Reading from files in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C code reading a file?

Consider a file named data.txt containing the text "Hello\nWorld". What will this program print?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    char buffer[20];
    if (fp == NULL) {
        printf("Error opening file\n");
        return 1;
    }
    fgets(buffer, sizeof(buffer), fp);
    printf("%s", buffer);
    fclose(fp);
    return 0;
}

A

Hello
World

B

World

C

Hello

D

Error opening file

💡 Hint

Remember that fgets reads one line or up to the buffer size.

The fgets function reads one line from the file, including the newline character. Since the first line is "Hello\n", it prints "Hello\n" only.

❓ Predict Output

intermediate

What is the output of this code reading integers from a file?

Given a file numbers.txt with content:

10 20 30

What does this program print?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("numbers.txt", "r");
    int a, b, c;
    if (fp == NULL) {
        printf("Cannot open file\n");
        return 1;
    }
    fscanf(fp, "%d %d %d", &a, &b, &c);
    printf("%d %d %d\n", a, b, c);
    fclose(fp);
    return 0;
}

A

10 20 30

B

10 20

C

Cannot open file

D

30 20 10

💡 Hint

fscanf reads formatted input from the file.

The fscanf reads three integers from the file in order and stores them in a, b, and c. The program prints them in the same order.

❓ Predict Output

advanced

What error does this code raise when reading a file?

What error will this program produce when run if missing.txt does not exist?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("missing.txt", "r");
    char c = fgetc(fp);
    printf("%c\n", c);
    fclose(fp);
    return 0;
}

A Segmentation fault

B Prints a random character

C Compilation error

D Prints EOF

💡 Hint

Check what happens if fopen returns NULL and you use the pointer without checking.

If the file does not exist, fopen returns NULL. Using fgetc on a NULL pointer causes a segmentation fault (crash).

🧠 Conceptual

advanced

How many characters are read by this code?

Given a file text.txt containing "abcde", how many characters will be read and printed by this program?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("text.txt", "r");
    int count = 0;
    int c;
    if (fp == NULL) return 1;
    while ((c = fgetc(fp)) != EOF) {
        putchar(c);
        count++;
    }
    fclose(fp);
    printf("\n%d\n", count);
    return 0;
}

A 6

B 5

C 4

D 0

💡 Hint

fgetc reads one character at a time until EOF.

The file contains 5 characters: 'a', 'b', 'c', 'd', 'e'. The loop reads all 5 characters and counts them. EOF is not counted.

🔧 Debug

expert

What is the value of variable 'line' after running this code?

Assume the file input.txt contains exactly 1234567890.

This program reads a line from input.txt. What will be the value of line after fgets?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("input.txt", "r");
    char line[10];
    if (fp == NULL) return 1;
    fgets(line, 10, fp);
    fclose(fp);
    printf("%s", line);
    return 0;
}

A "1234567890"

B "12345678"

C "123456789\n"

D "123456789"

💡 Hint

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

The buffer size is 10, so fgets reads up to 9 characters. The file content is "1234567890". It reads the first 9 characters: "123456789" and adds '\0'.

Practice

1. What does the fopen() function with mode "r" do in C?

easy

A. It deletes a file.

B. It opens a file for reading only.

C. It creates a new file.

D. It opens a file for writing only.

Solution

1. Step 1: Understand fopen modes

   The mode "r" in fopen means open the file for reading only.

2. Step 2: Check other modes

   Modes like "w" are for writing, "a" for appending, and none delete files.

3. Final Answer:

   It opens a file for reading only. -> Option B

4. Quick Check:

   fopen with "r" = open for reading [OK]

Hint: Remember: "r" means read only mode in fopen [OK]

Common Mistakes:

• Confusing "r" with write mode "w"
• Thinking fopen deletes or creates files with "r"
• Not checking if file exists before reading

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

easy

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

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

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

D. FILE *fp = fopen("data.txt");

Solution

1. Step 1: Check fopen syntax

   fopen requires two arguments: filename and mode as strings.

2. Step 2: Identify correct mode for reading

   "r" is the mode to open a file for reading.

3. Final Answer:

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

4. Quick Check:

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

Hint: Always use two arguments in fopen: filename and mode [OK]

Common Mistakes:

• Using "w" mode which opens for writing, not reading
• Omitting the mode argument in fopen
• Using "a" mode which opens for appending

3. What will be the output of the following C code if the file input.txt contains the line "Hello"?

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

medium

A. Hello\n

B. Hello

C. Hello\0

D. Error: file not found

Solution

1. Step 1: Understand fgets behavior

   fgets reads a line including the newline character if it fits in the buffer.

2. Step 2: Check file content and output

   The file has "Hello" followed by a newline, so fgets reads "Hello\n" and prints it.

3. Final Answer:

   Hello\n -> Option A

4. Quick Check:

   fgets includes newline if present = Hello\n [OK]

Hint: fgets reads newline character if space allows [OK]

Common Mistakes:

• Assuming fgets removes newline automatically
• Confusing null terminator with newline
• Not checking if file opened successfully

4. Identify the error in this code snippet that reads a character from a file:

FILE *fp = fopen("file.txt", "r");
char c;
c = fgetc(fp);
printf("%c", c);
fclose(fp);

medium

A. fclose should be called before reading.

B. fgetc should be replaced with fgets.

C. Variable c should be an int, not char.

D. Missing check if fopen returned NULL before reading.

Solution

1. Step 1: Check file open validation

   Always check if fopen returns NULL before reading to avoid errors.

2. Step 2: Review other parts

   fgetc is correct for reading one char; c as char is acceptable; fclose after reading is correct.

3. Final Answer:

   Missing check if fopen returned NULL before reading. -> Option D

4. Quick Check:

   Always check fopen result before file operations [OK]

Hint: Check fopen != NULL before reading file [OK]

Common Mistakes:

• Skipping NULL check after fopen
• Confusing fgetc and fgets usage
• Closing file too early

5. You want to read all lines from a file log.txt and print them. Which code snippet correctly does this in C?

hard

A. FILE *fp = fopen("log.txt", "r");
char c;
while((c = fgetc(fp)) != EOF) {
printf("%c", c);
}
fclose(fp);

B. FILE *fp = fopen("log.txt", "w");
char line[100];
while(fgets(line, 100, fp) != NULL) {
printf("%s", line);
}
fclose(fp);

C. FILE *fp = fopen("log.txt", "r");
char line[100];
while(fgets(line, 100, fp) != NULL) {
printf("%s", line);
}
fclose(fp);

D. FILE *fp = fopen("log.txt", "r");
char line[100];
while(fgets(line, 100, fp)) {
printf(line);
}
fclose(fp);

Solution

1. Step 1: Check file open mode

   Opening with "r" is correct for reading; "w" would overwrite the file.

2. Step 2: Validate reading loop and printing

   Using fgets with NULL check is correct; printf with "%s" is safe and prints lines properly.

3. Step 3: Review other options

   FILE *fp = fopen("log.txt", "w"); char line[100]; while(fgets(line, 100, fp) != NULL) { printf("%s", line); } fclose(fp); uses wrong mode "w"; FILE *fp = fopen("log.txt", "r"); char c; while((c = fgetc(fp)) != EOF) { printf("%c", c); } fclose(fp); reads char by char, not lines; FILE *fp = fopen("log.txt", "r"); char line[100]; while(fgets(line, 100, fp)) { printf(line); } fclose(fp); uses printf(line) without format string, unsafe.

4. Final Answer:

   Correct reading lines with fgets and printing with printf("%s", line). -> Option C

5. Quick Check:

   Use fgets + printf("%s") + fopen with "r" [OK]

Hint: Use fgets in loop with NULL check and printf("%s") [OK]

Common Mistakes:

• Opening file with wrong mode "w" for reading
• Using printf without format string
• Reading char by char when lines needed