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.