Introduction to Computer Programming 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 ICP: 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 ICP: Chapter 4: For Loops, Strings, and Tuples

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

7. 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 Range Function Examples ICP: 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? ICP: 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 ICP: 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! 0 1 2 3 4 anyword “A” “B” “C” “D” “E” -1 -5 -4 -3 -2 ICP: 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] ICP: 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 ICP: 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 ICP: Chapter 4: For Loops, Strings, and Tuples

14. #Runtime Error in Python word = “game” word[0] = “n” runtime error!!!!!!! String Immutability X name John Cleese Michael Palin ICP: 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 ICP: 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!!!!!!!!!!!!! ICP: 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] 0 5 1 2 3 4 “A” “B” “C” “D” “E” -1 -5 -4 -3 -2 ICP: 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 ICP: 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” ) ICP: 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 ICP: 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” ICP: 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 ICP: 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] ICP: Chapter 4: For Loops, Strings, and Tuples

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