String to Integer atoi in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of atoi with leading spaces and signs

What is the output of the following C code that uses atoi to convert a string with spaces and signs?

DSA C

#include <stdio.h>
#include <stdlib.h>

int main() {
    char str[] = "   -1234abc";
    int val = atoi(str);
    printf("%d\n", val);
    return 0;
}

A 0

B -1234

C 1234

D Syntax error

💡 Hint

atoi ignores leading spaces and stops at first non-digit after optional sign.

The atoi function skips leading spaces, reads the optional '-' sign, then converts digits until a non-digit character is found. So it converts " -1234abc" to -1234.

❓ Predict Output

intermediate

Result of atoi on string with no digits

What will be printed by this C program using atoi on a string with no digits?

DSA C

#include <stdio.h>
#include <stdlib.h>

int main() {
    char str[] = "abc";
    int val = atoi(str);
    printf("%d\n", val);
    return 0;
}

A 0

B Garbage value

C abc

D Runtime error

💡 Hint

atoi returns 0 if no valid conversion is possible.

Since the string "abc" does not start with digits or sign, atoi returns 0.

❓ Predict Output

advanced

Output of atoi with multiple signs

What is the output of this C code using atoi on a string with multiple signs?

DSA C

#include <stdio.h>
#include <stdlib.h>

int main() {
    char str[] = "--123";
    int val = atoi(str);
    printf("%d\n", val);
    return 0;
}

A Undefined behavior

B -123

C 123

D 0

💡 Hint

atoi stops conversion if invalid sign sequence is found.

The string "--123" is invalid for atoi because it expects at most one sign at the start. Conversion stops immediately and returns 0.

❓ Predict Output

advanced

Value returned by atoi on integer overflow

What is the output of this C program when atoi converts a string representing a number larger than INT_MAX?

DSA C

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>

int main() {
    char str[] = "999999999999999999999";
    int val = atoi(str);
    printf("%d\n", val);
    return 0;
}

A Implementation-defined value (overflow)

B 2147483647

C 0

D -1

💡 Hint

atoi does not handle overflow safely; result is undefined.

The atoi function does not check for overflow. Converting a very large number causes undefined behavior and the returned value depends on the system.

🧠 Conceptual

expert

Behavior of atoi with embedded null character

Consider the string literal "123\0abc" passed to atoi. What integer value does atoi return?

A 1230

B 0

C 123

D Runtime error

💡 Hint

Strings in C end at the first null character.

The embedded null character '\0' marks the end of the string. atoi reads only "123" and returns 123.

Practice

1. What does the atoi function do in C programming?

easy

A. Checks if a string contains only digits

B. Converts a string representing a number into an integer

C. Calculates the length of a string

D. Converts an integer into a string

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function stands for ASCII to integer, which means it converts a string of digits into an integer value.

2. Step 2: Compare options with definition

   Only Converts a string representing a number into an integer correctly describes this conversion from string to integer.

3. Final Answer:

   Converts a string representing a number into an integer -> Option B

4. Quick Check:

   atoi = string to integer conversion [OK]

Hint: Remember atoi means ASCII to integer conversion [OK]

Common Mistakes:

• Confusing atoi with itoa (integer to ASCII)
• Thinking atoi calculates string length
• Assuming atoi validates string digits

2. Which of the following is the correct syntax to use atoi in C?

easy

A. int num = atoi(char *str);

B. int num = atoi();

C. int num = atoi(int str);

D. int num = atoi(str);

Solution

1. Step 1: Check function parameter type

   The atoi function takes a single argument of type const char *, which is a string pointer.

2. Step 2: Validate correct syntax

   int num = atoi(str); correctly shows calling atoi with a string variable str without specifying type in call.

3. Final Answer:

   int num = atoi(str); -> Option D

4. Quick Check:

   atoi takes string variable as argument [OK]

Hint: Call atoi with just the string variable name [OK]

Common Mistakes:

• Including type in function call parentheses
• Calling atoi without arguments
• Passing integer instead of string

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

char str[] = "-123abc";
int val = atoi(str);
printf("%d", val);

medium

A. -123

B. 0

C. 123

D. Error

Solution

1. Step 1: Analyze atoi behavior with mixed string

   atoi converts initial numeric part including sign until non-digit character. Here, "-123" is converted, stops at 'a'.

2. Step 2: Determine output value

   So, val becomes -123 and printf prints -123.

3. Final Answer:

   -123 -> Option A

4. Quick Check:

   atoi stops at first non-digit, returns -123 [OK]

Hint: atoi reads number until first non-digit character [OK]

Common Mistakes:

• Expecting atoi to ignore leading sign
• Thinking atoi returns 0 on trailing letters
• Assuming runtime error due to letters

4. Identify the error in this code snippet using atoi:

char *str = NULL;
int num = atoi(str);
printf("%d", num);

medium

A. Passing NULL pointer to atoi causes undefined behavior

B. Missing header file

C. atoi cannot convert strings with digits

D. No error, code runs fine

Solution

1. Step 1: Check argument passed to atoi

   The code passes a NULL pointer to atoi, which expects a valid string pointer.

2. Step 2: Understand consequences of NULL argument

   Passing NULL leads to undefined behavior, often causing a crash or runtime error.

3. Final Answer:

   Passing NULL pointer to atoi causes undefined behavior -> Option A

4. Quick Check:

   atoi needs valid string pointer, NULL causes error [OK]

Hint: Never pass NULL to atoi; always check string pointer [OK]

Common Mistakes:

• Ignoring NULL pointer checks
• Assuming atoi handles NULL safely
• Confusing missing header with runtime error

5. You want to convert a string to integer but also detect if the string contains invalid characters after the number. Which approach is best in C?

hard

A. Use atoi and then check the rest of the string manually

B. Use atoi and assume input is always valid

C. Use strtol which provides end pointer to check leftover characters

D. Use printf to convert string to integer

Solution

1. Step 1: Understand limitation of atoi

   atoi converts initial number but does not provide info about leftover invalid characters.

2. Step 2: Use strtol for better control

   strtol converts string to long and returns pointer to first invalid character, allowing validation.

3. Step 3: Choose best approach

   Use strtol which provides end pointer to check leftover characters uses strtol with end pointer to detect invalid trailing characters safely.

4. Final Answer:

   Use strtol which provides end pointer to check leftover characters -> Option C

5. Quick Check:

   strtol + end pointer = safe string to int with validation [OK]

Hint: Use strtol with end pointer to detect invalid chars [OK]

Common Mistakes:

• Relying only on atoi for input validation
• Ignoring leftover characters after number
• Using printf incorrectly for conversion