String Traversal and Character Access in DSA C++ - Build from Scratch

String Traversal and Character Access

📖 Scenario: Imagine you are creating a simple program that reads a word and then looks at each letter one by one. This is like reading a word slowly to see each letter clearly.

🎯 Goal: You will build a program that stores a word, sets up a way to count letters, goes through each letter one by one, and finally prints all the letters separated by spaces.

📋 What You'll Learn

Create a string variable with a specific word

Create an integer variable to count letters

Use a for loop to go through each character in the string

Print all characters separated by spaces

💡 Why This Matters

🌍 Real World

Reading and processing words letter by letter is useful in text analysis, games like word puzzles, and input validation.

💼 Career

Understanding string traversal is fundamental for software developers working with text processing, user input handling, and data parsing.

1

Create the word string

Create a string variable called word and set it to the exact value "hello".

DSA C++

# Create a string variable called word and set it to "hello"
# Your code here

Hint

Use std::string to create the variable and assign the word "hello" to it.

2

Set up a counter for letters

Create an integer variable called length and set it to the length of the string word using word.length().

DSA C++

std::string word = "hello";
# Create an integer variable called length and set it to word.length()
# Your code here

Hint

Use word.length() to find how many letters are in the word.

3

Loop through each character

Use a for loop with an integer variable i starting at 0 and running while i < length. Inside the loop, access each character of word using word[i].

DSA C++

std::string word = "hello";
int length = word.length();
# Use a for loop with i from 0 to length-1 to access each character word[i]
# Your code here

Hint

Use for (int i = 0; i < length; i++) and inside the loop get the letter with word[i].

4

Print all letters separated by spaces

Inside the for loop, print each character word[i] followed by a space using std::cout. After the loop, print a newline with std::cout << std::endl;.

DSA C++

std::string word = "hello";
int length = word.length();
for (int i = 0; i < length; i++) {
    char letter = word[i];
    # Print letter followed by a space
}
# Print a newline after the loop
# Your code here

Hint

Use std::cout << letter << ' '; inside the loop and std::cout << std::endl; after the loop.

Practice

1. What does the expression str[i] do when str is a string in C++?

easy

A. Returns the length of the string

B. Accesses the character at index i in the string

C. Deletes the character at index i

D. Converts the string to uppercase

Solution

1. Step 1: Understand string indexing in C++

   In C++, strings are like arrays of characters. Using str[i] accesses the character at position i.

2. Step 2: Clarify what str[i] returns

   The expression returns the character stored at index i without modifying the string.

3. Final Answer:

   Accesses the character at index i in the string -> Option B

4. Quick Check:

   str[i] = character at i [OK]

Hint: Remember strings are arrays of chars; indexing gets a character [OK]

Common Mistakes:

• Thinking it returns string length
• Assuming it deletes characters
• Confusing with string case conversion

2. Which of the following is the correct way to loop through each character of a string str in C++?

easy

A. for (int i = 0; i <= str.length(); i++)

B. for (int i = 1; i < str.length(); i++)

C. for (int i = 0; i < str.length(); i++)

D. for (int i = 1; i <= str.length(); i++)

Solution

1. Step 1: Understand string length and indexing

   String indices start at 0 and go up to str.length() - 1. So loop must run while i < str.length().

2. Step 2: Check loop conditions

   for (int i = 0; i < str.length(); i++) uses i < str.length() starting from 0, which correctly covers all characters without going out of bounds.

3. Final Answer:

   for (int i = 0; i < str.length(); i++) -> Option C

4. Quick Check:

   Loop from 0 to length-1 [OK]

Hint: Loop from 0 to less than length to avoid out-of-bounds [OK]

Common Mistakes:

• Using <= causes out-of-bounds error
• Starting loop at 1 misses first character
• Confusing length with last index

3. What is the output of the following C++ code?

std::string s = "hello";
for (int i = 0; i < s.length(); i++) {
    if (s[i] == 'l') {
        std::cout << i << " ";
    }
}

medium

A. 0 1

B. 1 2

C. 3 4

D. 2 3

Solution

1. Step 1: Identify characters and their indices

   The string "hello" has characters: h(0), e(1), l(2), l(3), o(4).

2. Step 2: Check which indices have 'l'

   Characters at indices 2 and 3 are 'l', so those indices are printed.

3. Final Answer:

   2 3 -> Option D

4. Quick Check:

   Indices of 'l' = 2 3 [OK]

Hint: Check string indices carefully for matching characters [OK]

Common Mistakes:

• Confusing character positions
• Printing characters instead of indices
• Off-by-one errors in indexing

4. Identify the error in this code snippet that tries to print all characters of a string str:

for (int i = 0; i <= str.length(); i++) {
    std::cout << str[i];
}

medium

A. Using <= causes out-of-bounds access

B. Missing semicolon after for loop

C. str[i] cannot be used to access characters

D. Loop variable should be unsigned

Solution

1. Step 1: Analyze loop condition

   The loop runs while i <= str.length(), which means it tries to access str[str.length()].

2. Step 2: Understand string indexing limits

   Valid indices are from 0 to str.length() - 1. Accessing str[str.length()] is out-of-bounds and causes undefined behavior.

3. Final Answer:

   Using <= causes out-of-bounds access -> Option A

4. Quick Check:

   Loop must use <, not <= [OK]

Hint: Use < not <= to avoid accessing past string end [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming string length is last index
• Ignoring out-of-bounds errors

5. Given a string str, how can you create a new string containing only the vowels from str using traversal and character access in C++?

hard

A. Loop through str, check if each character is a vowel, and append it to a new string

B. Use str.substr() to extract vowels directly

C. Replace all consonants with spaces in str

D. Sort the string and pick vowels from the start

Solution

1. Step 1: Traverse the string character by character

   Use a loop to access each character in str using indexing.

2. Step 2: Check if character is a vowel and build new string

   If the character is one of 'a', 'e', 'i', 'o', 'u' (case-insensitive), append it to a new string variable.

3. Step 3: Return or print the new string

   After the loop, the new string contains only vowels from the original string.

4. Final Answer:

   Loop through str, check if each character is a vowel, and append it to a new string -> Option A

5. Quick Check:

   Filter vowels by checking each character [OK]

Hint: Check each char; add vowels to new string [OK]

Common Mistakes:

• Trying to extract vowels without looping
• Using substr incorrectly
• Sorting string changes order