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Indexing Strings

Mon, 05 Oct 2009 23:16:02 -0700

In the code below, the only part I'M having trouble with is word[position]. I know this is something really simple but I've always had trouble understanding these kinds of statements. I know that it print a random letter (position) from (word). What I don't know is why or how it does that. That's the part that never seems to get explained to me. How can you just put [] around part of a statement and everything just work right? Thanks.

# Random Access
# Demonstrates string indexing

import random

word = "index"
print "The word is: ", word, "\n"

high = len(word)
low = -len(word)

for i in range(10):
    position = random.randrange(low, high)
    print "word[", position, "]\t", word[position]

raw_input("\n\nPress the enter key to exit.")

Re: Indexing Strings

Thu, 08 Oct 2009 14:43:27 -0700

I'm not quite sure how to explain this if you already have it written out as 'word' and 'position'. Given a string "word" and an integer "i", word[i] returns the letter in the ith position. Remember that Python is zero-indexed, though, so for the word "index", you have
i -> 0
n -> 1
d -> 2
e -> 3
x -> 4
Note that there is not position 5, even though len(word) == 5.

Also, putting '-' in an index has a special meaning in Py. That makes your index start at the end of the word instead of at the beginning. So, you get

x -> -1
e -> -2
d -> -3
n -> -4
i -> -5

That's nice for when you need to get to the back of a string/list, but you don't know how long it is. I guess you could also use it to count backwards...

Re: Indexing Strings

Thu, 08 Oct 2009 23:26:59 -0700

Thanks for the help. I really appreciate it.