String input and output in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is commonly used in C to read a string from the user?

The scanf function is commonly used to read strings from the user in C, often with the format specifier %s.

beginner

How do you print a string stored in a variable in C?

Use the printf function with the format specifier %s to print a string variable, for example: printf("%s", str);

intermediate

What is a safe way to read a string input to avoid buffer overflow?

Use fgets instead of scanf to read strings safely by specifying the maximum number of characters to read, e.g., fgets(str, sizeof(str), stdin);

intermediate

What happens if you use scanf("%s", str); and the input is longer than the buffer?

It causes a buffer overflow, which can overwrite memory and lead to crashes or security issues because scanf does not limit input length.

beginner

How do you declare a string variable in C?

A string in C is an array of characters ending with a null character \0. For example: char str[100]; declares a string that can hold up to 99 characters plus the null terminator.

Which function is safer to read a string input in C?

A fgets

B scanf with %s

C gets

D printf

fgets is safer because it limits the number of characters read, preventing buffer overflow. gets is unsafe and deprecated. scanf with %s does not limit input size.

What format specifier is used to print a string in C?

A %d

B %s

C %f

D %c

The %s format specifier is used with printf to print strings.

What does the null character \0 signify in a C string?

A Space character

B Start of the string

C End of the string

D New line

The null character \0 marks the end of a string in C.

Which of these is a correct way to declare a string that can hold 50 characters?

A char str;

B int str[50];

C string str;

D char str[51];

Strings in C are arrays of characters, so char str[51]; declares a string with space for 50 characters plus the null terminator.

What is the risk of using scanf("%s", str); without limiting input size?

A Buffer overflow

B Syntax error

C No risk

D Memory leak

Using scanf without limiting input size can cause buffer overflow, which may crash the program or cause security issues.

Explain how to safely read and print a string in C.

Think about how to avoid buffer overflow and how strings end in C.

Describe the difference between using scanf and fgets for string input in C.

Consider safety and how input is stored.

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