String input and output in C - Mini Project: Build & Apply

String input and output

📖 Scenario: You are creating a simple program that asks a user for their name and then greets them.

🎯 Goal: Build a program that reads a user's name as input and then prints a greeting message including that name.

📋 What You'll Learn

Use char name[50]; to store the user's name

Use scanf to read the name from input

Use printf to display the greeting message

💡 Why This Matters

🌍 Real World

Reading and displaying text input is a basic skill used in many programs like user registration, chat apps, and command-line tools.

💼 Career

Understanding string input/output in C is essential for system programming, embedded systems, and software development roles.

1

Create a character array to store the name

Declare a character array called name with size 50 to hold the user's name.

C

#include 

int main() {
    char name[50];  // Your code here

    return 0;
}

Hint

Use char name[50]; inside main() to create space for the name.

2

Read the user's name using scanf

Use scanf with "%49s" to read the user's name into the name array.

C

#include 

int main() {
    char name[50];
    // Read the user's name
    scanf("%49s", name);  // Your code here

    return 0;
}

Hint

Use scanf("%49s", name); to safely read a string into name.

3

Print a greeting message with the name

Use printf to display the message Hello, <name>! where <name> is the value stored in name.

C

#include 

int main() {
    char name[50];
    scanf("%49s", name);
    // Print greeting message
    printf("Hello, %s!\n", name);  // Your code here

    return 0;
}

Hint

Use printf("Hello, %s!\n", name); to show the greeting with the user's name.

4

Run the program and display the greeting

Run the program and enter the name Alice when prompted. The program should print Hello, Alice!.

C

#include 

int main() {
    char name[50];
    scanf("%49s", name);
    printf("Hello, %s!\n", name);

    return 0;
}

// Run the program and check output

Hint

Type Alice as input and check that the output is Hello, Alice!

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