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.

Practice

1. Which function is used to read a single word string from the user in C?

easy

A. fgets(str, size, stdin);

B. printf("%s", str);

C. gets(str);

D. scanf("%s", str);

Solution

1. Step 1: Understand string input functions

   scanf with "%s" reads a single word string until whitespace.

2. Step 2: Identify correct function for single word input

   printf is for output, gets is unsafe, fgets reads whole line including spaces.

3. Final Answer:

   scanf("%s", str); -> Option D

4. Quick Check:

   Single word input = scanf("%s", str); [OK]

Hint: Use scanf with %s for single word input [OK]

Common Mistakes:

• Using printf instead of scanf for input
• Using fgets expecting single word input
• Using gets which is unsafe

2. Which of the following is the correct syntax to print a string variable str in C?

easy

A. printf("%d", str);

B. printf("%s", str);

C. scanf("%s", str);

D. gets(str);

Solution

1. Step 1: Identify the print format specifier for strings

   "%s" is used in printf to print strings.

2. Step 2: Match correct function and format

   printf with "%d" is for integers, scanf is for input, gets reads input.

3. Final Answer:

   printf("%s", str); -> Option B

4. Quick Check:

   Print string = printf("%s", str); [OK]

Hint: Use printf with %s to print strings [OK]

Common Mistakes:

• Using %d to print strings
• Using scanf instead of printf for output
• Using gets for printing

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

#include <stdio.h>
int main() {
    char str[20];
    scanf("%s", str);
    printf("%s", str);
    return 0;
}

If the user inputs: Hello World

medium

A. Hello

B. Hello World

C. World

D. Compilation error

Solution

1. Step 1: Understand scanf with %s behavior

   scanf("%s", str) reads input until first whitespace, so it reads only "Hello".

2. Step 2: Output printed by printf

   printf("%s", str) prints the stored string, which is "Hello".

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   scanf with %s stops at space = Hello [OK]

Hint: scanf %s reads only until space, not full line [OK]

Common Mistakes:

• Expecting scanf %s to read full line with spaces
• Confusing scanf with fgets
• Assuming output includes 'World'

4. Identify the error in the following code snippet for reading a full line including spaces:

#include <stdio.h>
int main() {
    char str[50];
    scanf("%s", str);
    printf("%s", str);
    return 0;
}

medium

A. char array size is too large

B. printf("%s", str) cannot print strings

C. scanf("%s", str) reads only until first space, not full line

D. Missing return type for main

Solution

1. Step 1: Analyze scanf with %s for input

   scanf("%s", str) stops reading input at first whitespace, so it cannot read full line with spaces.

2. Step 2: Check other parts of code

   printf("%s", str) correctly prints string, array size is sufficient, main has return type int.

3. Final Answer:

   scanf("%s", str) reads only until first space, not full line -> Option C

4. Quick Check:

   scanf %s stops at space, so not full line input [OK]

Hint: Use fgets to read full line with spaces [OK]

Common Mistakes:

• Assuming scanf %s reads full line
• Thinking printf can't print strings
• Confusing array size issues

5. How can you correctly read a full line of text including spaces into a string str of size 100 in C?

hard

A. fgets(str, 100, stdin);

B. scanf("%s", str);

C. printf("%s", str);

D. gets(str);

Solution

1. Step 1: Understand functions for reading strings with spaces

   scanf("%s", str) reads only until space, gets is unsafe and deprecated.

2. Step 2: Identify safe function to read full line

   fgets(str, 100, stdin) reads up to 99 characters including spaces safely.

3. Final Answer:

   fgets(str, 100, stdin); -> Option A

4. Quick Check:

   Use fgets for full line input with spaces [OK]

Hint: Use fgets to read lines with spaces safely [OK]

Common Mistakes:

• Using scanf %s for full line input
• Using gets despite safety issues
• Confusing printf with input functions