Appending to slices in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a slice in Go?

A slice is a flexible and dynamic view into the elements of an array. It can grow and shrink, unlike arrays which have fixed size.

beginner

How do you append an element to a slice in Go?

Use the built-in append() function. For example: slice = append(slice, element) adds element to the end of slice.

intermediate

What happens if the underlying array of a slice is too small when appending?

Go automatically creates a new, larger array, copies the existing elements, and appends the new element. The slice then points to this new array.

intermediate

Can you append multiple elements to a slice at once? How?

Yes. Use append(slice, elem1, elem2, ...) to add multiple elements. Or append another slice using append(slice, otherSlice...).

beginner

Why must you assign the result of append back to the slice variable?

Because append may return a new slice with a different underlying array, you must assign it back to keep the updated slice reference.

What does the append function return in Go?

A The original slice unchanged

B A new slice with the added elements

C An error if the slice is full

D The underlying array

append returns a new slice that includes the original elements plus the new ones.

How do you append all elements of one slice to another?

A append(slice1, slice2...)

B copy(slice1, slice2)

C slice1 + slice2

D append(slice1, slice2)

Use the ... operator to unpack slice2 elements when appending: append(slice1, slice2...).

If you append to a slice without assigning the result, what happens?

A The program crashes

B The slice is updated correctly

C The original slice remains unchanged

D The slice is cleared

append returns a new slice; if you don't assign it, the original slice stays the same.

What is the type of the second argument in append(slice, ...)?

A Only a single element

B Only an array

C Only a slice

D A single element or multiple elements

You can append one or more elements, or use ... to append all elements from another slice.

What happens internally when a slice's capacity is exceeded during append?

A A new larger array is allocated and elements copied

B An error is thrown

C The slice discards old elements

D The slice shrinks

Go allocates a new array with more capacity and copies existing elements when needed.

Explain how the append function works with slices in Go.

Think about what happens when the slice is full and how you keep the updated slice.

Describe how to append multiple elements or another slice to a slice in Go.

Remember the special syntax for unpacking slices.

Practice

1. What does the append function do in Go when used with slices?

easy

A. It adds new elements to the end of a slice, returning the updated slice.

B. It removes elements from the beginning of a slice.

C. It sorts the elements of a slice in ascending order.

D. It copies elements from one slice to another without changing size.

Solution

1. Step 1: Understand the purpose of append

   The append function is designed to add elements to the end of a slice in Go.

2. Step 2: Recognize the return value

   Append returns a new slice that includes the original elements plus the added ones.

3. Final Answer:

   It adds new elements to the end of a slice, returning the updated slice. -> Option A

4. Quick Check:

   append adds elements to slice end = A [OK]

Hint: Append adds elements at the end and returns the new slice [OK]

Common Mistakes:

• Thinking append removes elements
• Assuming append sorts the slice
• Not realizing append returns a new slice

2. Which of the following is the correct way to append the integer 5 to a slice named nums?

easy

A. nums.append(5)

B. append(nums, 5)

C. nums = append(nums, 5)

D. nums += 5

Solution

1. Step 1: Recall append syntax in Go

   Append returns a new slice, so you must assign it back to the original slice variable.

2. Step 2: Check each option

   nums = append(nums, 5) correctly assigns the result of append back to nums. Options A, B, and D don't work correctly.

3. Final Answer:

   nums = append(nums, 5) -> Option C

4. Quick Check:

   Assign append result back to slice = C [OK]

Hint: Always assign append result back to your slice variable [OK]

Common Mistakes:

• Not assigning append result back to slice
• Using method call syntax like nums.append()
• Trying to add element with += operator

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

nums := []int{1, 2, 3}
nums = append(nums, 4, 5)
fmt.Println(nums)

medium

A. [1 2 3]

B. Compilation error

C. [4 5]

D. [1 2 3 4 5]

Solution

1. Step 1: Understand initial slice and append

   nums starts as [1, 2, 3]. Append adds 4 and 5 to the end.

2. Step 2: Result after append

   After append, nums becomes [1, 2, 3, 4, 5]. Printing nums shows this full slice.

3. Final Answer:

   [1 2 3 4 5] -> Option D

4. Quick Check:

   Appending multiple elements adds all to slice end = D [OK]

Hint: Append can add multiple elements at once [OK]

Common Mistakes:

• Expecting append to replace elements
• Thinking append returns only new elements
• Confusing slice initialization syntax

4. Identify the error in this Go code snippet:

var letters []string
append(letters, "a")
fmt.Println(letters)

medium

A. append result is not assigned back to letters

B. letters slice is nil and cannot be appended to

C. Syntax error in append function call

D. fmt.Println cannot print slices

Solution

1. Step 1: Check slice initialization

   letters is a nil slice but append can still add elements to it.

2. Step 2: Check append usage

   Append returns a new slice; the code does not assign this back to letters, so letters remains nil.

3. Final Answer:

   append result is not assigned back to letters -> Option A

4. Quick Check:

   Always assign append result back to slice = B [OK]

Hint: Assign append result to slice even if slice is nil [OK]

Common Mistakes:

• Assuming append modifies slice in place
• Thinking nil slice cannot be appended to
• Believing fmt.Println can't print slices

5. Given a slice data := []int{1, 2, 3}, which code snippet correctly appends all elements from another slice extra := []int{4, 5, 6} to data?

hard

A. data = append(data, &extra...)

B. data = append(data, extra...)

C. data = append(data, extra)

D. data = append(data, *extra)

Solution

1. Step 1: Understand appending slices

   To append all elements of one slice to another, use the ... operator to unpack elements.

2. Step 2: Analyze options

   data = append(data, extra...) correctly uses append with extra unpacked as extra.... Options B, C, and D have syntax errors or incorrect usage.

3. Final Answer:

   data = append(data, extra...) -> Option B

4. Quick Check:

   Use ... to append all slice elements = A [OK]

Hint: Use ... to unpack slice elements when appending [OK]

Common Mistakes:

• Trying to append slice without ... (adds slice as single element)
• Using pointer or dereference incorrectly
• Trying to append slice as single element