python, how to write an iterative function

Question

I am quering a database for some paramaters which depend on a attribute called count! count can be incremented incase the 1st query does not return anything. Here is a sample code

sls = {(213.243, 55.556): {}, (217.193, 55.793): {}, (213.403, 55.369): {}}

for key in sls.keys:
  if not sls[key]:
     ra, dec = key[0], key[1]
     search_from_sourcelist(sl, ra,dec)

count = 1
def search_from_sourcelist(sl, ra,dec):
  dist = count/3600.0
  sls[(ra,dec)] = sl.sources.area_search(Area=(ra,dec,dist))
  return

Incase i run the method search_from_sourcelist, and it doesnt return anything, i would like to increment count, and do the query again. This is to be done for all keys in sls dictionary, untill all the keys have a value!!

You don't have a function anywhere in your code that calls itself, so I'm not sure you understand what "recursive" means. — Wooble
– Wooble, Commented Jun 28, 2011 at 13:19

Jakob Bowyer · Accepted Answer · 2011-06-28 13:26:20Z

4

Here is the most fundamental recursive function

def countdown(n):
    if n == 0:
        return "Blastoff"
    else:
        print "T minus %s" % n
        return countdown(n-1)

You will notice that countdown returns itself with a modified argument, in this case n but -1, so if you actually followed this all the way through you would get (-> indicates a call)

countdown(5) -> countdown(4) -> countdown(3) -> countdown(2) -> countdown(1) -> countdown(0) #stop

So now you understand what a recursive function looks like you realize you never actually return a call of your own function, thus your code is not recursive

We use recursion because we want to boil a task down to its simplest form then work from there, so a good example of this would be the mcnuggets problem. So you need to tell us what you are trying to achieve and how it can be made a smaller problem (or more importantly why.) Are you sure you cannot do this iteratively? remember you don't want to blow your stack depth because python is NOT tail recursive by standard

edited Jun 28, 2011 at 13:26

answered Jun 28, 2011 at 13:19

Jakob Bowyer

34.8k8 gold badges80 silver badges92 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user739807 Over a year ago

i am aware of that fact that there is no recursion, and that s what i need help with!

user739807 Over a year ago

perfect, i wanted an iterative method not recursion!

Johannes Charra · Accepted Answer · 2011-06-28 13:38:37Z

1

Recursion is useful when you find a way to reduce the initial problem to a "smaller version of itself".

The standard example is the factorial function

def fac(n):
    return n * fac(n-1) if n > 1 else 1

Here you reduce the problem of calculating the factorial of n to calculating the factorial of n-1.

In your code there is no such "reduction". You just increment a value and start the same problem over again. Thus, I recommend you solve it iteratively.

answered Jun 28, 2011 at 13:38

Johannes Charra

30k6 gold badges45 silver badges54 bronze badges

1 Comment

user739807 Over a year ago

oh yeah i actually dont need recusion! ThankS

inspectorG4dget · Accepted Answer · 2011-06-28 13:40:28Z

1

I'm not sure that you need a recursive algorithm for this.

Incase i run the method search_from_sourcelist, and it doesnt return anything, i would like to increment count, and do the query again. This can be done with a while loop as follows:

for key, value in sls.iteritems():
    if not value:
        ra, dec = key[0], key[1]
        count = 1
        while not search_from_sourcelist(sls, ra, dec):
            count += 1

But if you really do want to do this recursively, you can do it as follows, leave a comment and I'll write it up.

Further, you should look into your search_from_sourcelist function, as it always returns None

answered Jun 28, 2011 at 13:40

inspectorG4dget

115k30 gold badges159 silver badges253 bronze badges

Collectives™ on Stack Overflow

python, how to write an iterative function

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related