06/11/2014. Subjects. CS Applied Robotics Lab Gerardo Carmona :: makeroboticsprojects.com June / ) Beginning with Python

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1 CS Applied Robotics Lab Gerardo Carmona :: makeroboticsprojects.com June / 2014 Subjects 1) Beginning with Python 2) Variables 3) Strings 4) Basic arithmetic operators 5) Flow control 6) Comparison operators 7) Assignment Operators 8) Built in functions 9) Input data 10) Functions 11) Built in libraries 12) Lists 13) Tuples 14) Maps 1

2 About Python High level language: Easy to program. Interpreted language Interactive and script mode You can write files with code and then execute them Objective oriented Let users create and manipulate data structures (objects). Example If you type 1 + 1, the interpreter replies 2 >>> Python Terminal Command line 2

3 First program Type: >>>print Hello, World! Example of a print statement*. and have the same effect with text * In Python 3, print is a function, so the syntax is print( Hello, World ) Python GUI GUI Graphic User Interface Scripts You can write all your code in a file and then execute it. In the main window of python shell click on File>>New Window A new window opens, you can write all your code here, save it and execute it. Shortcut for new window: ctrl + n Shortcut for run script: F5 3

4 Variables A variable is a name that refers to a value. >>> message = Hi robotics class >>> n = 9 >>> pi = >>>print message Hi robotics class >>>print n 17 >>>print pi Variables names and keywords If you give a variable an illegal name, you get a syntax error: >>> 76trombones = big parade SyntaxError: invalid syntax >>>more@ = SyntaxError: invalid syntax >>>class = Advance programming SyntaxError: invalid syntax 4

5 Values and Types Hello, World! is a String If you aren t sure what type a value has, the interpreter can tell you: Floating-point >>> type( Hello, World! ) <type str > >>> type(17) <type int > >>> type( ) <type float > Strings Text between and are considered strings. But, what happened to the next codes? >>>print Hello, World! >>>print Hi, I m the professor >>>print And he said Hi everybody 5

6 Strings Text between and are considered strings. But, what happened to the next codes? >>>print Hello, World! >>>print Hi, I m the professor >>>print And he said Hi everybody Strings >>>print Hi I m Gerardo So in this case we need to rewrite the code to: >>>print Hi I m Gerardo What about the second example? >>>print "He said, "Aren't can't shouldn't wouldn't."" Strings When we have complex use of (double commas) and (simple commas) and when we want to write larger strings we can use (three simple commas at the beginning and end of each string) to solve this problem, for example: >>>print He said, "Aren't can't shouldn't wouldn't. 6

7 Strings An alternative to is using backslash (\) before each quotation mark within the string. This is called escaping. Escaping strings can make them harder to read, so it s probably better to use multiline strings. Some examples: >>>print 'He said, "Aren\'t can\'t shouldn\'t wouldn\'t." >>>print "He said, \"Aren't can't shouldn't wouldn't.\"" Embedding Values in Strings If you want to display a message using the contents of a variable, you can embed values in a string using %s, which is like a marker for a value that you want to add later. For example: >>>score = 100 >>>message = I scored %s points >>>print (message % score) I scored 100 points Embedding Values in Strings Other examples: >>>text = Hi %s, how are you? >>>name1 = Gerardo >>>name2 = Rachel >>>print (text % name1) Hi Gerardo, how are you? >>>print (text % name2) Hi Rachel, how are you? 7

8 Embedding Values in Strings Other examples: >>>nums = What did the number %s say to the number %s? Nice belt!! >>>print (nums % (0,8)) What did the number 0 say to the number 8? Nice belt!! Strings can be subscripted (indexed) Like C string index starts in zero. Examples: >>>text = abcdef >>>print text[0] a >>>print text[2:4] cd >>>print text[2:] cdef But Unlike a C string, Python strings cannot be changed. Assigning to an indexed position in the string results in an error. >>> x = 1234 >>>x[0] = "2" TypeError: 'str' object does not support item assignment However, creating a new string with the combined content is easy and efficient: >>>text = abcdef >>>print x + text[2] xc >>>print xyz + text[3:] xyzdef 8

9 Negative index Indices may be negative numbers, to start counting from the right. For example: >>>text = abcdef >>>print text[-1] f >>>print text[-2] e >>>print text[-2:] ef >>>print text[:-2] abcd Size of a string The built-in function len() returns the length of a string. Example: >>>text = abcdef >>>print len(text) 6 9

10 Arithmetic Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example + Addition - Adds values on either side of the operator a + b will give 30 - Subtraction - Subtracts right hand operand from left hand operand a - b will give -10 * Multiplication - Multiplies values on either side of the operator a * b will give 200 / Division - Divides left hand operand by right hand operand b / a will give 2 *Parentheses () can be used for grouping. Arithmetic Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example % Modulus - Divides left hand operand by right hand operand and b % a will give 0 returns remainder ** Exponent - Performs exponential (power) calculation on operators a**b will give 10 to the power 20 // Floor Division - The division of operands where the result is the quotient in which the digits after the decimal point are removed. 9//2 is equal to 4 and 9.0//2.0 is equal to 4.0 Examples with strings Operations with strings Try: >>> print (10 * abc ) >>> text1 = "abc" >>> text2 = "def" >>> print text1 + text2 + "!" 10

11 Examples with strings Answer >>> print (10 * 'abc') abcabcabcabcabcabcabcabcabcabc >>> text1 = "abc" >>> text2 = "def" >>> print text1 + text2 + "! abcdef! Examples with integers and floats Try: >>> print >>> print 10+4/2 >>> print 10/3 >>> print 10.0/3 >>> print 10/3.0 >>> print 10+2/4 Examples with integers and floats Answer >>> print >>> print 10+4/2 12 >>> print 10/3 3 >>> print 10.0/ >>> print 10/ >>> print 10+2/

12 Other examples The equal sign ('=') is used to assign a value to a variable. Afterwards, no result is displayed >>> width = 200 >>> height = 400 >>> print width + height 600 >>> x = y = z = 0 >>> print x, y, z >>> a,b = 5, 9 >>> print a,b 5 9 While loop We can write an initial sub-sequence of the Fibonacci series as follows: Fibonacci series: The sum of two elements defines the next Indentation is important and it s done by a tab or 4 spaces >>> a,b = 0,1 >>> while b < 10: print b, a,b = b, a+b Test your program with and without the comma 12

13 If statements Besides the while statement just introduced, Python knows the usual control flow statements known from other languages, with some twists. Perhaps the most well-known statement type is the if statement. For example: File: 01-if-statements.py For statements For loops are traditionally used when you have a piece of code which you want to repeat n number of times. Examples: >>>for x in range (0,3): print x, >>>for j in range(10): print j, >>>for i in range(5, 50, 10): print i, For statements Other examples: 02-for-statements.py 13

14 break statements The break statement, like in C, breaks out of the smallest enclosing for or while loop. for i in range (100): if i == 10: break print i, continue statement Continue works different, inside a loop you can skip some code and go directly to the next iteration. For example: for letter in 'Python': # First Example if letter == 'h': continue print 'Current Letter :', letter continue statement Other example: var = 10 # Second Example while var > 0: var = var -1 if var == 5: continue print 'Current variable value :', var 14

15 Comparison Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example == Checks if the value of two operands are equal or not, if yes then condition (a == b) is not true. becomes true.!= Checks if the value of two operands are equal or not, if values are not equal (a!= b) is true. then condition becomes true. <> Checks if the value of two operands are equal or not, if values are not equal (a <> b) is true. This is then condition becomes true. similar to!= operator. Continue Comparison Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example > Checks if the value of left operand is greater than the value of right (a > b) is not true. operand, if yes then condition becomes true. < Checks if the value of left operand is less than the value of right operand, if (a < b) is true. yes then condition becomes true. >= Checks if the value of left operand is greater than or equal to the value of (a >= b) is not true. right operand, if yes then condition becomes true. <= Checks if the value of left operand is less than or equal to the value of right (a <= b) is true. operand, if yes then condition becomes true. 15

16 Assignment Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example = Simple assignment operator, Assigns values from right side operands c = a + b will assign to left side operand value of a + b into c += Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand c += a is equivalent to c = c + a -= Subtract AND assignment operator, It subtracts right operand from the c -= a is equivalent to left operand and assign the result to left operand c = c - a *= Multiply AND assignment operator, It multiplies right operand with the c *= a is equivalent to left operand and assign the result to left operand c = c * a Assignment Operators Assume variable a holds 10 and variable b holds 20, then: Op. Description Example /= Divide AND assignment operator, It divides left operand with the right c /= a is equivalent to operand and assign the result to left operand c = c / a %= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand c %= a is equivalent to c = c % a **= Exponent AND assignment operator, Performs exponential (power) calculation on operators and assign value to the left operand c **= a is equivalent to c = c ** a //= Floor Division and assigns a value, Performs floor division on operators c //= a is equivalent to and assign value to the left operand c = c // a 16

17 Bitwise Operators Bitwise operator works on bits and perform bit by bit operation. Assume if a = 60; and b = 13; Now in binary format they will be as follows Op. Description Example & Binary AND Operator copies a bit to the result if it (a & b) will give 12 which is 0000 exists in both operands Binary OR Operator copies a bit if it exists in either (a b) will give 61 which is 0011 operand ^ Binary XOR Operator copies the bit if it is set in one (a ^ b) will give 49 which is 0011 operand but not both Bitwise Operators Bitwise operator works on bits and perform bit by bit operation. Assume if a = 60; and b = 13; Now in binary format they will be as follows Op. Description Example ~ Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~a ) will give -61 which is in 2's complement form due to a signed binary number. << Binary Left Shift Operator. The left operands value is moved a << 2 will give 240 which is left by the number of bits specified by the right operand >> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. a >> 2 will give 15 which is

18 Function range() It generates lists containing arithmetic progressions. Some examples: >>> range(5, 10) [5, 6, 7, 8, 9] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(-10, -100, -30) [-10, -40, -70] Creates a list of numbers, this function only works in Python 2.x, for python 3.x: >>> mylist = list(range(4)) [0, 1, 2, 3] We will learn about lists later Function len() Len() function returns the length of a string or array. Like for example: >>> text = abcdef >>> print len(text) 6 >>> nums = [2,3,7,8,10] >>> print len(nums) 5 Another example What s going to be printed? >>> items = [ leds, motors, raspberry pi, buttons ] >>> for j in range(len(items)): print j, items[j] 18

19 abs() Return the absolute value of a number. The argument may be a plain or long integer or a floating point number. If the argument is a complex number, its magnitude is returned. >>> abs(-100.2) >>> abs(3+2j) str() Convert to string. Try this! >>> age = 28 >>> print I m + age + years old Did it work? str() Now try this >>> age = 28 >>> print I m + str(age) + years old I m 28 years old How else can be programmed? 19

20 int() Returns an integer from a floating point number or string (must be number character). >>> x = 20 >>> y = int(x) >>> print y 20 >>> type(y) <type 'int'> float() Convert a string or a number to floating point. If the argument is a string, it must contain a possibly signed decimal or floating point number. >>> x = >>> y = float(x) >>> print y >>> type(y) <type float'> bool() Converts to a Boolean value. Returns false if no value or cero value is entered, true otherwise. >>> bool() False >>> bool(9) True >>> bool("0") True >>> bool("") False 20

21 bin() Returns a string with the binary representantion of a number >>> bin(7) 111 >>> bin( 8 ) 1000 ord() Returns ascii value of a char. >>> ord( A ) 65 Quick exercises A while loop can be used anywhere a for loop is used: i = 0 while i < 10: print(i) i = i + 1 # This is the same as: for i in range(10): print(i) 21

22 Input data With Python you can ask the user to type some data. Use command: raw_input Example: 03-input-data.py Tip: Using raw_input you get a string, so if you need to convert to int use int() function. 22

23 Project 1: Temperature converter Convert temperature between C, F and K C F Display a menu with all the options, user must select which option desires Ask for the temperature Display result Project 2: Factorial numbers Create a program that displays factorial number of a given number (do not use any library, write your own code). Numbers must be in range of 1 to 10, otherwise alert user. Project 3: Prime numbers Create a program that can tell you which number is prime from a range of 2 to

24 Other Projects Encrypt text: 04-encrypt.py and 05-get-text.py Defining a function To define a function use def name (parameters). For example: >>> def in_to_cm(in): cm = in*2.54 print cm Open 06-def.py file for more examples 24

25 Other functions examples See files Random number In order to generate a random number we need to import the file random.py by writing on our code import random. There are several pre-defined functions for random numbers, but we are going to focus only in one, random.randint(). Example, generate 10 random numbers between zero and nine: >>> import random >>> for i in range(10): x = random.randint(0,9) print x, Find more information: 25

26 time.sleep() Python has a functions that let us read cpu time. First we need to import the file time by writing import time. Time has a lot of functions, but we are going to focus on sleep which can suspend the calling thread for any given seconds. Try the next example: >>> import time >>> print Hello, >>> time.sleep(3) >>> print World! Find more information: time.clock() This function will return time elapsed in seconds. Try: >>> import time >>> print time.clock() >>> time.sleep(3) >>> print time.clock() 26

27 Project 4: Guess number game This game have 3 modes. Mode 1: Guess a number between 0 and 9 in less than 3 tries Mode 2: Guess a number between 0 and 99 in les than 7 tries. Mode 3: Guess a number between 0 and 99 and display time that it takes to guess. Project 4: Guess number game Requirements: Display menu and ask for game mode (user input) Use functions Generate random number Use time.clock() for mode 3 to calculate the time that the user takes to guess the number 27

28 List Lists Are More Powerful than strings. List can contain different elements, and these elements may be strings. Example: >>> food = ['pizza', 'lasagna', 'Pasta'] >>> print food ['pizza', 'lasagna', 'Pasta'] >>> print food[2] Pasta List We can change an item. >>>food[2] = 'cannelloni' >>>print food ['pizza', 'lasagna', 'canellonni'] And print some items from the list, say item from 0 to 1 (number 2 not included. >>>print food[0:2] ['pizza, 'lasagna'] 28

29 Types of lists Numbers >>>numbers = [2,9,3,7] >>>print numbers [2, 9, 3, 7] Numbers and strings >>>numbers_and_strings = [1, "cpu", 3, "monitor"] >>>print numbers_and_strings [1, 'cpu', 3, 'monitor'] Types of lists List of lists >>> list_of_lists = [food, numbers] >>> print list_of_lists [['pizza', 'lasagna', 'canellonni'], [2, 9, 3, 7]] Operations with lists Adding items to a list: >>> food.append('pasta') >>> print food ['pizza', 'lasagna', 'canellonni', 'pasta'] Removing items from a list >>> del food[1] >>> print food ['pizza', 'canellonni', 'pasta'] 29

30 Operations with lists Adding two lists >>> new_list = food + numbers >>> print new_list ['pizza', 'canellonni', 'pasta', 2, 9, 3, 7] Multiply a list by a number. For example >>> print numbers*2 [2, 9, 3, 7, 2, 9, 3, 7] Tuples A tuple is like a list that uses parentheses, as in this example: >>> example = ('1', 4, 'r', 67) >>> print example ('1', 4, 'r', 67) >>> print example[2] R >>> print example[0:2] ('1', 4) The main difference between a tuple and a list is that a tuple cannot change once you ve created it. 30

31 Tuples Why would you use a tuple instead of a list? Basically because sometimes it is useful to use something that you know can never change. If you create a tuple with two elements inside, it will always have those two elements inside. Not in order Maps In Python, a map(also referred to as a dict, short for dictionary) is a collection of things, like lists and tuples. The difference between maps and lists or tuples is that each item in a map has a key and a corresponding value. key value For example: >>> colors = {'Gerardo' : 'blue', 'Rachel' : 'purple', 'Santiago' : 'red'} >>> print colors {'Santiago': 'red', 'Gerardo': 'blue', 'Rachel': 'purple'} >>> print colors['gerardo'] blue 31

32 Maps To delete a value in a map, use its key. For example: >>> del colors['gerardo'] >>> print colors {'Santiago': 'red', 'Rachel': 'purple'} To replace a value use its key too. >>> colors['santiago'] = 'orange' >>> print colors {'Santiago': 'orange', 'Rachel': 'purple'} Maps As you can see, working with maps is kind of like working with lists and tuples, except that you can t join maps with the plus operator (+) or use append attribute to add items. 32

33 Project 5: Memory game Show a sequence of numbers. First show one number and ask for it, then show the first and second number and ask for them, and so on. You will need to clear the screen so the player can t see the previous numbers. One way to do it its by printing a few blank lines. You can do this in a function. Project 5: Memory game Requirements: Use functions Generate ten random numbers and save them in an array. Use time.clock() for mode 3 to calculate the time that the user takes to guess the number 33