Reading from files in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used to open a file for reading in C?

The fopen() function is used to open a file. To open a file for reading, use fopen(filename, "r").

beginner

How do you read a single character from a file in C?

Use the fgetc() function. It reads one character from the file and returns it as an int.

intermediate

What does fgets() do when reading from a file?

fgets() reads a line or up to a specified number of characters from a file into a string, stopping at a newline or EOF.

beginner

Why should you always check the return value of fopen()?

Because fopen() returns NULL if the file cannot be opened (e.g., file does not exist or no permission). Checking prevents errors later.

beginner

How do you properly close a file after reading in C?

Use the fclose() function with the file pointer to close the file and free resources.

Which mode should you use with fopen() to read a file?

A "r"

B "w"

C "a"

D "rw"

Use "r" mode to open a file for reading only.

What does fgetc() return when it reaches the end of a file?

A -1

B EOF

C NULL

D 0

fgetc() returns EOF (usually -1) when it reaches the end of the file.

Which function reads a whole line from a file into a string?

A fgetc()

B fprintf()

C fread()

D fgets()

fgets() reads a line or up to a specified number of characters from a file.

What should you do if fopen() returns NULL?

A Handle the error, e.g., show a message and stop reading

B Ignore it and continue

C Try to write to the file

D Close the file pointer

If fopen() returns NULL, the file could not be opened, so you should handle the error.

Which function closes a file in C?

A fileclose()

B close()

C fclose()

D endfile()

fclose() closes the file and releases resources.

Explain the steps to read text from a file in C.

Think about opening, reading, checking errors, and closing.

What are common errors to watch for when reading files in C?

Consider what can go wrong at each step.

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