Introduction to Computer Programming for Non-Majors

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1 Introduction to Computer Programming for Non-Majors CSC 2301, Fall 2014 Chapter 7 Part 2 The Department of Computer Science Quick review one-way or simple decision if <condition>: <body> two-way decision This is called an if-else statement: if <condition>: <statements> else: <statements> -2-1

2 Quick review Multi-Way Decisions if <condition1>: <case1 statements> elif <condition2>: <case2 statements> elif <condition3>: <case3 statements> else: <default statements> 4 Cont d Forming Simple Conditions Python Mathematics Meaning < < Less than <= Less than or equal to == = Equal to >= Greater than or equal to > > Greater than!= Not equal to

3 Exception Handling 5 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. Exception Handling 6 In the quadratic example, we checked the data before calling sqrt. Sometimes functions will check for errors and return a special value to indicate the operation was unsuccessful. E.g., a different square root operation might return a 1 to indicate an error (since square roots are never negative, we know this value will be unique)

4 Exception Handling 7 discrt = othersqrt(b*b - 4*a*c) if discrt < 0: print("no real roots. ) else:... Sometimes programs get so many checks for special cases that the algorithm becomes hard to follow. Programming language designers have come up with a mechanism to handle exception handling to solve this design problem. Exception Handling 8 The programmer can write code that catches and deals with errors that arise while the program is running, i.e., Do these steps, and if any problem crops up, handle it this way. This approach obviates the need to do explicit checking at each step in the algorithm

5 Exception Handling 9 # quadratic5.py # A program that computes the real roots of a quadratic equation. # Illustrates exception handling to avoid crash on bad inputs import math def main(): print("this program finds the real solutions to a quadratic\n") try: a, b, c = eval(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("\nthe solutions are:", root1, root2) except ValueError: print("\nno real roots") Exception Handling 10 The try statement has the following form: try: <body> except <ErrorType>: <handler> When Python encounters a try statement, it attempts to execute the statements inside the body. If there is no error, control passes to the next statement after the try except

6 Exception Handling 11 If an error occurs while executing the body, Python looks for an except clause with a matching error type. If one is found, the handler code is executed. The original program generated this error with a negative discriminant: Traceback (most recent call last): File "C:\Documents and Settings\Terry\My Documents\Teaching\W04\CS120\Textbook\code\chapter3\quadratic.py", line 21, in -toplevelmain() 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 -11- Exception Handling 12 The last line, ValueError: math domain error, indicates the specific type of error. Here s the new code in action: This program finds the real solutions to a quadratic Please enter the coefficients (a, b, c): 1, 1, 1 No real roots Instead of crashing, the exception handler prints a message indicating that there are no real roots

7 Exception Handling 13 The try except can be used to catch any kind of error and provide for a graceful exit. In the case of the quadratic program, other possible errors include not entering the right number of parameters ( unpack tuple of wrong size ), entering an identifier instead of a number (NameError), entering an invalid Python expression (TypeError). A single try statement can have multiple except clauses. Exception Handling 14 # quadratic6.py import math def main(): print("this program finds the real solutions to a quadratic\n") main() try: a, b, c = eval(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("\nthe solutions are:", root1, root2 ) except ValueError as excobj: if str(excobj) == "math domain error": print("no Real Roots") else: print("you didn't give me the right number of coefficients.") except NameError: print("\nyou didn't enter three numbers.") except TypeError: print("\nyour inputs were not all numbers.") except SyntaxError: print("\nyour input was not in the correct form. Missing comma?") except: print("\nsomething went wrong, sorry!")

8 Exception Handling 15 The multiple excepts act like elifs. If an error occurs, Python will try each except looking for one that matches the type of error. The bare except at the bottom acts like an else and catches any errors without a specific match. If there was no bare except at the end and none of the except clauses match, the program would still crash and report an error. Exception Handling 16 Exceptions themselves are a type of object. If you follow the error type with an identifier in an except clause, Python will assign that identifier the actual exception object

9 Study in Design: Max of Three 17 Now that we have decision structures, we can solve more complicated programming problems. The negative is that writing these programs becomes harder! Suppose we need an algorithm to find the largest of three numbers. Study in Design: Max of Three 18 def main(): x1, x2, x3 = eval(input("please enter three values: ")) # missing code sets max to the value of the largest print("the largest value is", max)

10 Strategy 1: Compare Each to All 19 This looks like a three-way decision, where we need to execute one of the following: max = x1 max = x2 max = x3 All we need to do now is preface each one of these with the right condition! Strategy 1: Compare Each to All 20 Let s look at the case where x1 is the largest. if x1 >= x2 >= x3: max = x1 Is this syntactically correct? Many languages would not allow this compound condition Python does allow it, though. It s equivalent to x1 x2 x

11 Strategy 1: Compare Each to All 21 Whenever you write a decision, there are two crucial questions: When the condition is true, is executing the body of the decision the right action to take? x1 is at least as large as x2 and x3, so assigning max to x1 is OK. Always pay attention to borderline values!! Strategy 1: Compare Each to All 22 Secondly, ask the converse of the first question, namely, are we certain that this condition is true in all cases where x1 is the max? Suppose the values are 5, 2, and 4. Clearly, x1 is the largest, but does x1 x2 x3 hold? We don t really care about the relative ordering of x2 and x3, so we can make two separate tests: x1 >= x2 and x1 >= x

12 Strategy 1: Compare Each to All 23 We can separate these conditions with and! if x1 >= x2 and x1 >= x3: max = x1 elif x2 >= x1 and x2 >= x3: max = x2 else: max = x3 We re comparing each possible value against all the others to determine which one is largest. Strategy 1: Compare Each to All 24 What would happen if we were trying to find the max of five values? We would need four Boolean expressions, each consisting of four conditions anded together. Yuck!

13 Strategy 2: Decision Tree 25 We can avoid the redundant tests of the previous algorithm using a decision tree approach. Suppose we start with x1 >= x2. This knocks either x1 or x2 out of contention to be the max. If the conidition is true, we need to see which is larger, x1 or x3. Strategy 2: Decision Tree

14 Strategy 2: Decision Tree 27 if x1 >= x2: if x1 >= x3: max = x1 else: max = x3 else: if x2 >= x3: max = x2 else max = x3 Strategy 2: Decision Tree 28 This approach makes exactly two comparisons, regardless of the ordering of the original three variables. However, this approach is more complicated than the first. To find the max of four values you d need ifelses nested three levels deep with eight assignment statements

15 29 Strategy 3: Sequential Processing How would you solve the problem? You could probably look at three numbers and just know which is the largest. But what if you were given a list of a hundred numbers? One strategy is to scan through the list looking for a big number. When one is found, mark it, and continue looking. If you find a larger value, mark it, erase the previous mark, and continue looking. Strategy 3: Sequential Processing

16 Strategy 3: Sequential Processing 31 This idea can easily be translated into Python. max = x1 if x2 > max: max = x2 if x3 > max: max = x3 Strategy 3: Sequential Programming 32 This process is repetitive and lends itself to using a loop. We prompt the user for a number, we compare it to our current max, if it is larger, we update the max value, repeat

17 Strategy 3: Sequential Programming 33 # maxn.py # Finds the maximum of a series of numbers def main(): n = eval(input("how many numbers are there? ")) # Set max to be the first value max = eval(input("enter a number >> ")) # Now compare the n-1 successive values for i in range(n-1): x = eval(input("enter a number >> ")) if x > max: max = x print("the largest value is", max) Strategy 4: Use Python 34 Python has a built-in function called max that returns the largest of its parameters. def main(): x1, x2, x3 = eval(input("please enter three values: ")) print("the largest value is", max(x1, x2, x3))

18 Some Lessons 35 There s usually more than one way to solve a problem. Don t rush to code the first idea that pops out of your head. Think about the design and ask if there s a better way to approach the problem. Your first task is to find a correct algorithm. After that, strive for clarity, simplicity, efficiency, scalability, and elegance. Some Lessons 36 Be the computer. One of the best ways to formulate an algorithm is to ask yourself how you would solve the problem. This straightforward approach is often simple, clear, and efficient enough

19 Some Lessons 37 Generality is good. Consideration of a more general problem can lead to a better solution for a special case. If the max of n program is just as easy to write as the max of three, write the more general program because it s more likely to be useful in other situations. Some Lessons 38 Don t reinvent the wheel. If the problem you re trying to solve is one that lots of other people have encountered, find out if there s already a solution for it! As you learn to program, designing programs from scratch is a great experience! Truly expert programmers know when to borrow

20 Chapter 8 Loop Structures and Booleans 39 Objectives To understand the concepts of definite and indefinite loops as they are realized in the Python for and while statements. To understand the programming patterns interactive loop and sentinel loop and their implementations using a Python while statement. Objectives 40 To understand the programming pattern end-of-file loop and ways of implementing such loops in Python. To be able to design and implement solutions to problems involving loop patterns including nested loop structures

21 Objectives 41 To understand the basic ideas of Boolean algebra and be able to analyze and write Boolean expressions involving Boolean operators. For Loops: A Quick Review 42 The for statement allows us to iterate through a sequence of values. for <var> in <sequence>: <body> The loop index variable var takes on each successive value in the sequence, and the statements in the body of the loop are executed once for each value

22 For Loops: A Quick Review 43 Suppose we want to write a program that can compute the average of a series of numbers entered by the user. To make the program general, it should work with any size set of numbers. We don t need to keep track of each number entered, we only need know the running sum and how many numbers have been added. For Loops: A Quick Review 44 We ve run into some of these things before! A series of numbers could be handled by some sort of loop. If there are n numbers, the loop should execute n times. We need a running sum. This will use an accumulator

23 For Loops: A Quick Review 45 Input the count of the numbers, n Initialize sum to 0 Loop n times Input a number, x Add x to sum Output average as sum/n For Loops: A Quick Review 46 # average1.py # A program to average a set of numbers # Illustrates counted loop with accumulator def main(): n = eval(input("how many numbers do you have? ")) sum = 0.0 for i in range(n): x = eval(input("enter a number >> ")) sum = sum + x print("\nthe average of the numbers is", sum / n) Note that sum is initialized to 0.0 so that sum/n returns a float!

24 For Loops: A Quick Review 47 How many numbers do you have? 5 Enter a number >> 32 Enter a number >> 45 Enter a number >> 34 Enter a number >> 76 Enter a number >> 45 The average of the numbers is 46.4 Indefinite Loops 48 That last program got the job done, but you need to know ahead of time how many numbers you ll be dealing with. What we need is a way for the computer to take care of counting how many numbers there are. The for loop is a definite loop, meaning that the number of iterations is determined when the loop starts

25 Indefinite Loops 49 We can t use a definite loop unless we know the number of iterations ahead of time. We can t know how many iterations we need until all the numbers have been entered. We need another tool! The indefinite or conditional loop keeps iterating until certain conditions are met. Indefinite Loops 50 while <condition>: <body> condition is a Boolean expression, just like in if statements. The body is a sequence of one or more statements. Semantically, the body of the loop executes repeatedly as long as the condition remains true. When the condition is false, the loop terminates

26 Indefinite Loops 51 The condition is tested at the top of the loop. This is known as a pre-test loop. If the condition is initially false, the loop body will not execute at all. Indefinite Loop 52 Here s an example of a while loop that counts from 0 to 10: i = 0 while i <= 10: print(i) i = i + 1 The code has the same output as this for loop: for i in range(11): print(i)

27 Indefinite Loop 53 The while loop requires us to manage the loop variable i by initializing it to 0 before the loop and incrementing it at the bottom of the body. In the for loop this is handled automatically. Indefinite Loop 54 The while statement is simple, but yet powerful and dangerous they are a common source of program errors. i = 0 while i <= 10: print(i) What happens with this code?

28 Indefinite Loop 55 When Python gets to this loop, i is equal to 0, which is less than 10, so the body of the loop is executed, printing 0. Now control returns to the condition, and since i is still 0, the loop repeats, etc. This is an example of an infinite loop. Indefinite Loop 56 What should you do if you re caught in an infinite loop? First, try pressing control-c If that doesn t work, try control-alt-delete If that doesn t work, push the reset button!

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Introduction to Computer Programming for Non-Majors

Introduction to Computer Programming for Non-Majors CSC 2301, Fall 2015 Chapter 7 Part 2 The Department of Computer Science Quick review one-way or simple decision if : two-way decision