COP 3223H meeting -*- Outline -*-

* Input and Output (I/O) in Python

  Input and output are handled by statements, since they change the
  state of the program's environment

** input/output devices/streams
   In this course we will focus on terminal-style interaction
   based on character streams (keyboards and monitors)...
     (it's simplest)
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    STANDARD INPUT AND OUTPUT DEVICES

In most operating systems (POSIX, Unix,...):

     stdin                stdout
     ------> [ Program ] ------>
                      \  stderr
                       \------->

  these are conventional names for I/O
  streams of characters

   stdin: standard input (often from a user)
            could be the terminal/console
            could be a file (prog <file)

   stdout: standard output (often to a user)
            could be the terminal/console
            could be a file (prog >file)

   stderr: standard error (usually to a user)
            usually a terminal/console/shell
            could be a file

In Unix (POSIX) system shells:

     prog < in > out 2>errs

runs the program with input from file "in"
output going into file "out" and errors
going into file "errs"

      prog1 <in1 | prog2 | prog3 >out3

sets up a "pipeline" where prog1, prog2,
and prog3 run in parallel,
    prog1's stdin is file in1
    prog2's stdin is prog1's stdout
    prog3's stdin is prog2's stdout
    prog3's stdout is out3
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        Unix also has more advanced ways of plumbing programs
        together, but this is a common pattern (pipes and filters)

** the problem, abstractly
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      THE PROBLEM OF INPUT AND OUTPUT

Input





Output






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        ...conveys some information from the external world
           (outside the computer) to the program
           - it must be encoded (or parsed)
             into some representation that the program can use

        ...conveys some information from the program to the extranal world,
           - it must be decoded (or unparsed)
             from the program's internal representation into
             a representation that conveys the intended information

        Q: Have you ever interacted with a program that was hard to
           talk to in a way that was understandable?
           A classic example is the error messages in modern GUIs,
           which come up with a button "OK" that to the program means
           "you acknowledge you have read this message" and to you
           probably means the opposite of "OK" (more like "Darn it!")

** output
*** printing
      printing uses character strings as an external representation
      for communication
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          OUTPUT IN PYTHON

print() function handles simple output
        (by default to sys.stdout)

this is like printf in C

Examples:
>>> print("hi")

>>> t = 7
>>> print("The value of t is:", t)

>>> t = t + 1
>>> print("Now t", t, sep='==')

>>> t = t+1
>>> print("And now t is", t)

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        Q: How did that space get in the output before 7?
        print separates its arguments with a space by default
        (but printf in C doesn't do that)
        Q: What does the sep keyword parameter do?
          specifies a string that separates the actuals in the output
        Note: keyword parameters are optional and have a default value
        (shown in IDLE's tool tips).
        They can be temporarily given a different value, in a call by
        adding sep='...' to the end of the call.

        C doesn't have any keyword parameters
        
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    OPTIONAL KEYWORD ARGUMENTS TO PRINT

NAME=<default>   Explanation
=========================================
sep=' '          separator printed between 
                   the arguments
end='\n'         terminator printed after
                   all the arguments

file=sys.stdout  output device

flush=False      forcibly flush the output
                   device?

Examples:

  print("Error!", file=sys.stderr,
        flush=True)
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      in C different functions handle these options (like fflush())

*** other ways to print
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     THERE ARE OTHER WAYS TO PRINT

We may see later:
   - write method on file objects
   - format strings
   - str.format()
   - printf style % operator formatting

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** character-based input in Python
   There are other important kinds of input, mouse clicks, for
   example, but character input is sufficient
   
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        READING A LINE OF INPUT
        WITH THE INPUT FUNCTION

Reading a line from sys.stdin:

>>> text = input("what number? ")

>>> text

>>> text + 3    # error!



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        Note that the newline in the input is stripped off,
        and that the result is a string!
        So the last bit is an error!

        similarly in C, but C doesn't have a function like "input" built-in

*** reading and parsing
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        PARSING READ INPUT

To convert a string into a number
or some other type, input is parsed.

Parsing an int:

   myint = int(input("number? "))

Parsing a float:

   myint = float(input("number? "))
   
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        Note I think it looks nice to put a space after the question
        mark

        Other types are similar, but we haven't gotten to them yet.

        In C one uses fscanf or sscanf to do these tasks.

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         FOR YOU TO DO

Write a Python program that reads 3 ints
from standard input and prints their sum.







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*** input from files
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        READING FROM A FILE

f = open(filename) # prepares to read from filename

f_contents = f.read()  # reads entire contents of f

See modules file and fileinput...

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        C uses library functions for these tasks