Writing to files in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C program writing to a file?

Consider the following C code that writes to a file named output.txt. What will be the content of the file after running this program?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (fp == NULL) {
        return 1;
    }
    fprintf(fp, "Hello\n");
    fprintf(fp, "World\n");
    fclose(fp);
    return 0;
}

A

The file contains:
Hello
World

B

The file contains:
HelloWorld

C The program crashes and no file is created

D The file is empty

💡 Hint

Look at how fprintf is used with newline characters.

The program opens the file in write mode, writes "Hello" followed by a newline, then "World" followed by a newline, then closes the file. So the file contains two lines: "Hello" and "World".

❓ Predict Output

intermediate

What error does this code produce when writing to a file?

What error will this C code produce when executed?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("readonly.txt", "r");
    fprintf(fp, "Trying to write\n");
    fclose(fp);
    return 0;
}

A Segmentation fault at fprintf

B Runtime error: writing to a file opened in read mode

C No error, file is written successfully

D Compilation error: fprintf requires a writable file pointer

💡 Hint

Think about the mode used to open the file and what fprintf expects.

The file is opened in read mode, so writing to it with fprintf causes a runtime error because the file pointer is not writable.

🔧 Debug

advanced

Why does this file writing code produce no output in the file?

The following C code is supposed to write "Data" to data.txt, but the file remains empty after running. What is the cause?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "w");
    if (fp == NULL) {
        return 1;
    }
    fputs("Data", fp);
    // Missing fclose(fp);
    return 0;
}

A The file pointer is NULL, so nothing is written

B The file is opened in write mode but no data is written

C fputs does not write to files, only to stdout

D The file is not closed, so output is buffered and not flushed to disk

💡 Hint

Think about how buffered output works and what happens if you don't close the file.

Output to files is buffered. Without calling fclose or fflush, the buffer may not be written to the file, leaving it empty.

📝 Syntax

advanced

Which option contains a syntax error in file writing code?

Identify the option that contains a syntax error preventing compilation.

A FILE *fp = fopen("file.txt", "w") if (fp == NULL) { return 1; } fprintf(fp, "Hello"); fclose(fp);

B FILE *fp = fopen("file.txt", "w"); if (fp == NULL) { return 1; } fprintf(fp, "Hello"); fclose(fp);

C FILE *fp = fopen("file.txt", "w"); if (fp != NULL) { fprintf(fp, "Hello"); fclose(fp); }

D ;)pf(esolcf ;)"olleH" ,pf(ftnirpf } ;1 nruter { )LLUN == pf( fi ;)"w" ,"txt.elif"(nepof = pf* ELIF

💡 Hint

Look carefully for missing punctuation in the code.

Option A is missing a semicolon after the fopen call, causing a syntax error.

🚀 Application

expert

How many bytes are written to the file by this program?

This C program writes a string to a file. How many bytes will the file contain after running?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("test.txt", "w");
    if (!fp) return 1;
    const char *str = "Line1\nLine2\n";
    int count = fprintf(fp, "%s", str);
    fclose(fp);
    return 0;
}

A 10 bytes

B 11 bytes

C 12 bytes

D 13 bytes

💡 Hint

Count all characters including newline characters in the string.

The string "Line1\nLine2\n" has 12 characters: 5 letters + 1 newline + 5 letters + 1 newline = 12 bytes.

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"