String input and output in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the string stored in name.

C

#include <stdio.h>

int main() {
    char name[20] = "Alice";
    printf([1], name);
    return 0;
}

Drag options to blanks, or click blank then click option'

A"%s\n"

B"%d\n"

C"%c\n"

D"%f\n"

Attempts:

3 left

💡 Hint

Common Mistakes

Using %d or %c instead of %s for strings.

Forgetting to add quotes around the format specifier.

Use "%s" in printf to print a string variable.

2fill in blank

medium

Complete the code to read a string input from the user into the array input.

C

#include <stdio.h>

int main() {
    char input[50];
    printf("Enter your name: ");
    scanf([1], input);
    printf("Hello, %s!\n", input);
    return 0;
}

Drag options to blanks, or click blank then click option'

A"%d"

B"%s"

C"%c"

D"%f"

Attempts:

3 left

💡 Hint

Common Mistakes

Using %d or %c instead of %s in scanf.

Adding & before the array name when reading strings.

Use "%s" in scanf to read a string input.

3fill in blank

hard

Fix the error in the code to correctly read a string with spaces from the user.

C

#include <stdio.h>

int main() {
    char fullName[100];
    printf("Enter your full name: ");
    fgets([1], 100, stdin);
    printf("Hello, %s", fullName);
    return 0;
}

Drag options to blanks, or click blank then click option'

AfullName[100]

B&fullName

CfullName

D*fullName

Attempts:

3 left

💡 Hint

Common Mistakes

Using & before the array name with fgets.

Using array indexing like fullName[100] which is out of bounds.

fgets requires the array name without & to read the string including spaces.

4fill in blank

hard

Fill both blanks to create a string copy using strcpy.

C

#include <stdio.h>
#include <string.h>

int main() {
    char source[] = "Hello";
    char destination[20];
    [1](destination, [2]);
    printf("Copied string: %s\n", destination);
    return 0;
}

Drag options to blanks, or click blank then click option'

Astrcpy

Bstrncpy

Csource

Ddestination

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping source and destination arguments.

Using strncpy without specifying length.

Use strcpy(destination, source) to copy the string from source to destination.

5fill in blank

hard

Fill all three blanks to concatenate two strings safely using strncat.

C

#include <stdio.h>
#include <string.h>

int main() {
    char str1[20] = "Hello, ";
    char str2[] = "World!";
    [1](str1, [2], [3]);
    printf("Concatenated string: %s\n", str1);
    return 0;
}

Drag options to blanks, or click blank then click option'

Astrncat

Bstr2

C6

Dstrlen(str2)

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect length value for strncat.

Swapping the order of arguments.

Use strncat(str1, str2, strlen(str2)) to concatenate str2 to str1 safely.

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