Lecture 33: Using File I/O

LECTURE 33: USING FILE I/O

CSC 107 – Programming For Science

Today’s Goal

Get familiar with opening & closing files Declaring variables for use with files Using variables to open files for reading &

writing Closing files and understand why you

should do so Understand what it means: ALL I/O is

file I/O Common bugs to avoid when coding with

files in C++ Get a better understanding of what >> & <<

do cin, cout are variables: know what their

types are

Image To Sharpen

I have a (fuzzy) 1024 x 768 picture to sharpen Only 786,432 numbers to type in to

yesterday's lab Will also need to click each pixel to update

with result

More Data Entry Positions

Testing improved jet designs for oeingB-ay Using program to simulate designs' lift &

drag 5 possible designs (each 150MB) to test

this iteration Once results available, will tweak & retest

designs Need room of touch typists for all this data

entry

This Is (Semi-Real) Problem

Large hadron collider eventually work as designed No black hole when smashing particles at

high speeds Creates 28.5 GB/min for nerds seeking

truth & beauty

This Is (Semi-Real) Problem

Large hadron collider eventually work as designed No black hole when smashing particles at

high speeds Creates 28.5 GB/min for nerds seeking

truth & beauty Hired trained monkeys to type all

244,813,135,872 bits

This Is (Semi-Real) Problem

Large hadron collider eventually work as designed No black hole when smashing particles at

high speeds Creates 28.5 GB/min for nerds seeking truth

& beauty Hired trained monkeys to type all

244,813,135,872 bits college students

Yeah, Right

Real world demands we use files for most I/O

Data files used to start and/or end most projects May contain: game levels, analysis results,

CCD pics Way to read & write files needed to be

useful

Program Basics For Files

All built-in file I/O code means adding to header

#include <fstream> Place with other #includes to use files in program Even if no files are used, no cost to adding this line

Must specify namespace of file I/O code, also If you really want, this can do this individually but…

using namespace std;…much easier and probably habit by now anyway

Opening a File

To use file, we need variable to use in program Numbers, cStrings, and booleans mixed in

the file Previous type (& arrays) do not make sense

to use C++ provides new types to refer to file

itself

Types Used For Files

Within program, may use file in 2 possible ways To read file, ifstream variables will be

needed Need variable of type ofstream to write to

file

File Variable Declaration

Types are new, but still just declaring variables Requires type & name of variable being

declared Names for human use; normal scoping

rules apply Cannot assign variables but can use as

parameters Cannot use in equations: files are NOT

numbersofstream fout;ifstream fin;ofstream bob, bill, babs, barb;ifstream thisIsAFileVariable;ofstream monkey = "foo.txt";ifstream fourFile = 4;

File Variable Declaration

Types are new, but still just declaring variables Requires type & name of variable being

declared Names for human use; normal scoping

rules apply Cannot assign variables but can use as

parameters Cannot use in equations: files are NOT

numbersofstream fout;ifstream fin;ofstream bob, bill, babs, barb;ifstream thisIsAFileVariable;ofstream monkey = "foo.txt";ifstream fourFile = 4;

File Variable Declaration

Types are new, but still just declaring variables Requires type & name of variable being

declared Names for human use; normal scoping

rules apply Cannot assign variables but can use as

parameters Cannot use in equations: files are NOT

numbersofstream fout;ifstream fin;ofstream bob, bill, babs, barb;ifstream thisIsAFileVariable;ofstream monkey = "foo.txt";ifstream fourFile = 4;

Name That File

Two ways to open a file once we have the name No matter which used, must know name of

file Defaults to current directory, if only a name

specified By including in name, can use other

directories No standard way to do this – depends on

the OS

Name That File

Two ways to open a file once we have the name No matter which used, must know name of

file Defaults to current directory, if only a name

specified By including in name, can use other

directories No standard way to do this – depends on

the OS

Opening the File

Can open file when variable declaredchar nameLoc[] = "bobbobbob";char sndName[] = "csi.txt";ifstream fin("image.dat");ofstream fout(nameLoc);ifstream bobism(sndName);ofstream cookies(“nomnomnom.txt");

Even after declaration, files can be openedifstream because;ofstream isaidso;because.open("mightMakes.right");cin >> sndName;isaidso.open(sndName);

Did I Do Good?

May not always be successful opening a file Cannot open and read file that does not

exist May not have permission to access a

certain file Cannot do impossible & write to

nonexistent drive Use built-in is_open function to check

this Called in manner similar to cout functions

like setf If variable attached to open file, function

returns true Returns false if variable not used to open

file, yet If attempt to open file fails, will also return

false

Examples of is_open

char sndName[];ifstream because, foo("foo.txt");ofstream isaidso, bar("snafu");cout << because.is_open() << endl;cout << foo.is_open() << endl;cout << bar.is_open() << endl;because.open("mightMakes.right");cin >> sndName;isaidso.open(sndName);cout << because.is_open() << endl;cout << isaidso.is_open() << endl;

Upon Opening The Location Is… Once open, read & write from start of

file As logical a choice as any other location to

start at If reading, can start getting all data from

file When writing to existing file what will

happen?

Oops!

Opening a File

Within program, may use file in 2 possible ways To read file, ifstream variables will be

needed Need variable of type ofstream to write to

file

Opening a File

Within program, may use file in 2 possible ways To read file, ifstream variables will be

needed Need variable of type ofstream to write to

file Open ofstream 2 different ways depending

on use

ofstream nukeIt("byebye.txt");ofstream begone;begone.open("erasedOld.dat");

ofstream keepIt("saved", ios::app);ofstream faithAlone;faithAlone.open("preserve", ios::app);

3

Closing File

Important to close files once you are done Program will delay saving data to make it

faster Crashes cause data loss if saves had been

waiting Until file is closed may be locked from other

uses Immediately saved on close, so insures

data is safe Can open more files if limit of open files

reachedofstream giants("nlChamp");ifstream yankees("evilEmpire");phillies.close();yankees.close();

Today's Key Point

Because of its history, all C++ I/O is file based Obvious when file is source of data or

target of write But also true when reading from keyboard Writing to screen also considered file I/O

Today's Key Point

You've been coding with files

since day 1

What Are cin & cout?

Statement needed for most file I/O#include <fstream>

To use cin & cout we must have statement:

#include <iostream>

What Are cin & cout?

Statement needed for most file I/O#include <fstream>

To use cin & cout we must have statement:

#include <iostream>

There is a method: similarity not an accident cin & cout are file variables defined by

system

What Are cin & cout?

Statement needed for most file I/O#include <fstream>

To use cin & cout we must have statement:

#include <iostream>

There is a method: similarity not an accident cin & cout are file variables defined by

system

Deep in Bowels of iostream

In iostream find 2 lines to be included in code:ifstream cin( );

ofstream cout( );

Already written code reading from a file Use ifstream like cin to read ASCII text in

a file Also know how to write ASCII text to a

file As with cout, ofstreams writes text to a

file

Read User's Typing With cin

Used to read one or more values at once:

cin >> variable;cin >> variable1 >> variable2;

Starts where last read stopped reading input

Automatically skips past whitespace Data type of variable determines what

is read Stops at first non-usable value found in the

input If input is not usable, will set variable equal

to 0

Read File W/ifstream Variable Used to read one or more values at

once:ifstream myFile;myFile >> variable;

myFile >> variable1 >> variable2; Starts where last read stopped reading

input Automatically skips past whitespace

Data type of variable determines what is read Stops at first non-usable value found in the

input If input is not usable, will set variable equal

to 0

Print to Screen Using cout

Easy to output: print text using cout

cout << “Hello World” << endl; Prints out whatever is placed between

quotes Value of variable printed if variable not in

quotes Use escape sequences for fancier text

output\n newline (move to start of next line)\t tab (go to next column that is multiple of 8)

Output using #include <iomanip> fancier

Print to File With ostream

Easy to output: output via ostream variable

ofstream outFile;outFile << “Hello World” << endl; Prints out whatever is placed between

quotes Value of variable printed if variable not in

quotes Use escape sequences for fancier text

output\n newline (move to start of next line)\t tab (go to next column that is multiple of 8)

Output using #include <iomanip> fancier

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main(void) {

int sum = 0; int val; cout << "-1 to quit or sum nums typed" << endl; cin >> val; while (val != -1) { sum += val; cout << val << endl; cin >> val; } return 0;}

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main(void) { ifstream fin; ofstream fout; int sum = 0; int val; fout << "-1 to quit or sum nums typed" << endl; fin >> val; while (val != -1) { sum += val; fout << val << endl; fin >> val; } return 0;}

And When We Run It?

#include <iostream>#include <fstream>using namespace std;int main(void) { ifstream fin; ofstream fout; int sum = 0; int val; fout << "-1 to quit or sum nums typed" << endl; fin >> val; while (val != -1) { sum += val; fout << val << endl; fin >> val; } return 0;}

And When We Run It?

Must Open File Before Using

How file opened unimportant, just that is open Open when declared, by giving file name as

cStringifstream bobIn("file.txt");ofstream whyNot(stringFromUser);

Open open later in program using open() routinebobIn.open(nameOfDataFile);whyNot.open("averagesWeCompute");

Variable must refer to open file or else it crashes Often add check with is_open() to protect

from crashs

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main(void) {

int sum = 0; int val; cout << "-1 to quit or sum nums typed" << endl; cin >> val; while (val != -1) { sum += val; cout << "Sum: " << val << endl; cin >> val; } return 0;}

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main() {

int sum = 0; int val; cout << "-1 to quit or sum nums typed" << endl; cin >> val; while (val != -1) { sum += val; cout << "Sum: " << val << endl; cin >> val; }

return 0;}

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main() { ifstream myFile("numbers.txt"); ofstream yNot("sums.out"); if (myFile.is_open() && yNot.is_open()) { int sum = 0; int val; yNot << "-1 to quit or sum nums typed" << endl; myFile >> val; while (val != -1) { sum += val; yNot << "Sum: " << val << endl; myFile >> val; } } return 0;}

See How Easy It Is?

#include <iostream>#include <fstream>using namespace std;int main() { ifstream myFile("numbers.txt"); ofstream yNot("sums.out"); if (myFile.is_open() && yNot.is_open()) { int sum = 0; int val; cout << "-1 to quit or sum nums typed" << endl; myFile >> val; while (val != -1) { sum += val; yNot << "Sum: " << val << endl; myFile >> val; } } return 0;}

Variables are Variables

Like all variables, can use files as parameters Argument must also be file variable for it to

compile Reading & writing continues through the

function Can only go forward in file no matter

what Within the function, "file position marker"

continues Cannot unring bell, does not overwrite file

on return As with cin to read, will not reread after

function

But…

Like all variables, can use files as parameters Argument must also be file variable for it to

compile Reading & writing continues through the

function Can only go forward in file no matter

what Within the function, "file position marker"

continues Cannot unring bell, does not overwrite file

on return As with cin to read, will not reread after

function

But…

Like all variables, can use files as parameters Argument must also be file variable for it to

compile Reading & writing continues through the

function Can only go forward in file no matter

what Within the function, "file position marker"

continues Cannot unring bell, does not overwrite file

on return As with cin to read, will not reread after

function

Butt…

Like all variables, can use files as parameters Argument must also be file variable for it to

compile Reading & writing continues through the

function Can only go forward in file no matter

what Within the function, "file position marker"

continues Cannot unring bell, does not overwrite file

on return As with cin to read, will not reread after

function

Variables are Variables

Like all variables, can use files as parameters Argument must also be file variable for it to

compile Reading & writing continues through the

function Can only go forward in file no matter

what Within the function, "file position marker"

continues Cannot unring bell, does not overwrite file

on return As with cin to read, will not reread after

function

PARAMETER MUST BE PASS-BY-REFERENCE

Your Turn

Get into your groups and try this assignment

For Next Lecture

Another use of data files in Section 17.1-17.3 Must we use text or are there faster

approaches? What was the point of learning binary

numbers? Could we predict size of file we read/write in

program?

Weekly Assignment uses Xcode this week Submit via e-mail; CloudCoder cannot use

files

Programming Assignment #3 available Friday