CATHERINE AND ANNIE Python: Part 4

Strings  Strings are interesting creatures. Although words are strings, anything contained within a set of quotes is treated as a string in Python  Examples: ‘38949’ is a string, as is ‘…\w{][{}’, as is ‘hello world!’  Spaces count as characters! So do punctuation marks  Computers treat strings like ‘lists’ of characters, all strung together  So you can access each character of a string using exactly the same syntax that you would to access an element in a list  In your folder, open the file that says “stringExample.py”  Right-click > Edit with IDLE  How do you think the for loop in this program works? What would happen if we didn’t include the comma after the print statement in the loop?

ASCII  ASCII stands for American Standard Code for Information Interchange (meaning it’s a way to communicate information)  In ASCII, each character, including numbers, punctuation, and white space characters are given a number, which is its ASCII value  You can see a full ASCII table at  There are two methods that you can use with ASCII:  ord(x) – gives you the ASCII value of x, a character  chr(x) – gives you the character that corresponds to x, an ASCII value  You can use ord and chr to do some very simple cryptography (ask Annie or me about it later if you want to know more)

Some useful string operators  + joins two strings together  Example: ‘soccer’ + ‘ball’ = ‘soccerball’  * repeats a string  Example: ‘cool’ * 3 = ‘coolcoolcool’  Indexing, works the same way indexing with lists does  word = ‘hello’, word[1] = ‘e’  Slicing  Used if you want to access a smaller section of a string; looks kind of like indexing  word[1:4] = ‘ell’ **NOTE: this did not return word[4]; slicing is exclusive: it goes up to, but does not include the second index

The string library  Just as there is a math library that you can use in your programs, there is also a string library that you can use  At the top of the program, you must type import string  This tells the computer that you want to see the functions contained in the string library file

More of the string library  Here’s a partial list of the functions contained in the string library:  string.capitalize(s) – capitalizes s (a string you would give the computer)  string.upper(s) – makes all the characters of s capital letters  string.strip(s) – removes all the white space (blank spaces, tabs, etc.) from the beginning and end of s  string.find(s, sub) – sub is a smaller string that you want to find within the larger string, s  string.count(s, sub) – counts the number of times sub occurs in s

Conditionals  Do you remember the work we did with logic gates yesterday?  Both of these are also used in Python  When might you want to use conditionals?

Boolean logic: a quick review  There are a few basic logic gates that we’re working with:  NOT: Requires one input, and makes the input opposite; turns true into false and false into true  OR: Requires two inputs, and as long as one input is true, then the OR gate returns true (or 1)  AND: Requires two inputs, and both must be true in order for the AND gate to be true (or 1)  Then there are the exclusive gates (which we don’t really have to worry about)  XOR: Works the same as OR, except that if both inputs are true, XOR is false  XAND: If one input is true, XAND is true. If both inputs are true, XAND is false  And last are the gates that combine NOT and either AND or OR  NAND: AND + NOT  NOR: OR + NOT For both of these, complete the OR or AND gate first, and then negate it using NOT

Some quick practice using the Python shell  Let’s open the Python shell and see this logic at work  Type in this: x = 5 then press Enter  Now type in x == 4 or x > 2 then press Enter. Did you get the result you were expecting?  Type in x == 5 and x > 8 then press Enter. Did you get the result you were expecting?

Conditional statements  Conditional statements are statements that depend on one or more conditions.  Conditional statements are used in programming when you want to complete a task, but only when certain conditions are met.  Example: You want to find the sum of two numbers, but only if they are both less than 5. How could you test that?  You (the programmer) set the condition, but the computer decides whether or not the condition is met!  There are a few different structures you can use with conditional statements in Python

If-statements  The simplest kind of conditional is a simple if- statement. Python evaluates the condition of the if- statement is true, the statement is completed. Otherwise, Python ignores it and moves on.  Syntax: if( ):  The condition can be anything you like, and there can be one or more than one. You can also use the logic gates OR and AND to test multiple conditions  Example:

If-else statements  If-else statements are used when you want the computer to perform an action even if the condition of your if-statement isn’t true.  For example: as long as two number are each less than five, find their sum. Otherwise, find their difference.  Coded:  Though an if-statement on its own will be ignored if its condition is false, an if-else statement will go straight to the else if the if statement’s condition is false  Notice that an else statement has no condition

If-elif-else statements  If-elif-else statements are used when there are three or more options  For example: A traffic light. IF the light is red, stop. ELSE IF the light is yellow, go. ELSE: go (because the only possibility left is that the light is green.)  The “elif” stands for else if  There is only ever one if statement and one else statement but there can be as many elif statements as you want.  Each condition is evaluated, beginning with the if and ending with the else. Whenever the computer reaches a statement for which the condition is true, it performs that code within that statements and then skips the rest of the statements

What structure would you use in each of these situations?  Determining where a point is on the Cartesian plane  If-elif-else  Determining the winner of a basketball game  If-else  Determining the number of days in a month  If-elif-else  Determining whether you passed a class or not  If-else

Your turn! Open the file called “Python – Lesson 4 Exercises” and complete the exercises