String Traversal and Character Access in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is string traversal in C?

String traversal in C means going through each character of a string one by one, usually using a loop, until the end of the string is reached.

beginner

How do you access the third character of a string named str in C?

You access it using str[2] because indexing starts at 0, so the first character is str[0], second is str[1], and third is str[2].

beginner

What marks the end of a string in C?

A string in C ends with a special character called the null character, written as '\0'. It tells the program where the string stops.

intermediate

Why is it important to check for the null character during string traversal?

Because the null character '\0' marks the end of the string. If you don't check for it, you might read beyond the string and get wrong data or cause errors.

beginner

Show a simple C code snippet to print each character of a string str on a new line.

#include int main() { char str[] = "hello"; int i = 0; while (str[i] != '\0') { printf("%c\n", str[i]); i++; } return 0; }

What does the expression str[i] represent in C when str is a string?

A The character at position i in the string

B The length of the string

C The memory address of the string

D The whole string

str[i] accesses the character at index i in the string str.

Which character indicates the end of a string in C?

A '\n'

B '\0'

C ' ' (space)

D '\t'

The null character '\0' marks the end of a string in C.

What will happen if you forget to check for the null character while traversing a string in C?

A The string length will increase

B The program will stop immediately

C The string will become empty

D You might read garbage values beyond the string

Without checking for '\0', traversal can go past the string and read invalid memory.

If char str[] = "abc";, what is the value of str[3]?

A '\0'

B 'a'

C 'c'

D Undefined

The string "abc" is stored as 'a', 'b', 'c', '\0'. So str[3] is the null character.

Which loop is commonly used to traverse a string until its end in C?

A for loop with condition <code>i < length</code>

B do-while loop without condition

C while loop checking <code>str[i] != '\0'</code>

D infinite loop

A while loop checking for str[i] != '\0' is common for string traversal.

Explain how to traverse a string in C and access each character safely.

Think about how to stop at the end of the string.

Describe why the null character is important in C strings and what could happen if it is ignored during traversal.

Consider what tells the program where the string ends.

Practice

1. What does the expression str[0] represent when str is a string in C?

easy

A. The first character of the string

B. The length of the string

C. The last character of the string

D. A pointer to the string

Solution

1. Step 1: Understand string indexing in C

   In C, strings are arrays of characters. Indexing starts at 0, so str[0] accesses the first character.

2. Step 2: Identify what str[0] returns

   It returns the character stored at the first position of the string, not the length or pointer.

3. Final Answer:

   The first character of the string -> Option A

4. Quick Check:

   str[0] = first character [OK]

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

Common Mistakes:

• Confusing character access with string length
• Thinking indexing starts at 1
• Mixing pointer and character access

2. Which of the following is the correct way to traverse a string str of length n in C using a for loop?

easy

A. for (int i = 1; i < n; i++)

B. for (int i = 0; i <= n; i++)

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

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

Solution

1. Step 1: Understand string indexing and loop bounds

   String indices in C start at 0 and go up to n-1 for length n.

2. Step 2: Choose loop condition to cover all characters

   The loop should run from 0 to n-1, so i < n is correct.

3. Final Answer:

   for (int i = 0; i < n; i++) -> Option C

4. Quick Check:

   Loop from 0 to n-1 = for (i=0; i<n; i++) [OK]

Hint: Loop from 0 to less than length for strings [OK]

Common Mistakes:

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

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

char str[] = "hello";
for (int i = 0; i < 5; i++) {
    printf("%c-", str[i]);
}

medium

A. hello-

B. h-e-l-l-o-

C. h e l l o

D. h-e-l-l-o

Solution

1. Step 1: Analyze the loop and print statement

   The loop runs from i=0 to 4, printing each character followed by a dash.

2. Step 2: Trace each iteration output

   Characters printed: 'h-', 'e-', 'l-', 'l-', 'o-'. Combined output is "h-e-l-l-o-".

3. Final Answer:

   h-e-l-l-o- -> Option B

4. Quick Check:

   Each char + '-' printed = h-e-l-l-o- [OK]

Hint: Look at printf format and loop range carefully [OK]

Common Mistakes:

• Ignoring the dash after each character
• Printing the whole string once instead of chars
• Confusing spaces with dashes

4. Identify the error in the following code snippet that tries to print each character of a string str.

char str[] = "world";
for (int i = 1; i <= 5; i++) {
    printf("%c", str[i]);
}

medium

A. Both A and B are errors

B. Loop condition should be i < 5 instead of <= 5

C. No error, code works fine

D. Loop starts at 1 and misses the first character

Solution

1. Step 1: Check loop start index

   Loop starts at 1, so it skips str[0] which is the first character.

2. Step 2: Check loop end condition

   Loop runs while i <= 5, but valid indices are 0 to 4. Accessing str[5] causes out-of-bounds error.

3. Final Answer:

   Both A and B are errors -> Option A

4. Quick Check:

   Start at 0 and use i < length to avoid errors [OK]

Hint: Loop from 0 to less than length to avoid errors [OK]

Common Mistakes:

• Starting loop at 1 and missing first char
• Using <= length causing out-of-bounds access
• Ignoring array indexing rules

5. Given a string str of unknown length, which of the following code snippets correctly prints all characters until the null terminator in C?

hard

A. for (int i = 0; str[i] != '\0'; i++) { printf("%c", str[i]); }

B. for (int i = 0; i < strlen(str); i++) { printf("%c", str[i]); }

C. while (*str != '\0') { printf("%c", *str); str++; }

D. All of the above

Solution

1. Step 1: Analyze for (int i = 0; str[i] != '\0'; i++) { printf("%c", str[i]); }

   Loops until character is not null terminator, printing each character. Correct method.

2. Step 2: Analyze for (int i = 0; i < strlen(str); i++) { printf("%c", str[i]); } and C

   for (int i = 0; i < strlen(str); i++) { printf("%c", str[i]); } uses strlen to get length, then loops printing characters. while (*str != '\0') { printf("%c", *str); str++; } uses pointer traversal until null terminator. Both are correct.

3. Step 3: Combine results

   All three methods correctly print the string characters until the null terminator.

4. Final Answer:

   All of the above -> Option D

5. Quick Check:

   All methods stop at '\0' and print chars [OK]

Hint: Any loop stopping at '\0' prints full string safely [OK]

Common Mistakes:

• Not checking for null terminator causing overflow
• Using wrong loop conditions
• Modifying string pointer incorrectly