Home Original page

Iterating over multiple lists (a newbie question)

Alex Martelli aleaxit at yahoo.com
Fri Jan 5 12:14:47 EST 2001 
"Victor Muslin" <victor at prodigy.net> wrote in message
news:3a54d0c5.523585234 at localhost...
    [snip]
> Suppose I want to parse command line arguments (never mind getops,
> since this could be some other situation not involving command line
> arguments). Here is how I could possibly parse command line arguments
> for command: "myprog -port 10 -debug":

If the situation does not involve command line arguments, its
general case would seem to be:
    -- I start with a list of strings, X
    -- I would like to make a list of strings, Y,
        which is the same as X _except_ that:
        -- for certain strings (say those in a set S),
        -- if a string in S is found as the non-last
            item of X,
        -- then I want to make the corresponding
            item of Y by "merging" that string in
            S with the following one in X

Applied to your example, that would make a list
X=['-port', '10', '-debug'] into another list
Y=['-port 10', '-debug'] (using ' ' as a separator
for the "merging") which you can then parse more
easily.  OK, then, one possibility:

def mergeSome(X, S, joiner=' '):
    Y=[]
    joinnext=0
    for x in X:
        if joinnext:
            Y[-1] += joiner+x
            joinnext=0
        else:
            Y.append(x)
            joinnext = x in S
    return Y

or, if one prefers to work with indices:

def mergeSome(X, S, joiner=' '):
    Y=[]
    i=0
    while i<len(X)-1:
        x = X[i]
        if x in S:
            Y.append(x+X[i+1])
            i += 2
        else:
            Y.append(x)
            i += 1
    if i<len(X):
        Y.append(X[i])
    return Y


Alex
More information about the Python-list mailing list