String Traversal and Character Access in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of character traversal with index

What is the output of the following C# code snippet that traverses a string and prints characters with their indices?

DSA C#

string s = "hello";
for (int i = 0; i < s.Length; i++)
{
    Console.Write(i + ":" + s[i] + " ");
}

A 0:h 1:e 2:l 3:o 4:l

B 0:h 1:e 2:l 3:l 4:o

C h e l l o

D h0 e1 l2 l3 o4

💡 Hint

Remember that string indices start at 0 and s[i] accesses the character at index i.

The loop runs from 0 to 4 (length 5). Each iteration prints the index, a colon, the character at that index, and a space. So output is '0:h 1:e 2:l 3:l 4:o '.

❓ Predict Output

intermediate

Output of foreach character traversal

What is the output of this C# code that uses foreach to traverse a string and print characters separated by commas?

DSA C#

string s = "world";
foreach (char c in s)
{
    Console.Write(c + ",");
}

A w,o,r,l,d,

B world,

C w o r l d

D w,o,r,l,d

💡 Hint

Each character is printed followed by a comma, including the last character.

The foreach loop prints each character followed by a comma. Since it does not skip the last comma, output ends with a comma.

🔧 Debug

advanced

Identify the error in string character access

What error does the following C# code produce when accessing characters in a string?

DSA C#

string s = "test";
for (int i = 0; i <= s.Length; i++)
{
    Console.Write(s[i]);
}

A IndexOutOfRangeException

B NullReferenceException

C SyntaxError

D No error, prints 'test'

💡 Hint

Check the loop condition and valid indices for string characters.

The loop runs from 0 to s.Length inclusive, but valid indices are 0 to s.Length - 1. Accessing s[s.Length] causes IndexOutOfRangeException.

❓ Predict Output

advanced

Output of reversed string traversal

What is the output of this C# code that prints characters of a string in reverse order?

DSA C#

string s = "abcde";
for (int i = s.Length - 1; i >= 0; i--)
{
    Console.Write(s[i]);
}

A edcb

B abcde

C edcba

D IndexOutOfRangeException

💡 Hint

Loop starts from last index and moves backward to 0.

The loop prints characters from index 4 to 0, resulting in 'edcba'.

🧠 Conceptual

expert

Number of characters accessed in nested loops

Consider this C# code snippet. How many characters are printed in total?

DSA C#

string s = "xyz";
for (int i = 0; i < s.Length; i++)
{
    for (int j = 0; j <= i; j++)
    {
        Console.Write(s[j]);
    }
}

A 3

B 7

C 9

D 6

💡 Hint

Count characters printed in each inner loop iteration and sum them.

Outer loop i=0 prints 1 char, i=1 prints 2 chars, i=2 prints 3 chars. Total = 1+2+3=6.

Practice

1. What does the expression str[2] return when string str = "hello";?

easy

A. The character 'o'

B. The character 'l'

C. The character 'e'

D. An error because strings cannot be indexed

Solution

1. Step 1: Understand zero-based indexing in strings

   In C#, string characters are accessed starting from index 0. So, str[0] = 'h', str[1] = 'e', str[2] = 'l'.

2. Step 2: Identify the character at index 2

   The character at index 2 in "hello" is 'l'.

3. Final Answer:

   The character 'l' -> Option B

4. Quick Check:

   str[2] = 'l' [OK]

Hint: Remember string indexing starts at 0 in C# [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Confusing characters at index 1 and 2
• Assuming strings can't be indexed

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

easy

A. for (char c = 0; c < str.Length; c++) { Console.WriteLine(str[c]); }

B. for (int i = 1; i <= str.Length; i++) { Console.WriteLine(str[i]); }

C. foreach (int c in str) { Console.WriteLine(c); }

D. for (int i = 0; i < str.Length; i++) { Console.WriteLine(str[i]); }

Solution

1. Step 1: Check loop indexing and range

   In C#, string indexing starts at 0 and goes up to Length - 1. So the loop must run from 0 to str.Length - 1.

2. Step 2: Validate each option

   for (int i = 0; i < str.Length; i++) { Console.WriteLine(str[i]); } uses correct indexing (0 to Length-1). for (int i = 1; i <= str.Length; i++) { Console.WriteLine(str[i]); } starts at 1 and uses <= which causes out-of-range error. foreach (int c in str) { Console.WriteLine(c); } tries to foreach over int, but string yields chars, so type mismatch. for (char c = 0; c < str.Length; c++) { Console.WriteLine(str[c]); } uses char as loop variable which is incorrect.

3. Final Answer:

   for (int i = 0; i < str.Length; i++) { Console.WriteLine(str[i]); } -> Option D

4. Quick Check:

   Loop from 0 to Length-1 for string traversal [OK]

Hint: Use for loop with i from 0 to str.Length - 1 [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= instead of < in loop condition
• Using wrong variable type in foreach

3. What will be the output of the following code?

string s = "code";
for (int i = 0; i < s.Length; i++)
{
    if (i % 2 == 0)
        Console.Write(s[i]);
}

medium

A. co

B. oe

C. cd

D. code

Solution

1. Step 1: Understand the loop and condition

   The loop runs from i=0 to i=3 (length 4). It prints characters only when i is even (0, 2).

2. Step 2: Identify characters at even indices

   At i=0, s[0] = 'c'; at i=2, s[2] = 'd'. So output is "cd".

3. Final Answer:

   cd -> Option C

4. Quick Check:

   Print chars at even indices: 'c' and 'd' [OK]

Hint: Print characters where index % 2 == 0 [OK]

Common Mistakes:

• Confusing even and odd indices
• Printing all characters ignoring condition
• Misreading string length

4. Identify the error in the following code snippet:

string text = "example";
for (int i = 0; i <= text.Length; i++)
{
    Console.Write(text[i]);
}

medium

A. The loop condition should be < instead of <=

B. The string cannot be indexed with []

C. Variable i should be of type char

D. Console.Write should be Console.WriteLine

Solution

1. Step 1: Analyze loop condition and indexing

   The loop runs from i=0 to i <= text.Length. Since string indices go from 0 to Length-1, accessing text[Length] causes an error.

2. Step 2: Correct the loop condition

   Change condition to i < text.Length to avoid out-of-range exception.

3. Final Answer:

   The loop condition should be < instead of <= -> Option A

4. Quick Check:

   Loop must run from 0 to Length-1 [OK]

Hint: Use i < str.Length to avoid index out of range [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming string indexing is 1-based
• Changing variable type unnecessarily

5. Given a string input, which code snippet correctly counts how many vowels (a, e, i, o, u) it contains?

hard

A. int count = 0;
foreach (char c in input.ToLower()) {
if ("aeiou".Contains(c)) count++;
}
Console.WriteLine(count);

B. int count = 0;
for (int i = 1; i <= input.Length; i++) {
if ("aeiou".Contains(input[i])) count++;
}
Console.WriteLine(count);

C. int count = 0;
foreach (int c in input) {
if ("aeiou".Contains((char)c)) count++;
}
Console.WriteLine(count);

D. int count = 0;
for (char c = 0; c < input.Length; c++) {
if ("aeiou".Contains(input[c])) count++;
}
Console.WriteLine(count);

Solution

1. Step 1: Check correct iteration over string characters

   int count = 0; foreach (char c in input.ToLower()) { if ("aeiou".Contains(c)) count++; } Console.WriteLine(count); uses foreach over chars in input.ToLower(), which is correct. int count = 0; for (int i = 1; i <= input.Length; i++) { if ("aeiou".Contains(input[i])) count++; } Console.WriteLine(count); uses 1-based indexing causing out-of-range error. int count = 0; foreach (int c in input) { if ("aeiou".Contains((char)c)) count++; } Console.WriteLine(count); uses foreach over int which is incorrect type. int count = 0; for (char c = 0; c < input.Length; c++) { if ("aeiou".Contains(input[c])) count++; } Console.WriteLine(count); uses char as loop variable which is invalid.

2. Step 2: Verify vowel check and counting

   int count = 0; foreach (char c in input.ToLower()) { if ("aeiou".Contains(c)) count++; } Console.WriteLine(count); checks if character is in "aeiou" string and increments count correctly.

3. Final Answer:

   Option A code snippet correctly counts vowels -> Option A

4. Quick Check:

   Use foreach char and check vowels with Contains [OK]

Hint: Use foreach char in lowercase string and Contains("aeiou") [OK]

Common Mistakes:

• Using wrong loop indices causing errors
• Incorrect variable types in loops
• Not converting string to lowercase before checking