caught.

try is used before a function that can throw an error.

Examples

Try to read a file. If it fails, print an error message.

Swift

do {
    try readFile()
    print("File read successfully")
} catch {
    print("Failed to read file")
}

Try to convert input. Catch error if conversion fails.

Swift

do {
    try convertInputToNumber()
    print("Conversion succeeded")
} catch {
    print("Conversion failed")
}

Sample Program

This program checks if a number is negative. If it is, it throws an error. The do-try-catch block catches the error and prints a message.

Swift

import Foundation

enum SimpleError: Error {
    case badNumber
}

func checkNumber(_ number: Int) throws {
    if number < 0 {
        throw SimpleError.badNumber
    }
}

do {
    try checkNumber(-5)
    print("Number is good")
} catch {
    print("Caught an error: number is negative")
}

Important Notes

You must use try before calling a function that can throw an error.

The catch block runs only if an error happens inside do.

You can have multiple catch blocks to handle different errors.

Summary

Use do-try-catch to handle errors safely.

try marks code that might fail.

catch handles errors without crashing.

Practice

1. What is the main purpose of using do-try-catch in Swift?

easy

A. To handle errors safely without crashing the program

B. To create loops that run multiple times

C. To define new functions

D. To declare variables with default values

Solution

1. Step 1: Understand the role of do-try-catch

   This structure is used to run code that might throw errors and handle those errors gracefully.

2. Step 2: Identify the purpose

   It prevents the program from crashing by catching errors and allowing alternative actions.

3. Final Answer:

   To handle errors safely without crashing the program -> Option A

4. Quick Check:

   Error handling = A [OK]

Hint: Remember: try means risky code, catch means safe backup [OK]

Common Mistakes:

• Confusing error handling with loops
• Thinking try-catch defines functions
• Assuming it declares variables

2. Which of the following is the correct syntax to start a do-try-catch block in Swift?

easy

A. try someFunction() do { catch handleError() }

B. try { do someFunction() } catch { handleError() }

C. catch { try someFunction() } do { handleError() }

D. do { try someFunction() } catch { handleError() }

Solution

1. Step 1: Recall correct do-try-catch syntax

   The correct syntax starts with do {}, inside which try is used before the function that might throw.

2. Step 2: Match syntax with options

   do { try someFunction() } catch { handleError() } correctly places do block, uses try inside, and follows with catch block.

3. Final Answer:

   do { try someFunction() } catch { handleError() } -> Option D

4. Quick Check:

   Correct syntax = D [OK]

Hint: Remember: do { try ... } catch { ... } is the pattern [OK]

Common Mistakes:

• Placing try outside do block
• Swapping catch and do blocks
• Missing curly braces

3. What will be the output of the following Swift code?

enum MyError: Error { case fail }

func risky() throws {
  throw MyError.fail
}

do {
  try risky()
  print("Success")
} catch {
  print("Caught an error")
}

medium

A. Success

B. Caught an error

C. No output

D. Compile-time error

Solution

1. Step 1: Analyze the function risky()

   This function always throws an error of type MyError.fail.

2. Step 2: Trace the do-try-catch block

   When try risky() runs, it throws an error, so the code jumps to the catch block and prints "Caught an error".

3. Final Answer:

   Caught an error -> Option B

4. Quick Check:

   Error thrown = catch block runs = B [OK]

Hint: If try throws, catch block runs, not code after try [OK]

Common Mistakes:

• Assuming code after try runs despite error
• Thinking no output occurs
• Confusing compile error with runtime error

4. Identify the error in this Swift code snippet:

do {
  try someFunction()
} catch error {
  print("Error: \(error)")
}

func someFunction() {
  print("Running")
}

medium

A. Function someFunction() must be marked with throws

B. Catch block syntax is incorrect

C. Missing do keyword

D. Print statement inside catch is invalid

Solution

1. Step 1: Check function declaration

   The function someFunction() is called with try, so it must be declared with throws to indicate it can throw errors.

2. Step 2: Identify mismatch causing error

   Since someFunction() is not marked throws, the compiler will raise an error.

3. Final Answer:

   Function someFunction() must be marked with throws -> Option A

4. Quick Check:

   try requires throws function = C [OK]

Hint: Only functions marked throws can be called with try [OK]

Common Mistakes:

• Ignoring throws keyword on function
• Thinking catch syntax is wrong
• Assuming print inside catch is invalid

5. Given this Swift code, what will be printed?

enum FileError: Error { case notFound, unreadable }

func readFile(_ name: String) throws -> String {
  if name == "missing.txt" { throw FileError.notFound }
  if name == "bad.txt" { throw FileError.unreadable }
  return "File content"
}

do {
  let content = try readFile("bad.txt")
  print(content)
} catch FileError.notFound {
  print("File not found")
} catch {
  print("Other error")
}

hard

A. File content

B. File not found

C. Other error

D. No output

Solution

1. Step 1: Analyze the function call

   Calling readFile("bad.txt") throws FileError.unreadable.

2. Step 2: Check catch blocks for matching error

   The first catch handles FileError.notFound, which does not match. The second catch is a general catch that handles all other errors, so it runs and prints "Other error".

3. Final Answer:

   Other error -> Option C

4. Quick Check:

   Specific catch misses, general catch runs = A [OK]

Hint: Specific catch runs only if error matches exactly [OK]

Common Mistakes:

• Assuming first catch runs for all errors
• Thinking no output occurs
• Confusing error types