CS162: Introduction to Computer Science II Streams 1 Streams A - - PDF document

1

1

CS162: Introduction to Computer Science II

Streams

Streams

• A stream is a flow of data
• Input stream: a stream going into your

program (eg. from a keyboard or file)

– cin is an input stream from the keyboard

• Output stream: a stream going out of

your program (eg. to the screen or file)

– cout is an output stream to the screen

2

2

File I/O

• We will be using text files
• You can create a text file using a text

editor like vi, emacs, vim or notepad

• If you take input from a file, you are

reading from it

• If you send output to a file, you are

writing to it

3

File I/O

• If you want to work with file I/O, you

need the following include:

#include <fstream>

• The two stream types you will work with

are:

ifstream (for input streams)

• fstream (for output streams)

4

3

Writing to a file

5

Writing to a file

#include <fstream> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt");
• utStream << "These are not the droids you

are looking for" << std::endl;

• utStream.close();

return 0; }

6

4

Writing to a file

#include <fstream> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt");
• utStream << "These are not the droids you

are looking for" << std::endl;

• utStream.close();

return 0; }

7

• This associates the file output.txt with

the output stream outStream.

• The file output.txt is created in the

same directory as the executable.

• If output.txt doesn’t exist, it is created.

If it already exists, it is overwritten

Writing to a file

#include <fstream> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt");
• utStream << "These are not the droids you

are looking for" << std::endl;

• utStream.close();

return 0; }

8

• Notice how everything is done

through outStream

• Use << to send output to a file (just

like you do with cout)

5

Writing to a file

#include <fstream> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt");
• utStream << "These are not the droids you

are looking for" << std::endl;

• utStream.close();

return 0; }

9

Closing the file does two things:

• Prevents your file from being corrupted
• If you want to read this file in the near

future, you need to close it

Writing to a file

• Recall that the line below overwrites output.txt

if it exists

• utStream.open("output.txt");
• If you want to append to the end of output.txt,

do the following:

– Add #include <iostream> – Change the call to open to be:

• utStream.open(“output.txt”,

std::ios::app);

10

6

Writing to a file

#include <fstream> #include <iostream> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt",std::ios::app);
• utStream << "These are not the droids you

are looking for" << std::endl;

• utStream.close();

return 0; }

11

Writing to a file

• If you run the filewriter program twice,

you end up with output.txt containing:

12

These are not the droids you are looking for These are not the droids you are looking for

7

Writing to a file

• Instead of two separate statements
• fstream outStream;
• utStream.open(“output.txt”,std::ios:app);
• You can combine them:
• fstream outStream(“output.txt”,std::ios::app);
• Output is buffered, meaning << doesn’t write

right away. You can force a write using flush

• utstream.flush()

13

Writing to a file

Sometimes opening a file for output can

• fail. Why? Lots of possibilities:
• Not enough disk space
• No write permissions

Need to check for file open failure (see next slide).

14

8

Writing to a file

#include <fstream> #include <iostream> #include <cstdlib> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt",std::ios::app);

if( outStream.fail() ) { std::cout << "Output file opening failed.\n"; exit(1); }

• utStream << "These are not the droids you are looking

for" << std::endl;

• utStream.close();

return 0; }

15

Writing to a file

#include <fstream> #include <iostream> #include <cstdlib> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt",std::ios::app);

if( outStream.fail() ) { std::cout << "Output file opening failed.\n"; exit(1); }

• utStream << "These are not the droids you are looking

for" << std::endl;

• utStream.close();

return 0; }

16

• Need this for the

exit() function

9

Writing to a file

#include <fstream> #include <iostream> #include <cstdlib> int main(int argc, char** argv) { std::ofstream outStream;

• utStream.open("output.txt",std::ios::app);

if( outStream.fail() ) { std::cout << "Output file opening failed.\n"; exit(1); }

• utStream << "These are not the droids you are looking

for" << std::endl;

• utStream.close();

return 0; }

17

• exit(1) means your program exited

with some error condition

• exit(0) means your program exited

without any errors

Formatting output

Formatting output

• Sometimes you need to output text to a

file with special formatting

• For example, how would you like to print

the number 2?

2, 2.0, 2.00, 2

• Use the setf() function on the outStream

before using <<

18

10

Formatting output

Examples:

• Prints floating point numbers in everyday

notation (not exponential notation)

• utStream.setf(ios::fixed);
• Print the decimal point
• utStream.setf(ios::showpoint);
• Puts 2 digits after the decimal point
• utStream.precision(2);
• See pg 536 for more formatting flags

19

Formatting output

Manipulators can also be used to format output eg.

• setw( ) sets the width

cout << setw(4) << 10 << setw(4) << 20

Prints out: 10 20 (each number takes up 4 spaces)

20

Must be called each time

11

Formatting output

• setprecision( ) sets the precision

cout << setprecision(2) << 10 << “ “ << 20 << endl;

Prints out: 10.00 20.00

21

Take effect and stays in effect

Formatting Output

• You can set multiple flags with a bitwise OR
• perator | eg.
• utStream.setf(ios::fixed |

ios::showpoint | ios::right);

• You can save your flags

long flagSettings =

• utStream.flags();
• You can set your flags:
• utStream.flags(flagSettings)
• You can unset a flag using unsetf(flag)
• n the output stream

22

12

Reading from a file

23

Reading from a file

• Suppose I have a file called input.txt

with the following lines:

24

1 2 3

13

Reading from a file

#include <fstream> #include <iostream> #include <cstdlib> int main(int argc, char** argv) { std::ifstream inStream; inStream.open("input.txt"); if( inStream.fail()) { std::cout << "Input file opening failed.\n"; exit(1); } int first, second, third; inStream >> first >> second >> third; std::cout << first << " " << second << " " << third << std::endl; inStream.close(); }

25

Reading from a file

int main(int argc, char** argv) { std::ifstream inStream; inStream.open("input.txt"); if( inStream.fail()) { std::cout << "Input file opening failed.\n"; exit(1); } int first, second, third; inStream >> first >> second >> third; std::cout << first << " " << second << " " << third << std::endl; inStream.close(); }

26

• input.txt now associated with inStream.
• Note: open() only accepts C-style

strings and not string objects. Use c_str() on a string object to get the C- style string

14

Reading from a file

int main(int argc, char** argv) { std::ifstream inStream; inStream.open("input.txt"); if( inStream.fail()) { std::cout << "Input file opening failed.\n"; exit(1); } int first, second, third; inStream >> first >> second >> third; std::cout << first << " " << second << " " << third << std::endl; inStream.close(); }

27

Opening a file for reading can fail eg. if it doesn’t exist

Reading from a file

int main(int argc, char** argv) { std::ifstream inStream; inStream.open("input.txt"); if( inStream.fail()) { std::cout << "Input file opening failed.\n"; exit(1); } int first, second, third; inStream >> first >> second >> third; std::cout << first << " " << second << " " << third << std::endl; inStream.close(); }

28

>> used to read in each line. Note: this assumes there are only 3 lines in the file input.txt.

15

Reading from a file

• Suppose you didn’t know how many

lines were in input.txt

• How do you read from a file and stop

when you hit the end of the file?

29

Reading from a file

• The extraction operator (instream >>

next) returns:

– true if there is more stuff to read and reads it into the variable next – false if you reached the end of the file and nothing is read into next

30

16

Reading from a file

int main(int argc, char** argv) { std::ifstream inStream; inStream.open("input.txt"); if( inStream.fail()) { std::cout << "Input file opening failed.\n"; exit(1); } std::string next; while(inStream >> next) { std::cout << next << " " << std::endl; } inStream.close(); }

31

Reading in from a file

• Instead of two lines:

ifstream inStream; inStream.open(“input.txt”);

• You can use one line:

ifstream inStream(“input.txt”);

32