Programming for Engineers in Python. Autumn

Transcription

1 Programming for Engineers in Python Autumn

2 Plan Administration: Course site Homework submission guidelines Working environment Python: Variables Editor vs. shell Homework 0 Python Cont. Conditional Statements (if/else) Prepare for Homework 1

3 Administration Yoav Ram Office hours: Wed. and Thu. by appointment only Location: Britannia 409 Noga Levy Office hours: by appointment only Location: 405a Shenkar building

4 Course Site - Moodle All relevant material: Slides for the lectures + recitations, homework, solutions, code examples. Automatic homework submission, (manual) grades. Forum anything you want to clarify from the lecture, recitations and homework. Announcements Instructions (how to set a working environment at home)

5 Homework Very important when learning to program! Therefore: Weekly hands-on assignments. Strict submission dates % of the final grade. Note that: Grades: 0 / 60 / 80 / 100. No appeals. Where can I work? Computer lab 06, open: 8:00 20:00, use /disk-on-key

6 Submission Guidelines Submission in singles! Should work on Python 2.7 No cheating! Guidelines in the course site. How to handle doesn t work situation: Go over the code Consult course slides Ask Google (a useful and legitimate source). Good forums to ask questions online: stackoverflow.com, underwar.co.il Check the forum Submit a question (reply within two working days)

7 Working Environment Install (at home): Windows 32 bit: msi Continue as in class Open: Start Menu All Programs Python 2.7 IDLE (Python GUI)

8 Working Environment Install (at home): Windows 32 bit: msi Continue as in class Open: Start Menu All Programs Python 2.7 IDLE (Python GUI)

9 IDLE Editor We want to save a sequence of commands and run it later in a new Python session. The Editor: - Write Python commands - Execute them in one key-press. Open the editor from the Shell: File New Window

10 IDLE Editor Cont. The new window is Untitled. First choose a title: In the new window: File Save as Chose a folder and a name. The name must end with.py

11 IDLE Editor Cont. Run Python: The output appears in the Shell window (a new Shell might open)

12 What are Variables? A location in the computer s memory. A variable - has a name (for access) - holds a value - has type according to its value - This is how data is handled

13 Numbers and their Types >>> 4 4 >>> type(4) <class 'int'> # integers type >>> >>> type( ) <class 'float'> # floating point ("reals") type Arithmetic operations: +, -, *, /, % (modulo), ** (power) What type is 8/5? And 8/5.0? Let s check

14 Variables and Assignments >>> n = 10 >>> m=(10+4)*5 The left hand side is a variable. The right hand side is an expression. The interpreter evaluates the expression and assigns its value to the variable. The variable's name is a sequence of letters and digits, starting with a letter. n m 10 70

15 Variables and Assignments: An Example Changing the value of a variable: >>> n=11 >>> n 11 Changing the type of a variable: >>> n= >>> n Variables can be used in expressions: >>> pi = >>> pi*

16 Variables and Assignments Cont. Referring to undefined variables leads to runtime error >>> check_this Traceback (most recent call last): File "<pyshell#16>", line 1, in <module> check_this NameError: name 'check_this' is not defined

17 Documentation and Variable Names Real computer programs include thousands of code lines, lots of variables. Readability is very important for code maintenance, it is a requirement in this course! Hence: Choose informative variable names: Not informative: astring, doessomething Informative: sumofexpenses, studentname, countwords Are i, j, m, n informative names? It depends! Documentation add remarks ( # ) before: Functions Logical units of code complex implementation

18 Strings We already met Strings in class. Strings are text sequences. They are actually an ordered list of characters

19 Strings Cont. >>> mssg1="let there" >>> mssg2=" be light" >>> mssg1+mssg2 'Let there be light What will the next expression print? >>> mssg1 + mssg2*2

20 Strings Access >>> a= Hello >>> a[1:3] 'el' >>> a[1:] 'ello' >>> a[-4:-2] 'el' >>> a[:-3] 'He' >>> a[-3:] 'llo H 0-5 e 1-4 l 2-3 l 3-2 o 4-1

21 Homework 0 Let the work begin: Download Homework 0 from the course website. Follow instructions Submit the file [your_id]_a0.py through the course website.

22 Boolean Variables and Operators Boolean variables: True or False. >>> a = True >>> b = True >>> c = False >>> d = False Generate complex Boolean expressions with logical operators: and / or / not : >>> a and b True >>> a and c False >>> a or c True >>> not a False

23 Variables - Status We saw the classes int, 'float', 'str, bool. Some operations allow mixing" variables of different types. Assignments: variable name = expression Subsequent assignments to the same variable can change its value and even its type. int integer numbers, float real numbers. True and False are Boolean constants or, and, not are Python's Boolean operators.

24 Python Comparison Operators We can compare: Numbers by their order Strings by lexicographical order Return True or False. Comparison types: ==!= > < >= <=

25 Python Comparison Operators >>> 5.0 == 5 True >>> 6!= 2*3 False >> not(-2 >= 1) True >>> (-2 >= 1) or (-2 <= -1) True >>> (-2 >= 1) and (-2 <= -1) False

26 Conditional Statements if <condition>: do something

27 Conditional Statements - Examples >>> if 54% 18 == 0: # the remainder of 54 divided by 18 print 54 is divisible by 18 print 54 is also divisible by 2 and 3 54 is divisible by is also divisible by 2 and 3 Following the if statement: Open a new scope = one tab to the right. Indicates the commands within the scope of this if. Note: tab!= four spaces, even if it looks identical!

28 Conditional Statements - else if <condition>: do something else: do something else

29 Conditional Statements - Examples if width == height: print found a square else: print found a rectangle width = height print now it is a square else is not part of the if scope! The commands under else are indented.

30 Nested Conditional Statements >>> n = 24 >>> if n % 4!= 0: print "n=",n print "n is not divisible by 4" else: if n % 3 == 0: # nested "if" print "n=",n print "n is divisible by 12" else: # nested "else" print "n=",n print "n is divisible by 4 but not by 3 n= 24 n is divisible by 12

31 Functions def function_name(input1, input2, ): command1 command2

32 Exercise Donuts: Input: count - an int representing the number of donuts Output: a string of the form 'Number of donuts: <count>. However, if the count is 10 or more, use the word 'many instead of the actual count. Examples: >>> donuts(5) Number of donuts: 5 >>> donuts(23) Number of donuts: many Function prototype: def donuts(count): # +++your code here+++

33 Solution def donuts(count): if count < 10: return 'Number of donuts: + str(count) else: return 'Number of donuts: many Return instead of print explanation in the next slide.

34 Exercise Unit Testing Test (1) Run my code with some input (2) Check the output My Code

35 Exercise Unit Testing Use the given implementation: - Download hw1.py - Fill in the necessary code instead of the remarks # +++your code here+++ - Instead of printing the code to the shell, return it to the testing function. - Run Module Example s output: OK got: 'Number of donuts: 4' expected: 'Number of donuts: 4' OK got: 'Number of donuts: 9' expected: 'Number of donuts: 9' OK got: 'Number of donuts: many' expected: 'Number of donuts: many' OK got: 'Number of donuts: many' expected: 'Number of donuts: many'