Adding and updating values in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add a new key-value pair to a map in Go?

You add a new key-value pair by assigning a value to a new key like this: mapName[newKey] = newValue.

beginner

How do you update an existing value in a Go map?

You update a value by assigning a new value to an existing key: mapName[existingKey] = newValue.

beginner

What happens if you assign a value to a key that does not exist in a Go map?

Go will add the key to the map with the assigned value. Maps grow dynamically when you add new keys.

intermediate

Can you add or update values in a nil map in Go?

No, a nil map cannot be written to. You must initialize the map before adding or updating values.

beginner

Show a simple Go code snippet that adds and updates values in a map.

m := make(map[string]int)
m["apple"] = 5  // add
m["apple"] = 10 // update

What is the correct way to add a new key "banana" with value 3 to a map named fruits in Go?

A fruits["banana"] = 3

B fruits.add("banana", 3)

C fruits.insert("banana", 3)

D fruits.put("banana", 3)

In Go, you add or update map values by assigning: map[key] = value.

If you assign a value to an existing key in a Go map, what happens?

A The map is cleared

B A new key is added

C An error occurs

D The value for that key is updated

Assigning a value to an existing key updates that key's value.

What happens if you try to add a key-value pair to a nil map in Go?

A The map is automatically initialized

B The key-value pair is added successfully

C Runtime panic occurs

D The operation is ignored silently

Writing to a nil map causes a runtime panic. You must initialize the map first.

Which function is commonly used to initialize a map before adding values?

A make()

B new()

C init()

D create()

The make() function initializes maps, slices, and channels in Go.

How do you update the value of key "orange" to 7 in a map named fruits?

A fruits.update("orange", 7)

B fruits["orange"] = 7

C fruits.set("orange", 7)

D fruits.change("orange", 7)

Updating a map value is done by assigning a new value to the key.

Explain how to add and update values in a Go map. Include what happens if the key does not exist.

Think about how assignment works with maps.

What must you do before adding or updating values in a Go map to avoid runtime errors?

Consider what happens if the map is nil.

Practice

1. What happens when you assign a value to a new key in a Go map using mapName[key] = value?

easy

A. A new key-value pair is added to the map.

B. The map is deleted.

C. The map becomes empty.

D. The program crashes.

Solution

1. Step 1: Understand map assignment

   Assigning a value to a new key in a map adds that key with the value.

2. Step 2: Differentiate from update

   If the key exists, the value updates; if not, a new pair is created.

3. Final Answer:

   A new key-value pair is added to the map. -> Option A

4. Quick Check:

   Adding new key = new pair added [OK]

Hint: Assigning to new key adds pair, not error or delete [OK]

Common Mistakes:

• Thinking map clears on new key assignment
• Confusing adding with deleting
• Assuming program crashes on new key

2. Which of the following is the correct syntax to update the value for key "age" to 30 in a Go map named person?

easy

A. person["age"] := 30

B. person["age"] = 30

C. person.age = 30

D. person.update("age", 30)

Solution

1. Step 1: Recall map update syntax

   In Go, map values are updated by assigning with = operator: map[key] = value.

2. Step 2: Check each option

   person["age"] = 30 uses correct syntax. person["age"] := 30 uses := which is for variable declaration, not map update. person.age = 30 uses dot notation which is invalid for maps. person.update("age", 30) uses a method that does not exist.

3. Final Answer:

   person["age"] = 30 -> Option B

4. Quick Check:

   Use = to update map value [OK]

Hint: Use =, not := or dot, to update map values [OK]

Common Mistakes:

• Using := instead of = for map update
• Using dot notation for map keys
• Trying to call update method on map

3. What is the output of this Go code?

m := map[string]int{"a": 1, "b": 2}
m["a"] = 5
fmt.Println(m["a"])

medium

A. 1

B. 2

C. 5

D. 0

Solution

1. Step 1: Analyze initial map values

   The map m starts with keys "a":1 and "b":2.

2. Step 2: Update value for key "a"

   m["a"] = 5 changes the value for "a" from 1 to 5.

3. Step 3: Print updated value

   fmt.Println(m["a"]) prints the updated value 5.

4. Final Answer:

   5 -> Option C

5. Quick Check:

   Updated value prints 5 [OK]

Hint: Updated map key prints new value, not old [OK]

Common Mistakes:

• Expecting old value instead of updated
• Confusing key "b" value with "a"
• Thinking print shows zero value

4. Identify the error in this Go code snippet that tries to add a new key-value pair to a map:

var m map[string]int
m["new"] = 10

medium

A. Key "new" is a reserved word and cannot be used.

B. Syntax error: missing semicolon after map declaration.

C. Value 10 must be a string, not int.

D. Map m is nil and not initialized before adding a key.

Solution

1. Step 1: Understand map initialization

   Declaring var m map[string]int creates a nil map. It must be initialized with make() before adding keys.

2. Step 2: Identify error cause

   Assigning m["new"] = 10 on a nil map causes runtime panic because map is not initialized.

3. Final Answer:

   Map m is nil and not initialized before adding a key. -> Option D

4. Quick Check:

   Maps must be initialized before adding keys [OK]

Hint: Always initialize maps with make() before adding keys [OK]

Common Mistakes:

• Assuming var declares initialized map
• Thinking key "new" is invalid
• Confusing value types for map

5. Given a map inventory := map[string]int{"apple": 5, "banana": 3}, write the correct Go code to add 2 more apples to the inventory without overwriting the existing count.

hard

A. inventory["apple"] += 2

B. inventory["apple"] = inventory["apple"] - 2

C. inventory["apple"] = 2

D. inventory.add("apple", 2)

Solution

1. Step 1: Understand current value

   Current count for "apple" is 5 in the map.

2. Step 2: Add 2 without overwriting

   Use += operator to add 2 to existing value: inventory["apple"] += 2.

3. Step 3: Check other options

   inventory["apple"] = 2 overwrites with 2, not add. inventory["apple"] = inventory["apple"] - 2 subtracts 2. inventory.add("apple", 2) uses a non-existent method.

4. Final Answer:

   inventory["apple"] += 2 -> Option A

5. Quick Check:

   Use += to add to map value [OK]

Hint: Use += to add to existing map value, not = [OK]

Common Mistakes:

• Overwriting value instead of adding
• Subtracting instead of adding
• Using non-existent map methods