Opening and closing files - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a file named "data.txt" for reading.

C

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "[1]");
    if (file == NULL) {
        perror("Error opening file");
        return 1;
    }
    fclose(file);
    return 0;
}

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

Ar

Ba

Cx

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using "w" which opens the file for writing and truncates it.

Using "a" which opens the file for appending.

Using "x" which creates a new file but fails if it exists.

The mode "r" opens the file for reading. If the file does not exist, fopen returns NULL.

2fill in blank

medium

Complete the code to close the opened file pointer named "fp".

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("log.txt", "w");
    if (fp == NULL) {
        return 1;
    }
    // Write something to the file
    fprintf(fp, "Hello, file!\n");
    [1](fp);
    return 0;
}

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

Afopen

Bclose

Copen

Dfclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using close(fp) which is not a standard C function for FILE pointers.

Using fopen(fp) which opens files, not closes.

Using open(fp) which is a system call, not for FILE pointers.

The function fclose(fp) closes the file associated with the file pointer fp.

3fill in blank

hard

Fix the error in the code to open a file for appending text.

C

#include <stdio.h>

int main() {
    FILE *file = fopen("output.txt", "[1]");
    if (file == NULL) {
        perror("Failed to open file");
        return 1;
    }
    fprintf(file, "Appending this line.\n");
    fclose(file);
    return 0;
}

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

Ar

Bw

Ca

Drw

Attempts:

3 left

💡 Hint

Common Mistakes

Using "r" which opens for reading only.

Using "w" which truncates the file.

Using "rw" which is not a valid mode string.

The mode "a" opens the file for appending. It writes data at the end of the file without truncating it.

4fill in blank

hard

Fill both blanks to open a file for writing and check if it opened successfully.

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("test.txt", "[1]");
    if (fp == [2]) {
        perror("Cannot open file");
        return 1;
    }
    fclose(fp);
    return 0;
}

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

Aw

Br

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using "r" mode which opens for reading, not writing.

Checking if fp equals 0 instead of NULL.

Using wrong mode strings.

Use "w" to open the file for writing. fopen returns NULL if it fails to open the file.

5fill in blank

hard

Fill all three blanks to open a file for reading, check for errors, and close it.

C

#include <stdio.h>

int main() {
    FILE *file = fopen("input.txt", "[1]");
    if (file == [2]) {
        perror("Error opening file");
        return 1;
    }
    // File operations here
    [3](file);
    return 0;
}

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

Aw

BNULL

Cfclose

Dr

Attempts:

3 left

💡 Hint

Common Mistakes

Using "w" mode which overwrites the file.

Checking if file pointer equals 0 instead of NULL.

Forgetting to close the file.

Open the file with mode "r" for reading. Check if the file pointer is NULL to detect errors. Use fclose to close the file.

Practice

1. What does the fopen function do in C programming?

easy

A. It closes an open file stream.

B. It opens a file and returns a pointer to the file stream.

C. It reads data from a file.

D. It writes data to a file.

Solution

1. Step 1: Understand the purpose of fopen

   The fopen function is used to open a file and returns a pointer to the file stream if successful.

2. Step 2: Differentiate fopen from other file functions

   Functions like fclose close files, fread reads data, and fwrite writes data. Only fopen opens files.

3. Final Answer:

   It opens a file and returns a pointer to the file stream. -> Option B

4. Quick Check:

   fopen = open file pointer [OK]

Hint: fopen opens files, fclose closes them [OK]

Common Mistakes:

• Confusing fopen with fclose
• Thinking fopen reads or writes data
• Not knowing fopen returns NULL on failure

2. Which of the following is the correct syntax to open a file named "data.txt" for writing in C?

easy

A. FILE *fp = fopen("data.txt", "w");

B. FILE *fp = fopen("data.txt", "r");

C. FILE *fp = open("data.txt", "w");

D. FILE fp = fopen("data.txt", "w");

Solution

1. Step 1: Identify correct fopen syntax

   The correct syntax uses FILE * pointer and fopen with filename and mode as strings.

2. Step 2: Check mode for writing

   Mode "w" opens the file for writing. "r" is for reading, and open is not a standard C function for files.

3. Final Answer:

   FILE *fp = fopen("data.txt", "w"); -> Option A

4. Quick Check:

   fopen + "w" = open for writing [OK]

Hint: Use FILE * and fopen with mode "w" to write [OK]

Common Mistakes:

• Using open instead of fopen
• Declaring FILE variable instead of pointer
• Using wrong mode like "r" for writing

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

FILE *fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    printf("File not found\n");
} else {
    printf("File opened successfully\n");
    fclose(fp);
}

medium

A. File opened successfully

B. No output

C. File not found

D. Compilation error

Solution

1. Step 1: Understand fopen with "r" mode on nonexistent file

   Opening a file for reading with "r" mode returns NULL if the file does not exist.

2. Step 2: Check the if condition

   The code checks if fp == NULL, then prints "File not found". Since file does not exist, this condition is true.

3. Final Answer:

   File not found -> Option C

4. Quick Check:

   fopen returns NULL if file missing [OK]

Hint: fopen returns NULL if file missing in "r" mode [OK]

Common Mistakes:

• Assuming fopen creates file in "r" mode
• Not checking for NULL before using file pointer
• Expecting "File opened successfully" always

4. Identify the error in the following code snippet:

FILE *fp;
fp = fopen("file.txt", "w");
printf("File opened\n");
// Missing fclose here

medium

A. fopen mode "w" is incorrect

B. File pointer is not checked for NULL

C. printf statement is incorrect

D. fclose is missing to close the file

Solution

1. Step 1: Check fopen usage and mode

   The fopen call with mode "w" is correct to open file for writing.

2. Step 2: Identify missing fclose

   The code opens the file but never closes it with fclose, which can cause resource leaks.

3. Final Answer:

   fclose is missing to close the file -> Option D

4. Quick Check:

   Always fclose after fopen [OK]

Hint: Always close files with fclose after opening [OK]

Common Mistakes:

• Ignoring fclose causes resource leaks
• Not checking fopen return value
• Thinking printf affects file handling

5. You want to append text to an existing file "log.txt" safely. Which code snippet correctly opens the file, checks for errors, writes "Hello\n", and closes the file?

hard

A. FILE *fp = fopen("log.txt", "a");
if (fp == NULL) {
perror("Error opening file");
} else {
fprintf(fp, "Hello\n");
fclose(fp);
}

B. FILE *fp = fopen("log.txt", "w");
if (fp != NULL) {
fprintf(fp, "Hello\n");
fclose(fp);
}

C. FILE *fp = fopen("log.txt", "r");
fprintf(fp, "Hello\n");
fclose(fp);

D. FILE *fp = fopen("log.txt", "a");
fprintf(fp, "Hello\n");
// no fclose

Solution

1. Step 1: Choose correct mode for appending

   Mode "a" opens file for appending without erasing existing content.

2. Step 2: Check for NULL and handle errors

   Always check if fopen returns NULL before writing. Use perror to show error if NULL.

3. Step 3: Write and close file properly

   Use fprintf to write text and fclose to close file to avoid leaks.

4. Final Answer:

   FILE *fp = fopen("log.txt", "a"); if (fp == NULL) { perror("Error opening file"); } else { fprintf(fp, "Hello\n"); fclose(fp); } -> Option A

5. Quick Check:

   Append mode + check NULL + fclose = safe file write [OK]

Hint: Use "a" mode, check NULL, fclose after writing [OK]

Common Mistakes:

• Using "w" mode erases file instead of appending
• Not checking fopen return value before writing
• Forgetting to close file with fclose