Steve,
On 17/11/18 03:52, Steve Keller wrote:
I have looked at generators, generator expressions, and iterators and
I try to get more familiar with these.
1. How would I loop over all (with no upper bound) integers or all
powers of two, for example?
In C it would be
for (int i = 0; ; i++) { ... } or for (int i = 1; ; i *= 2) { ... }
In Python, I could define a generator
def powers():
i = 1
while True:
yield(i)
i *= 2
for i in powers():
...
Is there a fundamental error here? Is the attempt to start from C (or
whatever) and reproduce C-code in Python?
Whilst it can be a useful early-learning path to translate "commands"
from one language to another, in order to commence actual usage quickly;
one should not try to use such kindergarten techniques to discuss
philosophy! First, learn the new language's idioms - a combination of
language-vocabulary and its underlying culture.
In this case, the comparison is between the for-command of a for-loop
(C), with Python's for-in structure. Python's idiom is to loop over the
contents of something: the characters within a string of characters, the
elements of a list, etc.
The C languages (and many others) have an idiom which controls the
repetition of a loop without regard to the data being processed, ie for
index = start-value, end-value, incremental-amount. This works
regardless of what the code is looping 'over', if indeed, anything.
In Python, we don't look at the "index", this is provided (ie "batteries
included"). Python looks at an instance of a collection object
(previously "sequence members") which is also an iterator (see your
previous questions). The mention of an iterable within a for-in
statement causes the production of an appropriate element-value at each
loop of the construct. Whereas with C one uses the index to choose which
item of data to consider during this iteration of the loop, eg
array[index]. In C the 'subject' of the loop is the index (or "offset").
In Python it is the collection, and iteratively, its elements.
This idiom removes a significant proportion of C (etc) programming
errors caused by the necessity to use "pointers" or indices, eg the
famous "out by one" error. Whereas in C it is necessary to use a pointer
to loop over the two choices, in Python we can immediately use, eg
for gender in [ "male", "female" ]: with no pointer and direct access to
the label-values, cf (define genders_array earlier, and then for index =
1, 2; this_loops_gender = genders_array( index ); use this_loops_gender.
To directly answer the narrow coding-question posed: in Python one
achieves "counting loops" using a while-construct, eg
count = 0 #or start-value
while ( count < maximum ):
etc
count += 1
The set-up, condition, and increment corresponding directly with the
descriptions of "start-value, end-value, and increment-amount" in C
(etc). In the case of "infinite loops: a common Python idiom is:
while True:
etc
with attendant start/increment code. In which case you enjoy all of the
benefits of a generator (eg no need to generate (and store) all of the
index values before looping commences) but without the abstraction in
the example code - simplicity before complexity!
Python also offers a second-level answer in the Python Standard Library.
The basic concepts of an iterator and/or the underlying technologies are
made available in "itertools" - just as you will find with many other
facilities within and around the language. There are a couple of
specific edge-cases (a deliberate, never-ending loop) within that
library. Enjoy!
Is it a valid approach to first solve a problem by 'thinking in C' and
only then trying to translate that 'solution' into Python? This approach
is where Star Wars' concept behind Yoda-speak comes from: some languages
sequence the words in a sentence quite differently from the preferred
subject-verb-object order of English:
Jack hits the ball.
Jack the ball hits.
The ball hit (by) Jack.
IMHO Python is NOT a 'better version of C' - for any definition of
"better", and regardless of anyone's enthusiastic preferences. There are
situations where it would be 'better' to use one in preference to the
other. Both ways! To write in Python, use Pythonic thinking, Python
constructs (one author talks of "Python's beautiful heart"), and the
idioms of Python; to realise its power!
--
Regards =dn
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