Throwing functions with throws in Swift - Step-by-Step Execution

Concept Flow - Throwing functions with throws

Define function with 'throws'

↓

Call function with 'try'

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Function runs

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No error

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Return value

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Continue

A function marked with 'throws' can produce an error. When calling it, use 'try' and handle errors with 'do-catch'.

Execution Sample

Swift

enum MyError: Error {
    case badInput
}

func checkNumber(_ num: Int) throws -> String {
    if num < 0 { throw MyError.badInput }
    return "Number is \(num)"
}

This function throws an error if the number is negative, otherwise returns a message.

Execution Table

Step	Action	Input	Condition	Result	Error Thrown	Output
1	Call checkNumber with 5	5	5 < 0?	False	No	"Number is 5"
2	Return from function				No	"Number is 5"
3	Call checkNumber with -3	-3	-3 < 0?	True	Yes: MyError.badInput
4	Catch error in do-catch				Yes	"Caught error: badInput"
5	End

💡 Execution stops after handling error or returning value.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
num	-	5	-3	-3
errorThrown	No	No	Yes	Yes
output	-	"Number is 5"		"Caught error: badInput"

Key Moments - 3 Insights

Why do we need to use 'try' when calling a throwing function?

Because the function can throw an error, 'try' tells Swift to handle that possibility, as shown in execution_table step 1 and 3.

What happens if the function throws an error?

The error is passed to the 'catch' block in the 'do-catch' structure, as seen in execution_table step 4.

Can the function return a value if it throws an error?

No, if an error is thrown, the function stops and does not return a value, shown in step 3 where output is empty.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after calling checkNumber with 5?

A "Number is 5"

B "Caught error: badInput"

C No output

D MyError.badInput

💡 Hint

Check row 1 and 2 in execution_table for output when input is 5.

At which step does the function throw an error?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Look at the 'Error Thrown' column in execution_table.

If we remove 'try' when calling the throwing function, what happens?

A The code compiles and runs normally

B Compilation error because 'try' is required

C The function never throws an error

D The error is automatically caught

💡 Hint

Swift requires 'try' to call throwing functions, as shown in concept_flow.

Concept Snapshot

Throwing functions use 'throws' to signal errors.
Call them with 'try' inside 'do-catch' blocks.
If error occurs, control jumps to 'catch'.
If no error, function returns normally.
Always handle errors to avoid crashes.

Full Transcript

In Swift, functions that can produce errors are marked with 'throws'. When you call these functions, you must use 'try' to acknowledge the possibility of an error. The call is usually inside a 'do-catch' block to handle errors gracefully. If the function runs without error, it returns a value. If it throws an error, the catch block runs. This prevents the program from crashing and lets you respond to problems. The execution table shows calling the function with a positive number returns a message, while calling it with a negative number throws an error that is caught and handled.