Reading from files in C

Introduction

Reading from files lets your program get information saved on your computer. This helps your program use data that was saved before or from other sources.

You want to load a list of names saved in a file to use in your program.

You need to read settings or preferences stored in a file to customize your program.

You want to process a text file with data like scores or messages.

You want to read a log file to check what happened earlier.

You want to read a file that a user selects to show its content.

Syntax

C

FILE *file_pointer;
file_pointer = fopen("filename.txt", "r");
if (file_pointer == NULL) {
    // handle error
}
// read from file
fclose(file_pointer);

fopen opens the file. The "r" means open for reading.

Always check if fopen returns NULL. That means the file could not be opened.

Examples

Open a file named data.txt for reading and check if it opened successfully.

C

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    printf("Cannot open file\n");
}

Read the file line by line and print each line.

C

char line[100];
while (fgets(line, sizeof(line), fp)) {
    printf("%s", line);
}

Read the file character by character and print each character.

C

int c;
while ((c = fgetc(fp)) != EOF) {
    putchar(c);
}

Sample Program

This program opens a file named example.txt for reading. It reads the file line by line and prints each line to the screen. If the file cannot be opened, it prints an error message.

C

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }

    char line[256];
    while (fgets(line, sizeof(line), file)) {
        printf("%s", line);
    }

    fclose(file);
    return 0;
}

Important Notes

Remember to close the file with fclose() to free resources.

fgets() reads one line at a time including the newline character.

Use fgetc() to read one character at a time if needed.

Summary

Use fopen() with "r" to open a file for reading.

Check if the file opened successfully by testing if the pointer is NULL.

Read file content using fgets() for lines or fgetc() for characters.

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