Array Traversal Patterns in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is array traversal?

Array traversal means visiting each element in an array one by one to read or process it.

beginner

How do you traverse an array from start to end in Go?

Use a for loop starting from index 0 up to the last index, accessing each element by its index.

beginner

What is the difference between forward and backward traversal?

Forward traversal visits elements from the first to the last. Backward traversal visits from the last to the first.

intermediate

Why might you use a step size greater than 1 when traversing an array?

To skip elements and visit every nth element, which can be useful for sampling or pattern checks.

intermediate

Explain the 'two-pointer' traversal pattern.

Two pointers start at different positions (often start and end) and move towards each other or in the same direction to solve problems efficiently.

Which Go loop is commonly used for array traversal?

A for loop

B while loop

C do-while loop

D foreach loop

Go uses 'for' loops for all looping needs, including array traversal.

What is the index of the last element in an array of length 10?

A 11

B 10

C 0

D 9

Array indices start at 0, so the last index is length - 1, which is 9.

What does backward traversal mean?

A Visiting elements from first to last

B Visiting elements from last to first

C Skipping every other element

D Visiting elements randomly

Backward traversal means visiting elements starting from the end towards the beginning.

In two-pointer traversal, where do the pointers usually start?

A Both at the start

B Both at the end

C One at start, one at end

D Random positions

Two-pointer technique often uses one pointer at the start and one at the end.

Why would you use a step size of 2 in array traversal?

A To visit only even-indexed elements

B To visit every element

C To visit only odd-indexed elements

D To visit elements in reverse

A step size of 2 starting at index 0 visits even-indexed elements.

Describe how to traverse an array forward and backward in Go.

Think about how the index changes in each direction.

Explain the two-pointer traversal pattern and give a simple example use case.

Imagine checking if a word reads the same forwards and backwards.

Practice

1. What does the following Go code snippet do?
for i := 0; i < len(arr); i++ { fmt.Println(arr[i]) }

easy

A. Prints each element of the array from start to end

B. Prints the array elements in reverse order

C. Prints only the first element of the array

D. Causes a runtime error due to index out of range

Solution

1. Step 1: Understand the loop structure

   The loop starts at index 0 and runs while i is less than the length of the array, increasing i by 1 each time.

2. Step 2: Analyze the print statement inside the loop

   Each iteration prints the element at index i, so all elements from start to end are printed in order.

3. Final Answer:

   Prints each element of the array from start to end -> Option A

4. Quick Check:

   Loop from 0 to len(arr)-1 prints all elements [OK]

Hint: Loop from 0 to len(arr) prints all elements in order [OK]

Common Mistakes:

• Thinking it prints in reverse order
• Assuming it prints only one element
• Confusing loop condition causing error

2. Which of the following is the correct syntax to traverse an array arr using a range-based for loop in Go?

easy

A. for value in arr { fmt.Println(value) }

B. for i := 0; i < len(arr); i++ { fmt.Println(arr[i]) }

C. for index, value := range arr { fmt.Println(index, value) }

D. foreach i in arr { fmt.Println(arr[i]) }

Solution

1. Step 1: Recall Go's range syntax

   Go uses for index, value := range arr to iterate over arrays or slices.

2. Step 2: Check each option

   for index, value := range arr { fmt.Println(index, value) } matches the correct syntax. for i := 0; i < len(arr); i++ { fmt.Println(arr[i]) } is valid but not range-based. Options C and D are invalid Go syntax.

3. Final Answer:

   for index, value := range arr { fmt.Println(index, value) } -> Option C

4. Quick Check:

   Range loop syntax = for index, value := range arr { fmt.Println(index, value) } [OK]

Hint: Use 'for index, value := range arr' for range loops in Go [OK]

Common Mistakes:

• Using Python or other language syntax
• Confusing traditional for loop with range loop
• Using invalid keywords like foreach

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

arr := []int{10, 20, 30, 40}
for i := len(arr) - 1; i >= 0; i-- {
    fmt.Print(arr[i], " ")
}

medium

A. 10 20 30 40

B. 40 30 20 10

C. Runtime error due to invalid index

D. No output

Solution

1. Step 1: Understand the loop range

   The loop starts at index len(arr)-1 (3) and decrements i until it reaches 0.

2. Step 2: Trace the printed values

   Indexes printed: 3, 2, 1, 0 corresponding to values 40, 30, 20, 10 printed with spaces.

3. Final Answer:

   40 30 20 10 -> Option B

4. Quick Check:

   Reverse loop prints elements backward [OK]

Hint: Loop from len(arr)-1 down to 0 prints array in reverse [OK]

Common Mistakes:

• Assuming forward order output
• Off-by-one errors causing runtime error
• Confusing loop condition direction

4. Identify the error in this Go code snippet that tries to print all elements of an array arr:

for i := 0; i <= len(arr); i++ {
    fmt.Println(arr[i])
}

medium

A. The print statement syntax is incorrect

B. The loop variable should start from 1 instead of 0

C. The array arr is not declared

D. The loop condition should be i < len(arr) to avoid index out of range

Solution

1. Step 1: Analyze the loop condition

   The loop runs while i <= len(arr), which means i can equal len(arr), an invalid index.

2. Step 2: Understand array indexing

   Array indexes go from 0 to len(arr)-1. Accessing arr[len(arr)] causes runtime error.

3. Final Answer:

   The loop condition should be i < len(arr) to avoid index out of range -> Option D

4. Quick Check:

   Index must be less than length, not less or equal [OK]

Hint: Use i < len(arr) to avoid out-of-range errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Starting loop at 1 instead of 0
• Ignoring array declaration errors

5. Given an array arr := []int{1, 2, 3, 4, 5}, which code snippet correctly prints only the elements at even indexes (0, 2, 4)?

hard

A. for i := 0; i < len(arr); i += 2 { fmt.Print(arr[i], " ") }

B. for i := 1; i < len(arr); i += 2 { fmt.Print(arr[i], " ") }

C. for _, v := range arr { if v%2 == 0 { fmt.Print(v, " ") } }

D. for i := len(arr)-1; i >= 0; i-- { if i%2 == 1 { fmt.Print(arr[i], " ") } }

Solution

1. Step 1: Understand the requirement

   We want to print elements at even indexes: 0, 2, 4.

2. Step 2: Check each option

   for i := 0; i < len(arr); i += 2 { fmt.Print(arr[i], " ") } increments i by 2 starting at 0, correctly accessing even indexes. for i := 1; i < len(arr); i += 2 { fmt.Print(arr[i], " ") } accesses odd indexes. for _, v := range arr { if v%2 == 0 { fmt.Print(v, " ") } } prints even values, not indexes. for i := len(arr)-1; i >= 0; i-- { if i%2 == 1 { fmt.Print(arr[i], " ") } } prints odd indexes in reverse.

3. Final Answer:

   for i := 0; i < len(arr); i += 2 { fmt.Print(arr[i], " ") } -> Option A

4. Quick Check:

   Increment by 2 from 0 prints even indexes [OK]

Hint: Increment index by 2 starting at 0 for even indexes [OK]

Common Mistakes:

• Confusing even indexes with even values
• Starting loop at 1 instead of 0
• Using incorrect loop increments