October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 1 Introduction to Computer Programming Chapter 4: For Loops, Strings, and Tuples Michael Scherger Department of Computer Science Kent State University

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 2 Contents Constructing for loops to move through a sequence Use the range() function Strings as sequences Tuples Sequence functions and operators Index and slice sequences

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 3 The Word Jumble Game Example: Word Jumble Game

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 4 Using For Loops A “for” loop is another control structure for looping –It uses “counter-controlled repetition” to determine when the loop is to terminate It will repeat the loop body for each element in a sequence Example: Loopy String

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 5 Understanding For Loops For loops iterate over every element in a sequence –A string is sequence of character elements Example: for letter in anyword: print letter

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 6 Counting With A For Loop Often programs need to count and use the current counter value for computation –For loops are best for counting Example: Counter The range() function returns a sequence of elements

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 7 Range Function Examples Counting Forward –range(5) –range(10, 20) Counting Backward –range(20, 0, -1) –range(20, 10, -1) Counting by Different Amounts –range(0, 50, 5) –range( 50, 0, -5) for ii in range(10, 20) print ii for ii in range( 20, 0, -1) print ii for ii in range(0, 50, 5) print ii

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 8 Sequence Operators and Functions With Strings Remember, a string is a type of sequence The function len() returns the length of a sequence (or string) The operator “in” test if an element is in a sequence Example: Message Analyzer –How could this be written with a loop?

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 9 Indexing Strings Looping through a sequence or string character by character is an example of sequential access The index operator [] allows for random access of a sequence –Syntax: anyword[ii] Example: Random Access

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 10 Indexing Strings An index is a position in the sequence –In Python position can be positive or negative –anyword[1] == anyword[-4] Run-time error if the index is out of range! “A”“B”“C”“D”“E” anyword

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 11 Indexing Strings Here is a code fragment to access a random sequence/string element anyphrase = “I’d like to have an argument” high = len(anyphrase) position = random.randrange(0, high) print anyphrase[position]

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 12 String Immutability Sequences are either mutable or immutable –Mutable means changeable –Immutable means unchangeable Strings are immutable sequences

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 13 String Immutability Consider the following example >>> name = “John Cleese” >>> print name John Cleese >>> name = “Michael Palin” >>> print name Michael Palin Did we change the string? –No, we just assigned a new string to name

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 14 String Immutability #Runtime Error in Python word = “game” word[0] = “n” runtime error!!!!!!! name John Cleese Michael Palin X

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 15 Building A New String If your program needs to build a new string it must do so using the string concatenation operator (either + or +=) Example: No Vowels Note the use of the CONSTANT in the program

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 16 Slicing Strings Indexing allows access to a single element “slice” from a sequence (string) A slice is any consecutive part of a sequence (string) Example: Pizza Slicer Do More Examples!!!!!!!!!!!!!

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 17 Slicing Strings Slicing is similar to indexing except you can get a sub-string from the string –use two index values –anystring[low:high] “A”“B”“C”“D”“E”

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 18 Creating Tuples A tuple is a sequence that can hold elements of any type –Tuples can hold integers, real numbers, strings, lists, dictionaries, and other tuples all at the same time –Tuples are immutable Example: Hero’s Inventory

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 19 Creating Tuples To create a empty tuple anytuple = () –an empty tuple is considered to be False if not anytuple print “The tuple is not empty” To create a tuple with elements days = (“Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”, “Sun” )

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 20 More on Tuples To print a tuple print anytuple –Python will display each element of the tuple surrounded by parenthesis To loop through each element in a tuple for item in anytuple print item

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 21 Using Tuples Example: Hero’s Inventory 2.0 Using the in operator with tuples –This will test “membership” if an element is in a tuple if “Mon” in days print “Monday is a days of the week”

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 22 Using Tuples Remember tuples are immutable –days[1] = “MON” will give a runtime error Tuples can be constructed or modified by using concatenation (just like strings) –Use the + or += operator to add new elements to the end

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 23 Using Tuples Indexing tuples –Just like indexing strings –Example: print days[1] print days[4] Slicing tuples –Just like slicing strings print days[2:3] print days[4:] print days [-4:-1]

October 4, 2005ICP: Chapter 4: For Loops, Strings, and Tuples 24 Word Jumble Game - Again Example: Word Jumble Game