Var for variables (mutable) in Swift - Step-by-Step Execution

Concept Flow - Var for variables (mutable)

Declare var x = 5

↓

Use x in code

↓

Change x = 10

↓

Use new x value

↓

End

This flow shows how a mutable variable is declared, used, changed, and used again with the new value.

Execution Sample

Swift

var x = 5
print(x)
x = 10
print(x)

Declare a mutable variable x, print it, change its value, then print the new value.

Execution Table

Step	Action	Variable x	Output
1	Declare var x = 5	5
2	Print x	5	5
3	Change x = 10	10
4	Print x	10	10
5	End of code	10

💡 Code ends after printing the updated value of x.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
x	undefined	5	10	10

Key Moments - 2 Insights

Why can we change the value of x after declaring it with var?

Because var declares a mutable variable, so x can be reassigned new values as shown in steps 1 and 3 of the execution_table.

What happens if we try to use x before declaring it?

The code would give an error because x is undefined before step 1; variables must be declared before use.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x after step 3?

A 5

B 10

C undefined

D 0

💡 Hint

Check the 'Variable x' column at step 3 in the execution_table.

At which step does the output first show the value 5?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Output' column in the execution_table to find when 5 is printed.

If we change 'var x = 5' to 'let x = 5', what would happen at step 3?

A Error because let variables cannot be changed

B x changes to 10 successfully

C x becomes 0

D No output is printed

💡 Hint

Recall that let declares constants which cannot be reassigned, unlike var.

Concept Snapshot

var declares a mutable variable in Swift.
You can assign a value and later change it.
Syntax: var variableName = value
Changing the value is allowed: variableName = newValue
Use var when you need to update the variable.

Full Transcript

This example shows how to use var in Swift to create a variable that can change. First, we declare x with var and assign 5. Then we print x, which outputs 5. Next, we change x to 10 and print again, outputting 10. The key point is var allows changing the variable's value after declaration. If we used let instead, changing x would cause an error. This helps understand mutable variables in Swift.

Practice

1. What does the var keyword do in Swift?

easy

A. It creates a variable whose value can change later.

B. It creates a constant value that cannot change.

C. It defines a function.

D. It declares a type alias.

Solution

1. Step 1: Understand the purpose of var

   The var keyword is used to declare variables that can be changed after they are set.

2. Step 2: Compare with constants

   Unlike let, which creates constants, var allows the value to be updated.

3. Final Answer:

   It creates a variable whose value can change later. -> Option A

4. Quick Check:

   var means mutable variable [OK]

Hint: Remember: var means variable, value can change [OK]

Common Mistakes:

• Confusing var with let (constant)
• Thinking var creates constants
• Mixing var with function declaration

2. Which of the following is the correct way to declare a mutable variable named score with initial value 10 in Swift?

easy

A. let score = 10

B. variable score = 10

C. const score = 10

D. var score = 10

Solution

1. Step 1: Identify the keyword for mutable variables

   In Swift, var declares a variable that can change, so use var score = 10.

2. Step 2: Check other options

   let creates a constant, variable and const are not valid Swift keywords.

3. Final Answer:

   var score = 10 -> Option D

4. Quick Check:

   Mutable variable uses var [OK]

Hint: Use var for variables, let for constants [OK]

Common Mistakes:

• Using let instead of var for mutable variables
• Using invalid keywords like variable or const
• Missing the equals sign

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

var count = 5
count = 10
print(count)

medium

A. 10

B. 5

C. Error: Cannot assign to constant

D. nil

Solution

1. Step 1: Understand variable assignment

   The variable count is declared with var and initially set to 5, so it can be changed.

2. Step 2: Update and print value

   The value is changed to 10 before printing, so print(count) outputs 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Mutable var updates value correctly [OK]

Hint: var allows value change, so print updated value [OK]

Common Mistakes:

• Thinking var acts like let (constant)
• Expecting original value instead of updated
• Confusing syntax causing errors

4. Identify the error in this Swift code:

var name = "Alice"
name = 42
print(name)

medium

A. Syntax error on variable declaration

B. No error, prints 42

C. Type mismatch error when assigning 42 to name

D. Runtime error when printing

Solution

1. Step 1: Check variable type inference

   Variable name is inferred as String because of initial value "Alice".

2. Step 2: Assign incompatible type

   Assigning an integer 42 to a String variable causes a type mismatch error.

3. Final Answer:

   Type mismatch error when assigning 42 to name -> Option C

4. Quick Check:

   Swift variables keep type, cannot assign different type [OK]

Hint: Variable type fixed by first value, cannot assign different type [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking var allows any type changes
• Confusing syntax error with type error

5. You want to keep track of a player's score that changes during the game. Which Swift code correctly declares and updates this score using var?

1. let score = 0
2. score = 10

3. var score = 0
4. score = 10

5. var score: String
6. score = 10

7. let score: Int
8. score = 10

hard

A. Lines 1 and 2

B. Lines 3 and 4

C. Lines 5 and 6

D. Lines 7 and 8

Solution

1. Step 1: Identify mutable variable declaration

   Lines 3 and 4 declare score with var and assign 0, then update to 10, which is correct.

2. Step 2: Check other options

   Lines 1 and 2 use let (constant), so line 2 causes error. Lines 5 and 6 declare score as String but assign integer 10, causing type mismatch error. Lines 7 and 8 use let constant and then try to assign, causing error.

3. Final Answer:

   Lines 3 and 4 -> Option B

4. Quick Check:

   Use var to declare and update variables [OK]

Hint: Declare with var and assign before update [OK]

Common Mistakes:

• Using let for variables that change
• Type mismatch with wrong type annotation
• Confusing var and let usage