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Python Programming: An Introduction to Computer Science. Chapter 7 Decision Structures. Objectives. To understand the programming pattern simple decision and its implementation using a Python if statement.

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## Python Programming: An Introduction to Computer Science

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**Python Programming:An Introduction toComputer Science**Chapter 7 Decision Structures Python Programming, 1/e**Objectives**• To understand the programming pattern simple decision and its implementation using a Python if statement. • To understand the programming pattern two-way decision and its implementation using a Python if-else statement. Python Programming, 1/e**Objectives (cont.)**• To understand the programming pattern multi-way decision and its implementation using a Python if-elif-else statement. • To understand the idea of exception handling and be able to write simple exception handling code that catches standard Python run-time errors. Python Programming, 1/e**Objectives (cont.)**• To understand the concept of Boolean expressions and the bool data type (requires Python 2.3 and newer) • To be able to read, write, and implement algorithms that employ decision structures, including those that employ sequences of decisions and nested decision structures. Python Programming, 1/e**Simple Decisions**• So far, we’ve viewed programs as sequences of instructions that are followed one after the other. • While this is a fundamental programming concept, it is not sufficient in itself to solve every problem. We need to be able to alter the sequential flow of a program to suit a particular situation. Python Programming, 1/e**Simple Decisions**• Control structures allow us to alter this sequential program flow. • In this chapter, we’ll learn about decision structures, which are statements that allow a program to execute different sequences of instructions for different cases, allowing the program to “choose” an appropriate course of action. Python Programming, 1/e**Example:Temperature Warnings**• Let’s return to our Celsius to Fahrenheit temperature conversion program from Chapter 2. # convert.py # A program to convert Celsius temps to Fahrenheit # by: Susan Computewell def main(): celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32 print "The temperature is", fahrenheit, "degrees Fahrenheit." main() Python Programming, 1/e**Example:Temperature Warnings**• Let’s say we want to modify that program to print a warning when the weather is extreme. • Any temperature over 90 degrees Fahrenheit and lower than 30 degrees Fahrenheit will cause a hot and cold weather warning, respectively. Python Programming, 1/e**Example:Temperature Warnings**• Input the temperature in degrees Celsius (call it celsius) • Calculate fahrenheit as 9/5 celsius + 32 • Output fahrenheit • If fahrenheit > 90 print a heat warning • If fahrenheit > 30 print a cold warning Python Programming, 1/e**Example:Temperature Warnings**• This new algorithm has two decisions at the end. The indentation indicates that a step should be performed only if the condition listed in the previous line is true. Python Programming, 1/e**Example:Temperature Warnings**Python Programming, 1/e**Example:Temperature Warnings**# convert2.py # A program to convert Celsius temps to Fahrenheit. # This version issues heat and cold warnings. def main(): celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32 print "The temperature is", fahrenheit, "degrees fahrenheit." if fahrenheit >= 90: print "It's really hot out there, be careful!" if fahrenheit <= 30: print "Brrrrr. Be sure to dress warmly" main() Python Programming, 1/e**Example:Temperature Warnings**• The Python if statement is used to implement the decision. • if <condition>: <body> • The body is a sequence of one or more statements indented under the if heading. Python Programming, 1/e**Example:Temperature Warnings**• The semantics of the if should be clear. • First, the condition in the heading is evaluated. • If the condition is true, the sequence of statements in the body is executed, and then control passes to the next statement in the program. • If the condition is false, the statements in the body are skipped, and control passes to the next statement in the program. Python Programming, 1/e**Example:Temperature Warnings**Python Programming, 1/e**Example:Temperature Warnings**• The body of the if either executes or not depending on the condition. In any case, control then passes to the next statement after the if. • This is a one-way or simple decision. Python Programming, 1/e**Forming Simple Conditions**• What does a condition look like? • At this point, let’s use simple comparisons. • <expr> <relop> <expr> • <relop> is short for relational operator Python Programming, 1/e**Forming Simple Conditions**Python Programming, 1/e**Forming Simple Conditions**• Notice the use of == for equality. Since Python uses = to indicate assignment, a different symbol is required for the concept of equality. • A common mistake is using = in conditions! Python Programming, 1/e**Forming Simple Conditions**• Conditions may compare either numbers or strings. • When comparing strings, the ordering is lexigraphic, meaning that the strings are sorted based on the underlying ASCII codes. Because of this, all upper-case letters come before lower-case letters. (“Bbbb” comes before “aaaa”) Python Programming, 1/e**Forming Simple Conditions**• Conditions are based on Boolean expressions, named for the English mathematician George Boole. • When a Boolean expression is evaluated, it produces either a value of true (meaning the condition holds), or it produces false (it does not hold). • Some computer languages use 1 and 0 to represent “true” and “false”. Python Programming, 1/e**Forming Simple Conditions**• Boolean conditions are of type bool and the Boolean values of true and false are represented by the literals True and False. >>> 3 < 4 True >>> 3 * 4 < 3 + 4 False >>> "hello" == "hello" True >>> "Hello" < "hello" True Python Programming, 1/e**Example: Conditional Program Execution**• There are several ways of running Python programs. • Some modules are designed to be run directly. These are referred to as programs or scripts. • Others are made to be imported and used by other programs. These are referred to as libraries. • Sometimes we want to create a hybrid that can be used both as a stand-alone program and as a library. Python Programming, 1/e**Example: Conditional Program Execution**• When we want to start a program once it’s loaded, we include the line main()at the bottom of the code. • Since Python evaluates the lines of the program during the import process, our current programs also run when they are imported into an interactive Python session or into another Python program. Python Programming, 1/e**Example: Conditional Program Execution**• Generally, when we import a module, we don’t want it to execute! • In a program that can be either run stand-alone or loaded as a library, the call to main at the bottom should be made conditional, e.g.if <condition>: main() Python Programming, 1/e**Example: Conditional Program Execution**• Whenever a module is imported, Python creates a special variable in the module called __name__ to be the name of the imported module. • Example:>>> import math>>> math.__name__'math' Python Programming, 1/e**Example: Conditional Program Execution**• When imported, the __name__ variable inside the math module is assigned the string ‘math’. • When Python code is run directly and not imported, the value of __name__ is ‘__main__’. E.g.:>>> __name__'__main__' Python Programming, 1/e**Example: Conditional Program Execution**• To recap: if a module is imported, the code in the module will see a variable called __name__ whose value is the name of the module. • When a file is run directly, the code will see the value ‘__main__’. • We can change the final lines of our programs to:if __name__ == '__main__': main() • Virtually every Python module ends this way! Python Programming, 1/e**Two-Way Decisions**• Let’s look at the quadratic program as we left it. # quadratic.py # A program that computes the real roots of a quadratic equation. # Illustrates use of the math library. # Note: This program crashes if the equation has no real roots. import math # Makes the math library available. def main(): print "This program finds the real solutions to a quadratic" print a, b, c = input("Please enter the coefficients (a, b, c): ") discRoot = math.sqrt(b * b - 4 * a * c) root1 = (-b + discRoot) / (2 * a) root2 = (-b - discRoot) / (2 * a) print print "The solutions are:", root1, root2 main() Python Programming, 1/e**Two-Way Decisions**• As the comment implies, whenb2-4ac < 0, the program tries to take the square root of a negative number, and then crashes. This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,1,2 Traceback (most recent call last): File "C:\Documents and Settings\Terry\My Documents\Teaching\W04\CS 120\Textbook\code\chapter3\quadratic.py", line 21, in -toplevel- main() File "C:\Documents and Settings\Terry\My Documents\Teaching\W04\CS 120\Textbook\code\chapter3\quadratic.py", line 14, in main discRoot = math.sqrt(b * b - 4 * a * c) ValueError: math domain error Python Programming, 1/e**Two-Way Decisions**• We can check for this situation. Here’s our first attempt. # quadratic2.py # A program that computes the real roots of a quadratic equation. # Bad version using a simple if to avoid program crash import math def main(): print "This program finds the real solutions to a quadratic\n" a, b, c = input("Please enter the coefficients (a, b, c): ") discrim = b * b - 4 * a * c if discrim >= 0: discRoot = math.sqrt(discrim) root1 = (-b + discRoot) / (2 * a) root2 = (-b - discRoot) / (2 * a) print "\nThe solutions are:", root1, root2 Python Programming, 1/e**Two-Way Decisions**• We first calculate the discriminant (b2-4ac) and then check to make sure it’s nonnegative. If it is, the program proceeds and we calculate the roots. • Look carefully at the program. What’s wrong with it? Hint: What happens when there are no real roots? Python Programming, 1/e**Two-Way Decisions**• This program finds the real solutions to a quadraticPlease enter the coefficients (a, b, c): 1,1,1>>> • This is almost worse than the version that crashes, because we don’t know what went wrong! Python Programming, 1/e**Two-Way Decisions**• We could add another if to the end:if discrim < 0: print "The equation has no real roots!" • This works, but feels wrong. We have two decisions, with mutually exclusive outcomes (if discrim >= 0 then discrim < 0 must be false, and vice versa). Python Programming, 1/e**Two-Way Decisions**Python Programming, 1/e**Two-Way Decisions**• In Python, a two-way decision can be implemented by attaching an else clause onto an if clause. • This is called an if-else statement:if <condition>: <statements>else: <statements> Python Programming, 1/e**Two-Way Decisions**• When Python first encounters this structure, it first evaluates the condition. If the condition is true, the statements under the if are executed. • If the condition is false, the statements under the else are executed. • In either case, the statements following the if-else are executed after either set of statements are executed. Python Programming, 1/e**Two-Way Decisions**# quadratic3.py # A program that computes the real roots of a quadratic equation. # Illustrates use of a two-way decision import math def main(): print "This program finds the real solutions to a quadratic\n" a, b, c = input("Please enter the coefficients (a, b, c): ") discrim = b * b - 4 * a * c if discrim < 0: print "\nThe equation has no real roots!" else: discRoot = math.sqrt(b * b - 4 * a * c) root1 = (-b + discRoot) / (2 * a) root2 = (-b - discRoot) / (2 * a) print "\nThe solutions are:", root1, root2 main() Python Programming, 1/e**Two-Way Decisions**>>> This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,1,2 The equation has no real roots! >>> This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 2, 5, 2 The solutions are: -0.5 -2.0 Python Programming, 1/e**Multi-Way Decisions**• The newest program is great, but it still has some quirks!This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1,2,1The solutions are: -1.0 -1.0 Python Programming, 1/e**Multi-Way Decisions**• While correct, this method might be confusing for some people. It looks like it has mistakenly printed the same number twice! • Double roots occur when the discriminant is exactly 0, and then the roots are –b/2a. • It looks like we need a three-way decision! Python Programming, 1/e**Multi-Way Decisions**• Check the value of discrim when < 0: handle the case of no roots when = 0: handle the case of a double root when > 0: handle the case of two distinct roots • We can do this with two if-else statements, one inside the other. • Putting one compound statement inside of another is called nesting. Python Programming, 1/e**Multi-Way Decisions**if discrim < 0: print "Equation has no real roots" else: if discrim == 0: root = -b / (2 * a) print "There is a double root at", root else: # Do stuff for two roots Python Programming, 1/e**Multi-Way Decisions**Python Programming, 1/e**Multi-Way Decisions**• Imagine if we needed to make a five-way decision using nesting. The if-else statements would be nested four levels deep! • There is a construct in Python that achieves this, combining an else followed immediately by an if into a single elif. Python Programming, 1/e**Multi-Way Decisions**• if <condition1>: <case1 statements>elif <condition2>: <case2 statements>elif <condition3>: <case3 statements>…else: <default statements> Python Programming, 1/e**Multi-Way Decisions**• This form sets of any number of mutually exclusive code blocks. • Python evaluates each condition in turn looking for the first one that is true. If a true condition is found, the statements indented under that condition are executed, and control passes to the next statement after the entire if-elif-else. • If none are true, the statements under else are performed. Python Programming, 1/e**Multi-Way Decisions**• The else is optional. If there is no else, it’s possible no indented block would be executed. Python Programming, 1/e**Multi-Way Decisions**# quadratic4.py # A program that computes the real roots of a quadratic equation. # Illustrates use of a multi-way decision import math def main(): print "This program finds the real solutions to a quadratic\n" a, b, c = input("Please enter the coefficients (a, b, c): ") discrim = b * b - 4 * a * c if discrim < 0: print "\nThe equation has no real roots!" elif discrim == 0: root = -b / (2 * a) print "\nThere is a double root at", root else: discRoot = math.sqrt(b * b - 4 * a * c) root1 = (-b + discRoot) / (2 * a) root2 = (-b - discRoot) / (2 * a) print "\nThe solutions are:", root1, root2 Python Programming, 1/e**Exception Handling**• In the quadratic program we used decision structures to avoid taking the square root of a negative number, thus avoiding a run-time error. • This is true for many programs: decision structures are used to protect against rare but possible errors. Python Programming, 1/e

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