Lecture - University of Rochesterrsarkis/csc161/_static/lectures/Writing Progra… · draft of the...

Richard E Sarkis CSC 161: The Art of Programming

Lecture

Writing Programs

Class Administrivia

Agenda

• To be able to understand and write Python statements to output information to the screen

• To assign values to variables

• To get numeric information entered from the keyboard

• To perform a counted loop

Software Development Process


The process of creating a program is often broken down into stages according to the information that is produced in each phase.


1. Analyze the Problem

2. Determine Specifications

3. Create a Design

4. Implement the Design

5. Test/Debug the Program

6. Maintain the Program


• Analyze the Problem Figure out exactly the problem to be solved. Try to understand it as much as possible.


• Determine Specifications • Describe exactly what your program will do.

• Don’t worry about how the program will work, but what it will do.

• Includes describing the inputs, outputs, and how they relate to one another.


• Create a Design • Formulate the overall structure of the program. • This is where the how of the program gets

worked out.

• You choose or develop your own algorithm that meets the specifications.


• Implement the Design • Translate the design into a computer language. • In this course we will use Python.


• Test/Debug the Program • Try out your program to see if it worked. • If there are any errors (bugs), they need to be

located and fixed. This process is called debugging.

• Your goal is to find errors, so try everything that might “break” your program!


• Maintain the Program • Continue developing the program in response to

the needs of your users.

• In the real world, most programs are never completely finished – they evolve over time.

Example Program: Temperature Converter

Example: Temperature Converter

• Analysis • The temperature is given in Celsius, user

wants it expressed in degrees Fahrenheit.



3. Create a Design





• Specification • Input: temperature in Celsius • Output: temperature in Fahrenheit

Tout°F = 9/5(Tin°C) + 32



3. Create a Design





• Design • Before we start coding, let’s write a rough

draft of the program in pseudocode

• Pseudocode is precise English that describes what a program does, step by step.

• Using pseudocode, we can concentrate on the algorithm rather than the programming language.



3. Create a Design





• Design: Pseudocode • Input the temperature in degrees Celsius • Calculate fahrenheit as (9/5)*celsius+32 • Output fahrenheit

• Now we need to convert this to Python!



3. Create a Design







3. Create a Design




# convert.py# A program to convert Celsius temps to Fahrenheit# by: Susan Computewell

def main(): celsius = float(input("What is the Celsius temperature? ")) fahrenheit = (9/5) * celsius + 32 print("The temperature is", fahrenheit, "degrees Fahrenheit.")

main()


• Implement • Once we write a program, we should test it!



3. Create a Design




>>> What is the Celsius temperature? 0 The temperature is 32.0 degrees Fahrenheit. >>> main() What is the Celsius temperature? 100 The temperature is 212.0 degrees Fahrenheit. >>> main() What is the Celsius temperature? -40 The temperature is -40.0 degrees Fahrenheit. >>>


• Maintain • Keep your code updated! • Come back after, say, a year and review

your code

• If others use it, you’ll support it for life!



3. Create a Design




Elements of Programs

Elements of Programs Identifiers

• Names are given to data values (celsius, fahrenheit), modules (convert), functions (main), etc.

• These names are called identifiers, and they're used to describe objects in Python


• Required: identifiers must begin with a letter or underscore (“_”), followed by any sequence of letters, digits, or underscores

• Identifiers are case sensitive, e.g. FooBar != foobar


• These are all different, valid names: XCelsiusSpamspamspAmSpam_and_EggsSpam_And_Eggs


• NameError is the error when you try to use an identifier without a value assigned to it

>>> x = 5>>> x5>>> print(x)5>>> print(spam)

Traceback (most recent call last): File "", line 1, in -toplevel- print spamNameError: name 'spam' is not defined>>>

Elements of Programs Reserved Words

• Some words are part of Python itself • These words are known as reserved words

• This means they are not available for you to use as a name for a variable, etc. in your program, e.g. and, del, for, is, raise, assert, elif, in

• For a complete list: https://docs.python.org/3/reference/lexical_analysis.html#keywords

https://docs.python.org/3/reference/lexical_analysis.html#keywords

Elements of Programs Literals

• Literals are values in source code that are written exactly as they are meant to be interpreted.

• Strings, integers, floating point numbers, boolean values, etc.

• 3.9, 24, True,“Cheezburger”

Elements of Programs Expressions

• The fragments of code that produce or calculate new data value(s) are called expressions

• Simple identifiers can also be expressions • They can consist of function calls, variable names,

literals, and operators, etc.


• Simpler expressions can be combined using operators.

• +, -, *, /, ** • Spaces are irrelevant within an expression. • The normal mathematical precedence applies.

((x1 – x2) / 2*n) + (spam / k**3)


3 + 5xy == 22

Elements of Programs Statements

• Statements are lines of code that 'do something' • Expressions are often parts of statements

print(42)if x == 22: do_y()returna = 7

Elements of Programs Statements

• Output Statements • A print statement can print any number of

expressions

• Successive print statements will display on separate lines

• A bare print will print a blank line

Elements of Programs

>>> print(3+4)7>>> print(3, 4, 3+4)3 4 7>>> print()

>>> print("The answer is", 3+4)The answer is 7>>>

Assignment Statements


• Simple Assignment =

• variable is an identifier, expr is an expression • The expression on the RHS is evaluated to produce

a value which is then associated with the variable named on the LHS


>>> x = 3.9 * x * (1-x) >>> T_out = 9/5 * (T_in) + 32 >>> x = 5


• Variables can be reassigned as many times as you want!

>>> myVar = 0>>> myVar0>>> myVar = 7>>> myVar7>>> myVar = myVar + 1>>> myVar8>>>


• Variables are like a box we can put values in • When a variable changes, the old value is erased

and a new one is written in

2.5. Assignment Statements 29

Here variable is an identifier and expr is an expression. The semantics of the assignment is thatthe expression on the right side is evaluated to produce a value, which is then associated with thevariable named on the left side.

Here are some of the assignments we’ve already seen:

x = 3.9 * x * (1 - x)

fahrenheit = 9 / 5 * celsius + 32

x = 5

A variable can be assigned many times. It always retains the value of the most recent assign-ment. Here is an interactive Python session that demonstrates the point:

>>> myVar = 0

>>> myVar

0

>>> myVar = 7

>>> myVar

7

>>> myVar = myVar + 1

>>> myVar

8

The last assignment statement shows how the current value of a variable can be used to update itsvalue. In this case I simply added one to the previous value. The chaos.py program from Chapter 1did something similar, though a bit more complex. Remember, the values of variables can change;that’s why they’re called variables.

Sometimes it’s helpful to think of a variable as a sort of named storage location in computermemory, a box that we can put a value in. When the variable changes, the old value is erasedand a new one written in. Figure 2.1 shows how we might picture the effect of x = x + 1 usingthis model. This is exactly the way assignment works in some computer languages. It’s also a verysimple way to view the effect of assignment, and you’ll find pictures similar to this throughout thebook.

x 10

Before After

11x

x = x + 1

Figure 2.1: Variable as box view of x = x + 1

Python assignment statements are actually slightly different from the “variable as a box” model.In Python, values may end up anywhere in memory, and variables are used to refer to them. As-signing a variable is like putting one of those little yellow sticky notes on the value and saying, “this


• However! Python doesn't overwrite these memory locations (boxes)

• Assigning a variable is more like putting a “sticky note” on a value and saying, “this is x”

30 Chapter 2. Writing Simple Programs

is x.” Figure 2.2 gives a more accurate picture of the effect of assignment in Python. An arrow isused to show which value a variable refers to. Notice that the old value doesn’t get erased by thenew one; the variable simply switches to refer to the new value. The effect is like moving the stickynote from one object to another. This is the way assignment actually works in Python, so you’ll seesome of these sticky-note style pictures sprinkled throughout the book as well.

x

After

11

10x

Before

10

x = x + 1

Figure 2.2: Variable as sticky note (Python) view of x = x + 1

By the way, even though the assignment statement doesn’t directly cause the old value of avariable to be erased and overwritten, you don’t have to worry about computer memory gettingfilled up with the “discarded” values. When a value is no longer referred to by any variable, it is nolonger useful. Python will automatically clear these values out of memory so that the space can beused for new values. This is like going through your closet and tossing out anything that doesn’thave a sticky note to label it. In fact, this process of automatic memory management is actuallycalled garbage collection.

2.5.2 Assigning Input

The purpose of an input statement is to get some information from the user of a program andstore it into a variable. Some programming languages have a special statement to do this. InPython, input is accomplished using an assignment statement combined with a built-in functioncalled input. The exact form of an input statement depends on what type of data you are trying toget from the user. For textual input, the statement will look like this:

= input()

Here is a string expression that is used to prompt the user for input; the prompt isalmost always a string literal (i.e., some text inside of quotation marks).

When Python encounters a call to input, it prints the prompt on the screen. Python then pausesand waits for the user to type some text and press the key. Whatever the user types isthen stored as a string. Consider this simple interaction:

>>> name = input("Enter your name: ")

0

In the C Languageint num = 0;

10

In the Python Languagenum = 0

10

Assignment Statements User Input

• The purpose of an input statement is to get input from the user and store it into a variable, e.g. entering a whole number (an integer) = int(input())

Assignment Statements User Input

• First the prompt is printed • The input part waits for the user to enter a value

and press

• The expression that was entered is evaluated to turn it from a string of characters into a Python value (a number)

• The value is assigned to the variable

Assignment Statements Simultaneous Assignment

• Several values can be calculated at the same time , , … = , , …

• Evaluate the expressions in the RHS and assign them to the variables on the LHS

• sum, diff = x+y, x-y


• How could you use this to swap the values for x and y?

• Why doesn’t this work? x = y  y = x

• We could use a temporary variable…


• We can swap the values of two variables quite easily in Python!

>>> x = 3>>> y = 4>>> print(x, y)3 4>>> x, y = y, x>>> print(x, y)4 3


• We can use this same idea to assign multiple variables from a single input statement!

• Use commas to separate the inputs:def spamneggs():   spam, eggs = eval(input("Enter # of slices of spam followed by # of eggs: "))   print ("You ordered", eggs, "eggs and", spam, "slices of spam. Yum!“)   >>> spamneggs()  Enter the number of slices of spam followed by the number of eggs: 3, 2  You ordered 2 eggs and 3 slices of spam. Yum!  >>>

Definite Loops

Definite Loops

• A definite loop executes a definite number of times, i.e., at the time Python starts the loop it knows exactly how many iterations to do.

• for in :  

• The beginning and end of the body are indicated by indentation

Definite Loops

• for in :  

• The variable after the for is called the loop index

• It is assigned each successive value in

Definite Loops>>> for i in [0,1,2,3]:

print (i)

0123>>> for odd in [1, 3, 5, 7]:

print(odd*odd)

192549

>>>

Definite Loops

• In chaos.py, what did range(10) do? >>> list(range(10))  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

• range is a built-in Python function that generates a sequence of numbers, starting with 0

• list is a built-in Python function that turns the sequence into an explicit list

• The body of the loop executes 10 times

Definite Loops

• for loops alter the flow of program execution, so they are referred to as control structures

36 Chapter 2. Writing Simple Programs

counted loops. Just be sure to use an identifier that you are not using for any other purpose.Otherwise you might accidentally wipe out a value that you will need later.

The interesting and useful thing about loops is the way that they alter the “flow of control” ina program. Usually we think of computers as executing a series of instructions in strict sequence.Introducing a loop causes Python to go back and do some statements over and over again. State-ments like the for loop are called control structures because they control the execution of otherparts of the program.

Some programmers find it helpful to think of control structures in terms of pictures calledflowcharts. A flowchart is a diagram that uses boxes to represent different parts of a program andarrows between the boxes to show the sequence of events when the program is running. Figure2.3 depicts the semantics of the for loop as a flowchart.

yes

more items in no

= next item

Figure 2.3: Flowchart of a for loop.

If you are having trouble understanding the for loop, you might find it useful to study theflowchart. The diamond shaped box in the flowchart represents a decision in the program. WhenPython gets to the loop heading, it checks to see if there are any (more) items left in the sequence.If the answer is yes, the loop index variable is assigned the next item in the sequence, and then theloop body is executed. Once the body is complete, the program goes back to the loop heading andchecks for another value in the sequence. The loop quits when there are no more items, and theprogram moves on to the statements that come after the loop.

Example Program: Future Value

Example: Future Value

• Analysis • Money deposited in a bank account earns

interest

• How much will the account be worth 10 years from now?

• Inputs: principal, interest rate • Output: value of the investment in 10 years



3. Create a Design





• Specifications • User enters the initial amount to invest, the

principal

• User enters an annual percentage rate, the interest

• The specifications can be represented like this…



3. Create a Design





• Program Future Value • Inputs:

principal The amount of money being invested, in dollars apr The annual percentage rate expressed as a decimal number.

• Output: The value of the investment 10 years in the future

• Relationship: Value after one year is given by principal * (1 + apr)

• This needs to be done 10 times.



3. Create a Design





• Design • Print an introduction

• Input the amount of the principal (principal)

• Input the annual percentage rate (apr)

• Repeat 10 times:

• principal = principal * (1 + apr)

• Output the value of principal



3. Create a Design





• Implementation • Each line translates to one line of Python

(in this case)

• Print an introduction print ("This program calculates the future")  print ("value of a 10-year investment.")

• Input the amount of the principal principal = float(input("Enter the initial principal: "))



3. Create a Design





• Implementation • Input the annual percentage rate

apr = float(input("Enter the annual interest rate: "))

• Repeat 10 times: for i in range(10):

• Calculate principal principal = principal * (1 + apr)

• Output the value of the principal at the end of 10 years print ("The value in 10 years is:", principal)



3. Create a Design







3. Create a Design




# futval.py# A program to compute the value of an investment# carried 10 years into the future

def main(): print("This program calculates the future value" " of a 10-year investment.")

principal = float(input("Enter the initial principal: ")) apr = float(input("Enter the annual interest rate: "))

for i in range(10): principal = principal * (1 + apr)

print ("The value in 10 years is:", principal)

main()




3. Create a Design




>>> main() This program calculates the future value of a 10-year investment. Enter the initial principal: 100 Enter the annual interest rate: .03 The value in 10 years is: 134.391637934 >>> main() This program calculates the future value of a 10-year investment. Enter the initial principal: 100 Enter the annual interest rate: .10 The value in 10 years is: 259.37424601

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