Programming for Engineers in Python Recitation 1 Plan

Programming for Engineers in Python Recitation 1

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

Administration Yoav Ram �Email: yoavram@post. tau. ac. il �Office hours: by appointment only �Location: Room 409, Britannia building �Noga Levy �Email: noga@post. tau. ac. il �Office hours: by appointment only �Location: Room 405 a, Shenkar building

Course Site - Moodle � http: //moodle. tau. ac. il � 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)

Homework � Very important when learning to program! Therefore: � Weekly “hands-on” assignments. � Strict submission dates. � 15 -20% 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 email/disk-on-key

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 � Google (a useful and legitimate source) � Check the forum in moodle � Submit a question to forum (reply within 48 hours)

Working Environment Install (at home): � Windows 32 bit: http: //www. python. org/ftp/python/2. 7. 2/python 2. 7. 2. msi � Continue as in class Open: Start Menu All Programs Python 2. 7 IDLE (Python GUI)

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

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’

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

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

Numbers and their Types >>> 4 4 >>> type(4) <class 'int'> # integers type >>> 3. 14159 >>> type(3. 14159) <class 'float'> # floating point ("reals") type Arithmetic operations: +, -, *, / , % (modulo), ** (power) What type is 8/5 ? And 8/5. 0 ? Let’s check…

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

Variables and Assignments: An Example Changing the value of a variable: >>> n=11 >>> n 11 Changing the type of a variable: >>> n=1. 3141 >>> n 1. 3141 Variables can be used in expressions: >>> pi = 3. 14159 >>> pi*2 + 1 7. 28318

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 Name. Error: name 'check_this' is not defined

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: i, j, m, n, a. String, does. Something … �Informative: sum. Of. Expenses, student. Name, count. Words … � Documentation – add remarks (‘#’) before: �Functions �‘Logical units’ of code �complex implementation

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

Strings – Cont. >>> mssg 1="Let there" >>> mssg 2=" be light" >>> mssg 1+mssg 2 'Let there be light‘ What will the next expression print? >>> mssg 1 + mssg 2*2

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

Homework 0 Let the work begin: � Download Homework 0 from the course website. � Follow instructions � Submit the file [your_name]_a 0_q 1. py through the course website. Questions?

Boolean Variables Comparison : � Numbers – by their order � Strings – by lexicographical order Returns boolean variabels: True or False. Comparison types: �= � != �> �< � >= � <=

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

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

Conditional Statements if <condition>: do something [else: do something else]

Conditional Statements Examples >>> if 54% 18 == 0: # the remainder of 54 divided by 18 print “ 54 is divisible by 18" else: print “ 54 is not divisible by 18" 54 is not divisible by 18 Indentation: � Following the if statement: Open a new scope = one tab to the right. � Indicates the commands within the scope of this if. � else - outside that scope. Note: tab != four spaces, even if it looks identical!

Functions - Reminder def function_name(input 1, input 2, …): command 1 command 2 …

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+++

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.

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

Exercise – Unit Testing Use the given implementation: - Download hw 1. 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'