Character Frequency Counting in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is character frequency counting?

It is the process of counting how many times each character appears in a given string or text.

beginner

Which data structure is commonly used to store character frequencies?

A map or dictionary where keys are characters and values are their counts.

intermediate

In Go, which type is best to represent a character frequency map?

map[rune]int because rune represents a Unicode character and int stores the count.

intermediate

Why is it important to use 'rune' instead of 'byte' for character frequency counting in Go?

Because 'rune' supports Unicode characters beyond ASCII, while 'byte' only supports single bytes which may not represent full Unicode characters.

beginner

What is the time complexity of counting character frequencies in a string of length n?

O(n), because each character is visited once to update its count.

What data structure is best for storing character counts in Go?

A int

B []int

C string

D map[rune]int

map[rune]int stores characters as keys and their counts as values.

Why use 'rune' instead of 'byte' for characters in Go?

A 'byte' supports Unicode

B 'byte' is faster

C 'rune' supports Unicode characters

D 'rune' is smaller

'rune' can represent all Unicode characters, 'byte' only ASCII.

What is the output of counting characters in the string "Go!"?

A {}

B {'G':1, 'o':1, '!':1}

C {'G':1, 'o':2, '!':1}

D {'G':2, 'o':1}

Each character appears once in "Go!".

What is the time complexity of character frequency counting?

A O(n)

B O(n^2)

C O(log n)

D O(1)

Each character is processed once, so O(n).

Which Go keyword is used to declare a map?

A map

B slice

C struct

D var

'map' declares a map type in Go.

Explain how to count character frequencies in a string using Go.

Think about looping over each character and storing counts in a map.

Why is using 'rune' important for character frequency counting in Go?

Consider the difference between ASCII and Unicode characters.

Practice

1. What does character frequency counting in a string help us find?

easy

A. How many times each character appears in the string

B. The length of the string

C. The first character of the string

D. The position of a character in the string

Solution

1. Step 1: Understand character frequency counting

   It means counting how many times each character shows up in a string.

2. Step 2: Compare with other options

   Length, first character, or position are different concepts, not frequency counting.

3. Final Answer:

   How many times each character appears in the string -> Option A

4. Quick Check:

   Frequency counting = count characters [OK]

Hint: Frequency means count of each character [OK]

Common Mistakes:

• Confusing frequency with string length
• Thinking frequency means position
• Mixing frequency with substring extraction

2. Which Go code snippet correctly initializes a map to count character frequencies?

easy

A. freq := make(map[rune]int)

B. freq := map[int]string{}

C. freq := []int{}

D. freq := make([]rune, 0)

Solution

1. Step 1: Identify correct map initialization for character counts

   We need a map with keys as characters (rune) and values as counts (int), so make(map[rune]int) is correct.

2. Step 2: Check other options

   freq := map[int]string{} uses wrong types (int keys, string values), C and D are slices, not maps.

3. Final Answer:

   freq := make(map[rune]int) -> Option A

4. Quick Check:

   Map for rune to int = freq map [OK]

Hint: Use map[rune]int for char frequency in Go [OK]

Common Mistakes:

• Using slice instead of map
• Wrong key or value types in map
• Not using make() for map initialization

3. What is the output of this Go code?

package main
import "fmt"
func main() {
  s := "hello"
  freq := make(map[rune]int)
  for _, ch := range s {
    freq[ch]++
  }
  fmt.Println(freq['l'])
}

medium

A. 3

B. 1

C. 2

D. 0

Solution

1. Step 1: Count occurrences of 'l' in "hello"

   The string "hello" has 'l' twice (positions 2 and 3).

2. Step 2: Check map frequency for 'l'

   freq['l'] increments twice, so freq['l'] = 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Count of 'l' in "hello" = 2 [OK]

Hint: Count repeated chars carefully [OK]

Common Mistakes:

• Counting only once
• Confusing index with count
• Using wrong character in print

4. Find the error in this Go code for counting character frequency:

package main
import "fmt"
func main() {
  s := "test"
  freq := map[rune]int{}
  for i := 0; i < len(s); i++ {
    freq[s[i]] += 1
  }
  fmt.Println(freq)
}

medium

A. fmt.Println cannot print maps

B. Map freq is not initialized properly

C. Loop should use range instead of index

D. Using s[i] as byte instead of rune causes wrong map key type

Solution

1. Step 1: Check map key type and s[i] type

   Map keys are rune but s[i] is byte, causing type mismatch.

2. Step 2: Understand why this is an error

   Go does not automatically convert byte to rune for map keys, so this causes a compile error.

3. Final Answer:

   Using s[i] as byte instead of rune causes wrong map key type -> Option D

4. Quick Check:

   Byte vs rune key mismatch error [OK]

Hint: Use rune(s[i]) or range loop for correct key type [OK]

Common Mistakes:

• Ignoring byte vs rune difference
• Assuming map auto-converts keys
• Thinking map initialization is wrong

5. Given a string, how can you find the character with the highest frequency using Go maps?

hard

A. Use a slice to store counts and pick the last element

B. Count frequencies in a map, then iterate map to find max count character

C. Sort the string and pick the first character

D. Print the string length as the highest frequency

Solution

1. Step 1: Count character frequencies using a map

   Use a map[rune]int to count how many times each character appears.

2. Step 2: Iterate the map to find the character with the highest count

   Loop through the map keys and values, track the max count and corresponding character.

3. Final Answer:

   Count frequencies in a map, then iterate map to find max count character -> Option B

4. Quick Check:

   Map count + max search = highest frequency char [OK]

Hint: Use map to count, then find max by iteration [OK]

Common Mistakes:

• Sorting string does not give frequency
• Using slice without mapping chars
• Confusing length with frequency