The end to all language wars and the great unity API to come!

Steven D'Aprano steve+comp.lang.python at pearwood.info
Wed Jul 6 16:55:39 CEST 2011


rantingrick wrote:

> On Jul 6, 6:44 am, Steven D'Aprano <steve
> +comp.lang.pyt... at pearwood.info> wrote:
> 
>> A control structure is a structure which controls the program flow.
>> Control structures include:
>>
>> * jumps (goto, gosub, comefrom, exceptions, break, continue)
>>
>> * loops (for, while, repeat...until)
>>
>> * conditional branches (if, case/switch)
> 
> -------------------------------------------
> THIS CODE RESULTS IN A CONTROL STRUCTURE!
> 
> --> lst.sort(lambda x,y: cmp(x[1], y[1]))

No it doesn't.

How does it change the program flow? You call the sort method, it sorts, and
execution continues at the next statement. Regardless of whether you supply
a cmp function or not, the program flow is identical:

ENTER SORT ROUTINE
PERFORM SORTING
EXIT SORT ROUTINE

There is no control transferred. It is a linear program flow: in, do the
job, out again. Since it doesn't modify the program flow, it is not a
control structure.

"Perform sorting" is a black box. It could have loops, branches,
unconditional exists. It could have COMEFROM statements up the wazoo, if it
were implemented in a language with COMEFROM (like Intercal). None of that
matters two bits: the caller cannot use sort to modify the execution
sequence around it, therefore it's not a control structure. No matter how
much the sort routine jumps around internally, you can't use that change
program flow around it.

print surely is implemented with a loop: it has to loop over a string and
write it to stdout. Would you say that therefore print is a control
structure:

ENTER PRINT STATEMENT
PERFORM PRINTING
EXIT PRINT STATEMENT

One entry, one exit.



-- 
Steven




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