File pointers in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this file pointer code?

Consider the following C code that writes and reads from a file using file pointers. What will be printed on the screen?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("test.txt", "w+");
    fputs("Hello, world!", fp);
    fseek(fp, 7, SEEK_SET);
    char buffer[6];
    fgets(buffer, 6, fp);
    printf("%s\n", buffer);
    fclose(fp);
    return 0;
}

A world

B world!

C Hello

D Hello,

💡 Hint

Remember that fgets reads up to n-1 characters and adds a null terminator.

The code writes "Hello, world!" to the file, then moves the file pointer to position 7 (the 'w' in "world!"). fgets(buffer, 6, fp) reads 5 characters plus the null terminator. So it reads "world" (5 chars) and stops before the '!'.

❓ Predict Output

intermediate

What does this code print after reading a file?

Given this code snippet, what will be the output?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    if (!fp) {
        printf("File not found\n");
        return 1;
    }
    fseek(fp, 0, SEEK_END);
    long size = ftell(fp);
    printf("%ld\n", size);
    fclose(fp);
    return 0;
}

A 0

B The size of the file in bytes

C File not found

D Undefined behavior

💡 Hint

Think about what ftell returns after seeking to the end of the file.

The code opens the file in read mode, moves the file pointer to the end using fseek, then uses ftell to get the current position, which equals the file size in bytes.

🔧 Debug

advanced

Why does this code cause a runtime error?

Examine the code below. It attempts to read a line from a file. Why does it cause a runtime error?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("input.txt", "r");
    char *line;
    if (fp == NULL) {
        return 1;
    }
    fgets(line, 100, fp);
    printf("%s", line);
    fclose(fp);
    return 0;
}

A The pointer 'line' is uninitialized and points to an undefined memory location

B The fgets call uses wrong parameters

C The file pointer 'fp' is not checked for NULL

D The file 'input.txt' does not exist

💡 Hint

Think about where line points before calling fgets.

The variable line is a pointer but it is not initialized to point to valid memory. Using fgets to write into it causes undefined behavior and usually a runtime crash.

📝 Syntax

advanced

Which option causes a compilation error?

Which of the following code snippets will cause a compilation error related to file pointers?

A FILE *fp; fp = fopen("file.txt", "r"); fclose(fp);

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

C FILE fp; fp = fopen("file.txt", "r"); fclose(&fp);

D FILE *fp = fopen("file.txt", "r"); if(fp) fclose(fp);

💡 Hint

Remember how FILE and FILE * types are used.

Option C declares fp as a FILE struct, but fopen returns a pointer to FILE. Assigning a pointer to a struct variable causes a compilation error.

🚀 Application

expert

How many characters are read by this code?

Given the file "example.txt" contains the text "ABCDE\nFGHIJ", how many characters will be printed by this program?

C

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

int main() {
    FILE *fp = fopen("example.txt", "r");
    char buffer[5];
    int count = 0;
    while (fgets(buffer, sizeof(buffer), fp)) {
        printf("%s", buffer);
        count += strlen(buffer);
    }
    fclose(fp);
    printf("\nTotal chars: %d\n", count);
    return 0;
}

A 12

B 10

C 9

D 11

💡 Hint

Remember that fgets reads up to n-1 characters and includes the newline if present.

The file has 11 characters including the newline. The code reads in chunks of 4 characters plus null terminator. It reads "ABCD", then "E\nFG", then "HIJ". The total characters printed and counted are 11.

Practice

1. What does a file pointer in C represent?

easy

A. A variable that stores file contents

B. A reference to a file opened in the program

C. A function to read files

D. A type of file format

Solution

1. Step 1: Understand what a file pointer is

   A file pointer is a special pointer that points to a file stream opened by the program.

2. Step 2: Differentiate from other options

   It is not the file contents or a function, but a reference to the file for reading or writing.

3. Final Answer:

   A reference to a file opened in the program -> Option B

4. Quick Check:

   File pointer = Reference to opened file [OK]

Hint: File pointer points to file stream, not contents [OK]

Common Mistakes:

• Thinking file pointer stores file data
• Confusing file pointer with file reading function
• Assuming file pointer is a file format

2. Which of the following is the correct way to open a file for reading in C?

easy

A. FILE *fp = fopen("file.txt", "r");

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

C. fopen FILE *fp = "file.txt", "r";

D. FILE *fp = open("file.txt", "r");

Solution

1. Step 1: Recall fopen syntax

   The fopen function returns a pointer to FILE, so the variable must be FILE *fp.

2. Step 2: Check each option

   FILE *fp = fopen("file.txt", "r"); correctly declares FILE *fp and calls fopen with filename and mode "r".

3. Final Answer:

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

4. Quick Check:

   Correct fopen syntax = FILE *fp = fopen("file.txt", "r"); [OK]

Hint: fopen returns FILE pointer, so use FILE * variable [OK]

Common Mistakes:

• Declaring FILE variable instead of pointer
• Using open() instead of fopen()
• Incorrect order of fopen parameters

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

FILE *fp = fopen("test.txt", "w");
fprintf(fp, "Hello\n");
fclose(fp);

fp = fopen("test.txt", "r");
char buffer[10];
fgets(buffer, 10, fp);
printf("%s", buffer);
fclose(fp);

medium

A. Hello\n

B. Compilation error

C. test.txt

D. Hello

Solution

1. Step 1: Write and close file

   The code opens test.txt for writing, writes "Hello\n", then closes the file.

2. Step 2: Read and print file content

   It reopens test.txt for reading, reads a line into buffer, then prints it. fgets reads until newline or buffer limit.

3. Final Answer:

   Hello (printed as Hello with newline) -> Option D

4. Quick Check:

   File write/read prints "Hello" [OK]

Hint: fgets reads line including newline, printf prints it [OK]

Common Mistakes:

• Expecting literal \n printed
• Confusing fprintf and printf output
• Forgetting to close file before reopening

4. Identify the error in this code snippet:

FILE *fp;
fp = fopen("data.txt", "r");
char c = fgetc(fp);
while (c != EOF) {
    putchar(c);
    c = fgetc(fp);
}
fclose(fp);

medium

A. fgetc returns int, not char, causing wrong EOF check

B. fopen mode "r" is incorrect for reading

C. Missing fclose call

D. putchar cannot print characters

Solution

1. Step 1: Check fgetc return type

   fgetc returns int to allow EOF (-1) detection, storing in char loses EOF info.

2. Step 2: Understand EOF check failure

   Comparing char c to EOF fails because c cannot hold EOF value properly, causing infinite loop or wrong output.

3. Final Answer:

   fgetc returns int, not char, causing wrong EOF check -> Option A

4. Quick Check:

   Use int for fgetc result to detect EOF [OK]

Hint: Store fgetc result in int to detect EOF correctly [OK]

Common Mistakes:

• Using char instead of int for fgetc result
• Ignoring EOF check causing infinite loops
• Assuming putchar can't print chars

5. You want to append text "World" to an existing file "hello.txt" without deleting its content. Which is the correct way to open the file?

hard

A. FILE *fp = fopen("hello.txt", "r");

B. FILE *fp = fopen("hello.txt", "w");

C. FILE *fp = fopen("hello.txt", "a");

D. FILE *fp = fopen("hello.txt", "r+");

Solution

1. Step 1: Understand file modes

   "a" mode opens file for appending, preserving existing content and writing at the end.

2. Step 2: Compare other modes

   "w" overwrites file, "r" is read-only, "r+" allows read/write but does not append automatically.

3. Final Answer:

   FILE *fp = fopen("hello.txt", "a"); -> Option C

4. Quick Check:

   Append mode = "a" [OK]

Hint: Use "a" mode to add without erasing file [OK]

Common Mistakes:

• Using "w" which erases file content
• Using "r" which is read-only
• Confusing "r+" with append mode