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.