CSCI26I File I/O in Detail ? Review Programs can read and write - - PowerPoint PPT Presentation

CSCI26I

File I/O in Detail

?

Review

• Programs can read and write files to disk
• And from any “stream” !
• #include <fstream>
• ifstream my_file_object(“filename.ext”)
• Use while loops to repeatedly read from

the stream

• close() all filestreams

You Will Always...

• Include the fstream library
• Declare your filestreams (and open them)
• Check for an I/O error
• While not at the end of file, do stuff
• Close the file

I/O Boilerplate

#include <cstdlib> #include <iostream> #include <fstream> using namespace std; int main() { char x; // or int x, double x, etc. ifstream mydata("FILENAME"); if(!mydata) {

• cerr << "Error" << endl;
• system("PAUSE");
• exit(1);

} while(myfile >> x) { // do marvelous things } myfile.close(); return 0; }

Reading Data from Streams

Variable types matter! Values in the file matter!

Example

1 2 200 300 1.1 2.1 x cc 320 420

datafile.txt

int main() { int x, y; ifstream data("data.txt"); while (data >> x >> y) { cout << x << " " << y << endl; } data.close(); system("PAUSE"); return 0; }

How >> and Streams Work

First, remember to think of this data “streaming” to your program.

1 2 200 300 320 420

datafile.txt

How >> and Streams Work

420 320 300 200 2 1

ifstream numbaz(“datafile.txt”);

1 2 200 300 320 420

datafile.txt

How >> and Streams Work

420 320 300 200 2 1

numbaz int x; while (numbaz >> x) { // have fun }

cursor

“Computer, read the next numeric characters after the cursor up until a whitespace character and assuming it’s numeric, assign the value to x.”

How >> and Streams Work

420 320 300 200 2 1

numbaz int x; while (numbaz >> x) { // have fun }

cursor

“Computer, read the next numeric characters after the cursor up until a whitespace character and assuming it’s numeric, assign the value to x.”

whee!

Writing Files

• fstream myoutput(“filename.ext”);

“Computer, create an output filestream called myoutput and save its contents to filename.ext.”

• fstream myoutput(“filename.ext”);

myoutput << “obladee obladah life goes on”; myoutput.close();

Use It Like cout

cout << “i am the walrus”;

“Computer, push this string to the screen.” “Computer, push this string to my file.”

Homework

• Read Etter p144 - 145 and 4.5
• Complete assignment 12_shiftCipher

Mmmmoooooooooo!