Reading from files in C - Mini Project: Build & Apply

Reading from files

📖 Scenario: You are creating a simple program to read a list of names from a text file and display them on the screen. This is like opening a guest list and showing each guest's name one by one.

🎯 Goal: Build a C program that opens a file called names.txt, reads each line (each name), and prints all the names to the screen.

📋 What You'll Learn

Open the file names.txt for reading

Read each line from the file using fgets

Print each name to the screen

Close the file after reading

💡 Why This Matters

🌍 Real World

Reading from files is a common task in many programs, such as loading user data, configuration settings, or lists of items.

💼 Career

Understanding file input/output is essential for software developers, especially when working with data files, logs, or user inputs stored in files.

1

Create a file pointer and open the file

Write a line of code to create a file pointer called file and open the file "names.txt" in read mode using fopen.

C

# Your code here

Hint

Use FILE *file = fopen("names.txt", "r"); to open the file for reading.

2

Create a buffer to hold each line

Add a character array called line with size 100 to store each line read from the file.

C

#include 

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

// Your code here

Hint

Declare char line[100]; to hold each name line.

3

Read each line from the file using fgets

Write a while loop that uses fgets(line, 100, file) to read each line until the end of the file.

C

#include 

FILE *file = fopen("names.txt", "r");
char line[100];

// Your code here

Hint

Use while (fgets(line, 100, file)) to read lines until no more lines are left.

4

Print each line and close the file

Inside the while loop, write printf("%s", line); to print each name. After the loop, close the file using fclose(file);.

C

#include 

FILE *file = fopen("names.txt", "r");
char line[100];

while (fgets(line, 100, file)) {
    // Your code here
}

// Your code here

Hint

Use printf("%s", line); inside the loop and fclose(file); after the loop.

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