Note

This appendix documents the pcinput module from the printed book, as reference material for readers who run the exercises in their own environment, outside Dodona. The module is not available on Dodona, so you cannot import it in a solution that you submit. On Dodona you read the input with input() and cast it to the type you need: int(input(prompt)) instead of getInteger(prompt), float(input(prompt)) instead of getFloat(prompt), and input(prompt) instead of getString(prompt) or getLetter(prompt). The functions below keep asking until the user enters something valid, and that is not needed on Dodona, where the input of an exercise comes from a fixed test case and is thus always valid already.

In many of the exercises in this book, it is useful to have a function available that accepts inputs of a certain type. I created a module called pcinput which contains a number of such functions. During many of the exercises in this book, I assume that you have that module available. To get it, download it from http://www.spronck.net/pythonbook, or copy the code below to a file called “pcinput.py,” and make sure that it is located in same folder where you keep the files with your own code.

Note that these functions are rather ugly as they print error messages if something is wrong. However, nicer functions would be more difficult to use (you would have to know about exceptions, which are not covered until Chapter 18). For the purpose of learning to code Python, they work fine.

Each of the functions asks the user to supply a value of a certain type (a float, an integer, a string, or a capital letter), and returns that value. You can call each of the functions with a string as argument, that will be used as prompt.

def getFloat( prompt ):
    while True:
        try:
            num = float( input( prompt ) )
        except ValueError:
            print( "That is not a number -- please try again" )
            continue
        return num

def getInteger( prompt ):
    while True:
        try:
            num = int( input( prompt ) )
        except ValueError:
            print( "That is not an integer -- please try again" )
            continue
        return num

def getString( prompt ):
    line = input( prompt )
    return line.strip()

def getLetter( prompt ):
    while True:
        line = input( prompt )
        line = line.strip()
        line = line.upper()
        if len( line ) != 1:
            print( "Please enter exactly one character" )
            continue
        if line < 'A' or line > 'Z':
            print( "Please enter a letter from the alphabet" )
            continue
        return line