String input and output in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C code snippet?

Consider the following C program. What will it print when run?

C

#include <stdio.h>

int main() {
    char str[10];
    scanf("%s", str);
    printf("Hello, %s!", str);
    return 0;
}

A Compilation error

B Hello, !

C Hello, world!\n

D Hello, world!

💡 Hint

Think about what scanf with %s reads and how printf prints it.

The scanf with %s reads a word until whitespace. If the input is 'world', it stores it in str. Then printf prints 'Hello, world!'.

❓ Predict Output

intermediate

What does this code print?

What is the output of this C program?

C

#include <stdio.h>

int main() {
    char s[6] = "Hello";
    printf("%s\n", s);
    return 0;
}

A Hello

B Hello\n

C Hello\0

D Compilation error

💡 Hint

Remember how strings are stored and printed in C.

The string "Hello" fits in the array s with the null terminator. printf prints the string until the null character, so output is Hello.

❓ Predict Output

advanced

What is the output of this code with fgets?

What will this C program print if the user inputs: Hello World\n?

C

#include <stdio.h>

int main() {
    char buffer[6];
    fgets(buffer, 6, stdin);
    printf("%s", buffer);
    return 0;
}

A Hello World

B Hello

C Hello\n

D Hello\0

💡 Hint

fgets reads up to size-1 characters including newline if present.

fgets reads up to 5 characters (6-1) including the newline if it fits. Input is 'Hello World\n', so buffer contains 'Hello\0'. printf prints 'Hello'.

❓ Predict Output

advanced

What error does this code cause?

What happens when this C code runs?

C

#include <stdio.h>

int main() {
    char str[5];
    scanf("%s", str);
    printf("%s", str);
    return 0;
}

A Runtime error: segmentation fault always

B Compilation error due to missing semicolon

C Buffer overflow if input longer than 4 characters

D Prints input safely

💡 Hint

Think about how scanf reads strings and array size.

The array str can hold 4 characters plus null terminator. If input is longer than 4 characters, scanf writes beyond array bounds causing buffer overflow.

🧠 Conceptual

expert

How many characters are read by fgets in this code?

Given this code, how many characters maximum will fgets read from input?

C

#include <stdio.h>

int main() {
    char input[10];
    fgets(input, sizeof(input), stdin);
    return 0;
}

A 9 characters plus the null terminator

B 10 characters plus the null terminator

C Exactly 10 characters, no null terminator

D 8 characters plus the null terminator

💡 Hint

Remember how fgets uses the size parameter.

fgets reads up to size-1 characters and adds a null terminator. Here size is 10, so it reads up to 9 characters plus null terminator.