Compsci 6/101, Fall 2011, Lab 3

This lab has the following parts:

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String Slicing and Dicing

Assume you have typed the following lines into your Python Console: 

a = "computational thinking"
b = "duke university"
c = "python code"

Use the slicing operator, string concatenation, string multiplication (by an int, as in "abc" * 3), or indexing to form each of the words below. Each use of any operator (slicing, indexing, concatenation, and multiplication) has a cost of one. Indicate the cost of your construction as well as the operators you used. The first two are done for you to serve as examples.

  1. "honk" formed from c[3:6]+b[2]

    This has a cost of three: one slice, one catenation, one index.

  2. "puking" formed from a[3:5]+a[18:]

    Again the cost is three: two slices and one catenation with +.

  3. Create "docking" and provide its cost.
  4. Create "cocoa" and provide its cost.
  5. Create "thought" and provide its cost.
  6. Create "ratatatat" and provide its cost.
  7. Create "duke is cool" and provide its cost.

Vocabulary

A method is a function invoked on an object. For example, the table below shows some common string methods.
string method purpose
s.upper() returns string upper case version of string s
s.count(sub) returns int number of (non-overlapping) occurrences of sub in s
s.endswith(sub) returns boolean depending on whether s ends with sub
s.find(sub) returns int: first index at which sub occurs in s or -1 if no occurrences
s.split() returns list of s split on whitespace
s.split(sep) returns list of s split on sep, a delimiter
s.strip() returns copy of s withOUT leading and trailing whitespace

 

list method purpose
lst.count(elt) returns number of occurrences of elt in lst
lst.index(elt) returns first/least index at which elt occurs in lst, generates error if elt not in lst
lst.append(elt) append elt to end of lst, None returned

Functions are applied to objects. For example, len returns an int: the length of the given string or list or other sequence/iterable. So len("apple") is 5 and len([1,2,3]) is 3. For example, the table below shows some common string methods.

function call result returned type returned
len([1,2,3,"apple"]) 4 int
sum([1,2,3,4]) 10 int
max([5,4,1,2,9,3]) 9 same as list elements
max("ape", "bee", "zebra", "wildebeast"] "zebra" same as list elements
min([5,4,1,2,9,3]) 1 same as list elements
sorted([5,4,1,2,7]) [1,2,4,5,7] list
reversed([1,2,3,6]) [6,3,2,1] list

Vocabulary Questions

  1. What do you think the name of the string method is that returns the lowercase version of a string?
  2. The function max returns the maximal value of a string as well as a list. What is max("science") and why?
  3. What is the value of reversed(sorted([5,4,1,2,8])) and why?
  4. What is the value of sorted(lst).index(min(lst)) for any list lst? Why? What's the value if min is replaced by max?
  5. Why is the value of st.endswith(st[-3:]) guaranteed to always be true?
  6. When is the value of st.upper().endswith(st) true?