Arithmetic operators and overflow in Swift - Step-by-Step Execution

Concept Flow - Arithmetic operators and overflow

Start with two numbers

↓

Apply arithmetic operator

↓

Check for overflow?

No→Return result

Yes↓

Handle overflow (wrap, trap, or report)

↓

End

This flow shows how Swift performs arithmetic operations and checks if the result fits in the number type, handling overflow if needed.

Execution Sample

Swift

var a: UInt8 = 250
let b: UInt8 = 10
let c = a &+ b

Adds two UInt8 numbers using overflow addition (&+), which wraps around on overflow.

Execution Table

Step	Variable a	Variable b	Operation	Result	Overflow Detected	Action
1	250	10	a &+ b	260 (exceeds 255)	Yes	Wrap around using &+
2	250	10	Wrap 260 to 4 (260 - 256)	4	No	Result stored in c
3	N/A	N/A	End	Final c = 4	N/A	Execution stops

💡 Overflow detected because 260 is greater than UInt8 max 255; wrapping result to 4.

Variable Tracker

Variable	Start	After Operation	Final
a	250	250	250
b	10	10	10
c	N/A	4	4

Key Moments - 2 Insights

Why does the result wrap around instead of causing an error?

Because the &+ operator in Swift performs overflow addition, it wraps the value instead of trapping, as shown in execution_table step 1 and 2.

What happens if we use + instead of &+ when adding 250 and 10?

Using + would cause a runtime error (trap) on overflow, unlike &+ which wraps. This is why &+ is used to allow overflow safely (not shown in table but important).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of c after wrapping?

A 260

B 10

C 4

D 250

💡 Hint

Check the 'Result' column in step 2 where wrapping occurs.

At which step does the overflow get detected?

A Step 3

B Step 1

C Step 2

D No overflow detected

💡 Hint

Look at the 'Overflow Detected' column in the execution table.

If variable a was 100 instead of 250, what would happen to the overflow?

A No overflow would occur

B Overflow would still occur

C Program would crash

D Result would be zero

💡 Hint

Refer to variable_tracker and consider 100 + 10 within UInt8 range.

Concept Snapshot

Swift arithmetic operators like + can trap on overflow.
Use &+ (overflow addition) to wrap values instead.
UInt8 max is 255; adding beyond wraps around.
Overflow operators prevent runtime crashes by wrapping.
Always choose operator based on desired overflow behavior.

Full Transcript

This example shows how Swift handles arithmetic operations with overflow. We start with two UInt8 numbers, 250 and 10. Adding them normally would exceed the maximum value 255 for UInt8. Using the overflow addition operator &+, Swift detects the overflow and wraps the result around, storing 4 instead of causing an error. The execution table traces each step: detecting overflow, wrapping the value, and storing the final result. Variables a and b remain unchanged, while c holds the wrapped result. Key points include understanding the difference between + and &+ operators and how overflow is handled safely in Swift.

Practice

1. What happens in Swift when you add two Int8 numbers and the result exceeds the maximum value of Int8 without using overflow operators?

easy

A. The program causes a runtime error and stops.

B. The result wraps around automatically without error.

C. The result is set to zero.

D. Swift converts the result to a larger integer type automatically.

Solution

1. Step 1: Understand Int8 limits

   Int8 can hold values from -128 to 127. Adding beyond 127 causes overflow.

2. Step 2: Behavior without overflow operators

   Swift checks for overflow and triggers a runtime error if it happens without using special operators.

3. Final Answer:

   The program causes a runtime error and stops. -> Option A

4. Quick Check:

   Overflow without &+ causes error [OK]

Hint: Overflow without &+ causes runtime error in Swift [OK]

Common Mistakes:

• Thinking Swift wraps values automatically without &+
• Assuming Swift converts to bigger type automatically
• Believing result resets to zero on overflow

2. Which of the following is the correct syntax to add two UInt8 numbers allowing overflow in Swift?

easy

A. let result = a &+ b

B. let result = a ++ b

C. let result = a + b

D. let result = a add b

Solution

1. Step 1: Recall overflow operators in Swift

   Swift uses &+ for addition with overflow allowed.

2. Step 2: Check each option

   let result = a &+ b uses &+ correctly; others are invalid or normal addition without overflow.

3. Final Answer:

   let result = a &+ b -> Option A

4. Quick Check:

   Use &+ for overflow addition [OK]

Hint: Use &+ for overflow addition in Swift [OK]

Common Mistakes:

• Using + instead of &+ for overflow
• Using ++ which is invalid in Swift
• Trying non-existent operators like 'add'

3. What is the output of the following Swift code?

let maxValue: UInt8 = 255
let result = maxValue &+ 1
print(result)

medium

A. 256

B. Runtime error

C. 255

D. 0

Solution

1. Step 1: Understand UInt8 max value and overflow

   UInt8 max is 255. Adding 1 with &+ causes overflow wrapping to 0.

2. Step 2: Evaluate the code output

   Using &+ allows overflow, so 255 + 1 wraps to 0 without error.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   UInt8 overflow wraps to 0 with &+ [OK]

Hint: &+ wraps UInt8 overflow to 0 [OK]

Common Mistakes:

• Expecting 256 which is out of UInt8 range
• Expecting runtime error without &+
• Thinking result stays 255

4. Identify the error in this Swift code snippet:

var x: Int8 = 120
x = x + 10
print(x)

medium

A. No error, prints 130

B. Syntax error: cannot assign to variable

C. Runtime error due to overflow

D. Compile-time error: type mismatch

Solution

1. Step 1: Check Int8 range and operation

   Int8 max is 127. Adding 10 to 120 results in 130, which is overflow.

2. Step 2: Understand Swift overflow behavior without &+

   Normal + operator causes runtime error on overflow.

3. Final Answer:

   Runtime error due to overflow -> Option C

4. Quick Check:

   Int8 overflow without &+ causes runtime error [OK]

Hint: Int8 overflow without &+ triggers runtime error [OK]

Common Mistakes:

• Assuming it prints 130 ignoring overflow
• Thinking it's a syntax or compile error
• Believing variable assignment is invalid

5. You want to safely multiply two Int8 numbers that might overflow. Which Swift code correctly handles overflow and prints the wrapped result?

let a: Int8 = 100
let b: Int8 = 2
// Fill in the blank
print(result)

hard

A. let result = a * b

B. let result = a &* b

C. let result = a *& b

D. let result = multiply(a, b)

Solution

1. Step 1: Recall overflow multiplication operator

   Swift uses &* to multiply with overflow allowed.

2. Step 2: Check options for correct syntax

   let result = a &* b uses &* correctly; others are invalid or normal multiplication without overflow.

3. Final Answer:

   let result = a &* b -> Option B

4. Quick Check:

   Use &* for overflow multiplication [OK]

Hint: Use &* to multiply with overflow in Swift [OK]

Common Mistakes:

• Using * instead of &* for overflow
• Using invalid operator *&
• Assuming a multiply() function exists