Lecture 23

Interacting With Files

Why is interacting with files important?:

files allow data to persist after the program has finished executing
all programs prior to now can only store data for the length of time the program is executing

How to interact with files?:

declare a file pointer FILE *file_pointer;
- read more about FILE here: http://www.cplusplus.com/reference/cstdio/FILE/
- review the lecture on pointers: Pointers Lecture
attach a file to the file pointer file_pointer = fopen("test.txt", "r");
- read more about fopen here: http://www.cprogramming.com/tutorial/cfileio.html
- understand the different modes to open a file:
  - r: open for reading
  - w: open for writing (file need not exist)
  - a: open for appending (file need not exist)
  - r+: open for reading and writing, start at beginning
  - w+: open for reading and writing (overwrite file)
  - a+: open for reading and writing (append if file exists)
read/write to the file using fscanf/fprintf
- read more about fscanf here: http://www.cplusplus.com/reference/cstdio/fscanf/
- read more about fprintf here: http://www.cplusplus.com/reference/cstdio/fprintf/
close the file and file pointer fclose(file_pointer);
- read more about fclose here: http://www.cplusplus.com/reference/cstdio/fclose/

Open and Close a File

A simple example of opening and closing a file:

This program opens and closes a file and displays a message if file was successfully or unsuccessfully opened

open_close_file.c:

NULL

NULL is a way to detect if file was opened correctly or not
you could just compare the file pointer to 0, but NULL is more descriptive
read more about NULL pointer here: http://en.wikipedia.org/wiki/Pointer_%28computer_programming%29#Null_pointer
read more about NULL pointer vs 0 here: http://stackoverflow.com/questions/176989/do-you-use-null-or-0-zero-for-pointers-in-c

File Location

the file location parameter in fopen("data/test.txt", "r"); is relative to where the executable is ran from
the file location is not relative to where the open_close_file.c file is located
if pwd returns
```
code/files
```
and I run a program that opens fopen("data/test.txt", "r");, then the program is going to look for
```
code/files/data/test.txt
```

Read From a File

A simple example of reading a file:

This program reads from a file and displays the file contents to the user:

read_file.c:

data/test.txt:

Things to note:

fscanf behaves very similarly to scanf

Another example of reading a file:

This program reads from a file using a while loop and displays the file contents to the user:

read_file_with_while.c:

data/test.txt:

Things to note:

EOF means "end of file"
EOF is just a macro constant for -1
you could just compare -1, but EOF is more descriptive

Writing to a File

A simple example of writing to a file:

This program creates a files and then writes to it:

write_file.c:

Things to note:

fprintf behaves very similarly to printf

Write a Program to Copy One File's Content to Another File

Assignment

Homework Problem 23.1, at the end of this problem you should submit to Laulima:

a file named UHusername_23_1.c
a file named 23_1.input
- zhaol_23_1.c
- zhaol_23_1_functions.c
- zhaol_23_1_functions.h
Write a program that parses a text file (23_1.input) that contains the following lyrics:

Test Case 1:

Successfully opened file
The word mama showed up 10 time(s)

The program displays the number of times "mama" shows up in the text file, using arabic numbers (e.g. 1, 3, 20) and not words (e.g. one, three, twenty)
- all the text is lowercase
- there are no words that contain the sequence of characters m,a,m,a, such as mamaliga
- any function listed at http://www.mrx.net/c/readfunctions.html
- any function listed at http://en.wikipedia.org/wiki/C_string_handling#Functions
- http://stackoverflow.com/questions/3825668/c-c-checking-for-null-pointer
- strcmp.c:
- strstr.c:

Optional Assignments

Optional assignments are not graded and does not need to be turned in. It is intended to give you more practice (if you want it) using the programming concepts discussed in lecture. Some optional assignments are intended to introduce related concepts (if you care about it) not discussed in this course to broaden your knowledge about programming in general.

If the optional assignment is modifying a required assignment, be sure to leave your previous assignment work untouched by copying them to a new folder for the optional assignment and modify only the files in the new folder

Update 23.1 to take into account for multiple "mama"s on a single line

Modify the program so it will count the following as 5 "Mama"s:

Mama, Mama you know I love you
Mama, Mama you're the queen of my heart
Your love is like tears from the stars
Mama I just want you to know lovin' you is like food to my soul

Update 23.1 to use regular expressions (aka regex)

regex is text parsing syntax that has been ported to many different programming languages
You can learn the regex syntax at regexone
C has a regex library that you can use for this optional assignment
If getting regex working in C is too much of a hassle, you can also just try regex for the above text parsing assignments at rubular. Rubular is the Ruby programming language's version of regex, and it will allow you to test regex capabilities that are common to all regex libraries in other languages.