Deleting the first element of a list

Duncan Booth duncan at NOSPAMrcp.co.uk
Thu Oct 3 10:36:30 CEST 2002


Bruce Dawson <bruce_deletethis_dawson at cygnus-software.com> wrote in 
news:3D9BC6F0.4080500 at cygnus-software.com:

> Then it should be much better to go:
> 
> myList.reverse()
> while myList:
>      # Do Something with the last element
>      del myList[len(myList)-1]
> 
> This changes it from O(n^2) to O(n), which could make it run thousands 
> of times faster.

Ignoring any questions of speed, it may make the code somewhat clearer to 
rewrite this as:

myList.reverse()
while myList:
     element = myList.pop()
     # Do Something with  element

Alternatively, in many cases you can just avoid the removal of individual 
items. For example if the list contains items to process, and during the 
processing it is possible that more items are added:

while myList:
    work, myList = myList, []
    for item in work:
        process(item)

-- 
Duncan Booth                                             duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?



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