Writing to files in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used to open a file for writing in C?

The fopen() function is used to open a file. To open a file for writing, you use fopen(filename, "w").

beginner

How do you write a string to a file in C?

You use the fprintf() or fputs() functions. For example, fprintf(file, "%s", "Hello") writes the string "Hello" to the file.

beginner

What must you always do after finishing writing to a file?

You must close the file using fclose(file) to save changes and free resources.

intermediate

What happens if you open a file with mode "w" and the file already exists?

The existing file is cleared (emptied) before writing starts. If the file does not exist, a new file is created.

beginner

How can you check if a file was opened successfully?

After calling fopen(), check if the returned pointer is NULL. If it is NULL, the file did not open successfully.

Which mode opens a file for writing and clears its contents if it exists?

A "w"

B "r"

C "a"

D "r+"

Mode "w" opens a file for writing and clears existing content or creates a new file.

What does fclose() do in file handling?

A Deletes the file

B Opens the file for reading

C Reads data from the file

D Closes the file and saves changes

fclose() closes the file and ensures all data is saved properly.

Which function writes formatted text to a file?

A fread()

B fprintf()

C fclose()

D fopen()

fprintf() writes formatted text to a file.

How do you check if fopen() failed to open a file?

A Check if the file exists

B Check if the file size is zero

C Check if the returned pointer is NULL

D Check if the file is empty

If fopen() returns NULL, it means the file could not be opened.

Which function writes a string to a file without formatting?

A fputs()

B fprintf()

C fread()

D fclose()

fputs() writes a string directly to a file without formatting.

Explain the steps to write text to a file in C.

Think about opening, writing, and closing the file.

What happens if you open a file with "w" mode and the file already exists?

Consider how "w" mode treats existing files.

Practice

1. What does the mode "w" mean when used with fopen in C?

easy

A. Open a file for writing, creating it if it doesn't exist, or overwriting if it does.

B. Open a file for reading only.

C. Open a file for appending data at the end.

D. Open a file in binary mode for reading and writing.

Solution

1. Step 1: Understand fopen modes

   The mode "w" in fopen means open for writing. If the file exists, it will be overwritten. If it doesn't exist, it will be created.

2. Step 2: Compare with other modes

   Other modes like "r" are for reading, "a" for appending, and "wb" for binary write. "w" specifically means write with overwrite or create.

3. Final Answer:

   Open a file for writing, creating it if it doesn't exist, or overwriting if it does. -> Option A

4. Quick Check:

   fopen mode "w" = write with overwrite [OK]

Hint: Remember: "w" means write and overwrite file [OK]

Common Mistakes:

• Confusing "w" with "a" (append mode)
• Thinking "w" opens file for reading
• Assuming "w" preserves existing content

2. Which of the following is the correct way 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 = fopen("data.txt", "a");

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

Solution

1. Step 1: Identify correct function and mode

   In C, to open a file for writing, use fopen with mode "w". The function is fopen, not open.

2. Step 2: Check each option

   FILE *fp = fopen("data.txt", "r"); uses mode "r" (read), not writing. FILE *fp = fopen("data.txt", "a"); uses "a" (append). FILE *fp = open("data.txt", "w"); uses open, which is not standard C file open function.

3. Final Answer:

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

4. Quick Check:

   Use fopen with "w" to write [OK]

Hint: Use fopen with "w" to write files in C [OK]

Common Mistakes:

• Using open() instead of fopen()
• Using wrong mode like "r" or "a"
• Forgetting to use double quotes around filename

3. What will be the output in the file "output.txt" after running this code?

FILE *fp = fopen("output.txt", "w");
fprintf(fp, "Hello %s!", "World");
fclose(fp);

medium

A. World!

B. Hello %s!

C. Hello World!

D. Compilation error

Solution

1. Step 1: Understand fprintf usage

   fprintf writes formatted text to the file. The format string "Hello %s!" expects a string argument, which is "World" here.

2. Step 2: Determine file content

   The file will contain the formatted string with %s replaced by "World", so the file content is "Hello World!".

3. Final Answer:

   Hello World! -> Option C

4. Quick Check:

   fprintf formats string correctly [OK]

Hint: fprintf formats and writes text to file [OK]

Common Mistakes:

• Thinking %s prints literally
• Forgetting to pass argument for %s
• Assuming fprintf prints to console

4. Identify the error in this code snippet that writes "Test" to a file:

FILE *fp;
fp = fopen("test.txt", "w");
fprintf(fp, "Test");

medium

A. fopen mode "w" is incorrect for writing

B. File name should be a variable, not a string literal

C. fprintf syntax is wrong, needs format specifier

D. Missing fclose(fp) to close the file

Solution

1. Step 1: Check file opening and writing

   The file is opened correctly with fopen and mode "w". fprintf syntax is correct to write "Test".

2. Step 2: Check for missing file closure

   The code does not call fclose(fp), which is necessary to close the file and flush buffers properly.

3. Final Answer:

   Missing fclose(fp) to close the file -> Option D

4. Quick Check:

   Always close files with fclose [OK]

Hint: Always fclose after writing to file [OK]

Common Mistakes:

• Forgetting fclose causes data loss
• Confusing fopen modes
• Thinking fprintf needs format specifier always

5. You want to write multiple lines to a file "log.txt" in C. Which code snippet correctly writes these lines and ensures the file is properly handled?

1) FILE *fp = fopen("log.txt", "w");
fprintf(fp, "Line 1\n");
fprintf(fp, "Line 2\n");
fclose(fp);

2) FILE *fp = fopen("log.txt", "r");
fprintf(fp, "Line 1\n");
fclose(fp);

3) FILE *fp = fopen("log.txt", "w");
fprintf(fp, "Line 1\nLine 2\n");
// missing fclose

4) FILE *fp = fopen("log.txt", "a");
fprintf(fp, "Line 1\n");
fclose(fp);

hard

A. Options 1 and 4

B. Option 1 only

C. Option 3 only

D. Option 2 only

Solution

1. Step 1: Analyze each option's correctness

   Option 1 opens file with "w" (write), writes two lines separately, and closes file properly. This is correct.

2. Step 2: Check other options

   Option 2 opens with "r" (read), so writing causes error. Option 3 writes lines but misses fclose, risking data loss. Option 4 opens with "a" (append), writes only one line, but the goal is multiple lines.

3. Final Answer:

   Option 1 only -> Option B

4. Quick Check:

   "w" + multiple fprintf + fclose [OK]

Hint: Use "w" mode and always fclose [OK]

Common Mistakes:

• Using "r" mode when writing
• Forgetting fclose after writing
• Not understanding difference between "w" and "a"