Writing to files in C - Mini Project: Build & Apply

Writing to files

📖 Scenario: Imagine you want to save a list of your favorite fruits to a file on your computer so you can look at it later or share it with friends.

🎯 Goal: You will write a simple C program that creates a file and writes a list of fruits into it, one fruit per line.

📋 What You'll Learn

Create an array of strings with exact fruit names

Open a file for writing with the exact filename

Write each fruit to the file on its own line

Close the file properly

Print a confirmation message after writing

💡 Why This Matters

🌍 Real World

Saving data to files is common in many programs, like saving user preferences, logs, or reports.

💼 Career

Understanding file writing is essential for software developers working on data storage, configuration files, or any program that needs to save information.

1

Create the fruit list

Create a string array called fruits with these exact entries: "Apple", "Banana", "Cherry", "Date", "Elderberry".

C

# Create the array of fruits
// Your code here

Hint

Use char *fruits[] = { ... }; to create the array of fruit names.

2

Open the file for writing

Add a variable called file of type FILE * and open a file named "fruits.txt" for writing using fopen.

C

#include 

char *fruits[] = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

// Open the file "fruits.txt" for writing
// Your code here

Hint

Use FILE *file = fopen("fruits.txt", "w"); to open the file for writing.

3

Write fruits to the file

Use a for loop with an integer variable i from 0 to 4 to write each fruit from fruits[i] to the file using fprintf, adding a newline after each fruit.

C

#include 

char *fruits[] = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

int main() {
    FILE *file = fopen("fruits.txt", "w");
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    // Write each fruit to the file
    // Your code here

Hint

Use for (int i = 0; i < 5; i++) and inside the loop fprintf(file, "%s\n", fruits[i]);.

4

Close the file and print confirmation

Close the file using fclose(file) and then print "Fruits saved to fruits.txt" using printf. End the main function with return 0;.

C

#include 

char *fruits[] = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

int main() {
    FILE *file = fopen("fruits.txt", "w");
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    for (int i = 0; i < 5; i++) {
        fprintf(file, "%s\n", fruits[i]);
    }

    // Close the file and print confirmation
    // Your code here

Hint

Use fclose(file); to close the file and printf("Fruits saved to fruits.txt\n"); to print the message.

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"