In the Absence of a GoTo Statement: Newbie needs menu-launcher to choose amongst sub-programs

[Alex Martelli]

"Ron Stephens" <rdsteph at earthlink.net> wrote in message
news:3AD5EEEF.C4CB8AF2 at earthlink.net...
> Sacrilege, I know. ;-))) But sometimesIi wonder why no modern language
> will let me have a simple goto statement when nothing else will do as
> well...this is a rhetorical statement only...

Answering rhetorical questions is one of my favorite hobbies, and,
here, the answer is: "because, in a well-designed language, there
is no situation in which ``nothing else will do as well''":-).


> I have written and am writing a series of similar small programs to help
> a user choose amongst several different alternatives. The programs
    ...
> user can choose the general program, or else a specific program. Once
> the user chooses, the appropriate mini-progran should launch.

OK.  Now, is each of these programs in a separate .py file, or
is each a function, each with a different name, inside the same
main .py file?

> So, now  I consider procedural menu program using raw_input. Still, how
> do I go to the appropriate sub-program when the user chooses one? With

In the "separate files" case, assuming you want to run just
one program as chosen by the user:


program_names = ('one', 'two', 'three', 'four')
num_programs = 1+len(program_names)

prompt = "Please enter a number between 1 and %s" % num_programs

while 1:
    input_string = raw_input(prompt)
    try:
        input_code = int(input_string)
    else:
        if 1<=input_code<=num_programs:
            program = program_names[input_code-1] + '.py'
            execfile(program)
            break


The "several functions" case is not too different:

def one():
    # your code goes here

def two():
    # your code goes here

# etc, then

functions = (one, two, three, four)
num_programs = 1+len(functions)

prompt = "Please enter a number between 1 and %s" % num_programs

while 1:
    input_string = raw_input(prompt)
    try:
        input_code = int(input_string)
    else:
        if 1<=input_code<=num_programs:
            function = functions[input_code-1] + '.py'
            function()
            break


> goto's this would be trivial. Now, I consider a nested set of "if "
> statements that determines which number was chosen, but still how do I
> "jump" to the correct sub-program??? Even if I define the subprograms as
> functions even, how do I jump to them???

You don't really "jump" in this approach -- you do _invoke_ your
code (in a file or function), but, when that is done (if without
raising exceptions), you get back to the point from where you
had invoked it.  Which is why I have the "break" statement, to
end the 'while 1' loop when the operation of the subprogram
is finished.  (If errors were to be expected and caught, the
function-call would be placed inside a try/except statement).


Alex