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The fencepost problem

Archive - Originally posted on "The Horse's Mouth" - 2006-01-10 03:03:12 - Graham Ellis

If you erect a fence of 10 panels, you'll need 11 fenceposts to hold it up. And if you write a program that joins together 10 elements on a line, you'll only need 9 separators between them - this is known as the "fencepost problem".

If you write a simple program loop to output each element from a list followed by a separator, you'll end with a spurious separator on the end of the line. Here's what I mean (in Python).

posts = ["apple","banana","cherry"]

panel = "and"

for place in posts:

   print place,panel,

print

which gives the result

apple and banana and cherry and

It would be possible - but long winded - to print out the connecting panel only for the second and subsequent posts:
posts = ["apple","banana","cherry"]

panel = "and"

i = 0

for place in posts:

   if i: print panel,

   i += 1

   print place,

print

but surely there's a better way?? Yes - there is.


In Python, the join method lets me connect a list of strings using another string as the glue between them. My example becomes:
posts = ["kiwi","orange","pineapple"]

panel = " with "

salad = panel.join(posts)

print salad

which gives the result

kiwi with orange with pineapple


Link - full source code


In Perl, the join function performs the same task:
@salad = ("Cherry","Damson",Fig");

print (join(" and ",@salad));



With PHP, the function you'll use is implode:
$salad = array("tangerine","pear","grape");

$fsal = implode(" and ",$salad);