Do-try-catch execution flow in Swift - Step-by-Step Execution

Concept Flow - Do-try-catch execution flow

Start

↓

Enter do block

↓

Execute try statement

↓

Success?

Yes→Continue after do

↓

Exit do-try-catch

↓

Error thrown?

Yes→Catch matching error

↓

Handle error

↓

Exit do-try-catch

The program enters the do block, tries to run code that might throw an error, and if an error occurs, it jumps to the matching catch block to handle it.

Execution Sample

Swift

enum MyError: Error {
    case badInput
}

func throwError() throws {
    throw MyError.badInput
}

do {
    try throwError()
    print("Success")
} catch MyError.badInput {
    print("Caught bad input error")
}

This code tries to run a function that may throw an error and catches a specific error if thrown.

Execution Table

Step	Action	Try Statement Result	Catch Block Entered	Output
1	Enter do block	N/A	No
2	Execute try throwError()	Throws MyError.badInput	Yes, catch MyError.badInput
3	Handle error in catch	N/A	Yes	Caught bad input error
4	Exit do-try-catch	N/A	No

💡 Error thrown in try, caught in catch, then execution exits do-try-catch block

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Output	""	""	"Caught bad input error"	"Caught bad input error"

Key Moments - 2 Insights

Why does the code jump to the catch block instead of printing "Success"?

Because the try statement threw an error (Step 2 in execution_table), the normal flow is interrupted and the catch block runs to handle the error.

What happens if no error is thrown inside the do block?

If no error is thrown, the catch block is skipped and the code after try runs normally, like printing "Success" (not shown in this trace).

Visual Quiz - 3 Questions

Look at the execution table, what is the output after Step 3?

A "Caught bad input error"

B "Success"

C "" (empty string)

D Runtime error

💡 Hint

Check the Output column at Step 3 in the execution_table

At which step does the catch block start executing?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the Catch Block Entered column in execution_table

If the try statement did not throw an error, what would happen?

A Catch block runs anyway

B Program crashes

C Code after try runs, catch block skipped

D Execution stops immediately

💡 Hint

Refer to key_moments explanation about no error thrown

Concept Snapshot

do {
  try someFunction()
  // code if no error
} catch ErrorType {
  // handle error
}

- Code inside do runs first
- try runs code that may throw
- If error thrown, jump to matching catch
- If no error, catch skipped
- After catch or do, execution continues

Full Transcript

In Swift, the do-try-catch structure lets you run code that might cause an error. You start with do, then try to run code that can throw an error. If it throws, the program jumps to the catch block that matches the error type to handle it. If no error happens, the catch block is skipped and the code after try runs normally. This flow helps keep your program safe from crashes by managing errors clearly.

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