File pointers in C - 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 *fp;
    fp = fopen("data.txt", "[1]");
    if (fp == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }
    fclose(fp);
    return 0;
}

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

Aa

Bw

Cr

Drw

Attempts:

3 left

💡 Hint

Common Mistakes

Using "w" which overwrites the file instead of reading.

Using "rw" which is not a valid mode in C.

The mode "r" opens the file for reading. Other modes like "w" are for writing, "a" for appending.

2fill in blank

medium

Complete the code to write the string "Hello" to the file using fprintf.

C

#include <stdio.h>

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

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

AWorld

BText

CFile

DHello

Attempts:

3 left

💡 Hint

Common Mistakes

Writing a different string than "Hello".

Forgetting to include the newline character if needed.

"Hello" is the string to write to the file as per the instruction.

3fill in blank

hard

Fix the error in the code to correctly read a line from the file into buffer.

C

#include <stdio.h>

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

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

Abuffer

B&buffer

Cfp

D*buffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using &buffer which is a pointer to the array, not a char pointer.

Using *buffer which is the first character, not a pointer.

fgets expects a pointer to a char array, so passing "buffer" is correct.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths for words longer than 3 characters.

C

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

int main() {
    char *words[] = {"cat", "elephant", "dog", "giraffe"};
    int lengths[4];
    int count = 0;
    for (int i = 0; i < 4; i++) {
        if (strlen(words[i]) [1] 3) {
            lengths[count] = strlen(words[i]);
            count[2];
        }
    }
    for (int j = 0; j < count; j++) {
        printf("%d\n", lengths[j]);
    }
    return 0;
}

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

A>

B++

C<

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator which selects wrong words.

Decrementing count instead of incrementing.

The condition checks if word length is greater than 3, and count is incremented with ++.

5fill in blank

hard

Fill all three blanks to create a file pointer, open a file for appending, and write a line.

C

#include <stdio.h>

int main() {
    FILE *[1];
    [1] = fopen("log.txt", "[2]");
    if ([1] == NULL) {
        return 1;
    }
    fprintf([1], "Log entry\n");
    fclose([1]);
    return 0;
}

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

Afp

Ba

Dr

Attempts:

3 left

💡 Hint

Common Mistakes

Using "r" mode which opens file for reading only.

Using different variable names inconsistently.

Use variable name "fp" for the file pointer, open file in append mode "a", and use "fp" to write and close.

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