On Tue, 2006-03-28 at 06:15 -0800, Sathyaish wrote: > And that the "extra-memory" operation I've given above is expensive, I > believe. Is there an efficient way to do it? >
How big is your string? For short strings (i.e. where large means you don't have enough RAM to hold one extra copy.) >>> "Abc"[::-1] 'cbA' >>> Also, anytime you reach for a for-loop to build a string step by step, you are making a mistake. Consider your example. strText = "foo" strTemp = "" for chr in strText: strTemp = chr + strTemp Each loop you are copying the string again, the timing behavior of your function is O(n^2). If you are really concerned about memory allocation, well, I don't know if you realize this, but every time you call strTemp = chr + strTemp you are throwing away your old copy and building a new copy. Ouch. Forgive me for preaching, but you just committed the grievous act of premature optimization. Don't worry about that first malloc, if Python is going to call malloc, it has a lot of opportunity to do so later. And malloc isn't as evil as you make it out to be. One of the advantages of using a high level language is you get to leave the issue of how to implement the small stuff up to the language designer and focus on the bigger picture - algorithmic appropriateness and overall correctness. In my experience I've found that when under time pressure python programs tend to out perform C because doing it right is so much easier in the former. As for mutability, immutability is a big virtue and performance gain. If I have two pointers to immutable strings, once I compare them I can know for eternity which is larger, so long as I don't let go of my references to them. Thus I can use them as keys in a complicated and efficient data structure. If Python strings were mutable the best implementation we could hope for dict would be a linked list. Also, consider some other side effects of mutable strings. >> s = "Abc" >> myfancy_structre.add_somehow( s ) >> t = s[::-1] >> print s Abc >> print t cbA Now if strings were mutable: >> s = "Abc" >> myfancy_structre.add_somehow( s ) >> s.reverseme() >> print s cbA Other users of s between assignment and reversal (like myfancy_structure) might not be happy that is was reversed when they next must use it. Cheers - Adam DePrince -- http://mail.python.org/mailman/listinfo/python-list
