Great Ideas in Computer Science

(10.    Substrings (continued))

        Remember:

            substr(T,N,L)

        is a Pascal function that outputs
        the substring of T
        that begins at position N
        and that has length = L.

        Suppose that S := 'Algorithm'.
        For each of the following,
            (i)    What do they compute?
           (ii)    Can you describe what they do in a
                    general way?
 

    d-    substr(S, 1, length(S)-1)
    e-    substr(S, 2, length(S)-1)
    f-    substr(S, 2, length(S)-2)

Answers:

d-    if S = 'Algorithm', then this returns 'Algorith';
        i.e., it returns all but the last character of S

        Why?  Because it takes S, finds the 1st character
        (which is 'A'), computes "length(S)-1" (which is
        9-1, which is 8), and then returns the substring
        that begins with 'A' and is 8 characters long.

e-    if S = 'Algorithm', then this returns 'lgorithm';
        i.e., it returns all but the first character of S

        Why?  Because it takes S, finds the 2nd
        character (which is 'l'), computes "length(S)-1"
        (which, as we just saw, is 8), and then returns
        the substring that begins with 'l' and is 8
        characters long.

f-    This takes S, finds the 2nd character (which,
        as we just saw, is 'l'), computes "length(S)-2"
        (which is 9-2, which is 7), and then returns
        the substring that begins with 'l' and is 7
        characters long.

        i.e., if S = 'Algorithm', then this returns 'lgorith';
        i.e., it returns all but the first and last
              characters of S

If you are still not sure about the English descriptions
of what these do in general, try several different
strings as input so that you can begin to see the
patterns.

Here are 2 more to try:

g-    substr(S, length(S)    , 1)
h-    substr(S, length(S-1), 2)

Click here for the answers and the rest of these
lecture notes.