introduction to python programming course 2

Recap Python programming of the previous week • Python script files rules: one statement per line, each statement starting in the 1st column • A simple program contains 3 steps: Input, Processing, Output • Input involves assigning values to variables. It can be done with assignment lines (such as a = 12) or by reading the values from the keyboard (such as a = int(input(‘Enter the value of a:’)) ). Note that int() and input() are Python functions. • Processing involves operations with the variables • Output to the black screen involves the use of the print() function Slides are adapted from prof Radu I Campeanu

Calculating mathematical expressions • Assignment statements • Variables/Objects • Floating point variables • Mathematical operators and expressions • Using the math module

Assignment statements • In a program, a variable is a named item, such as a, used to hold a value. • An assignment statement assigns a value to a variable, such as a = 4. • ‘=‘ has nothing to do with mathematics. In computer programming it means assignment, i.e. it gives the value 4 to the variable a. • a = b means that the value of b is copied into a; as a result both variables a and b have the same value. • a = a+1 means that the value of a is incremented by 1. An alternative is to write a +=1 (the meaning of ‘+=‘ is increment by the value on the right)

Objects • Python variables are also called objects. • Each Python object has three properties: value, type, and identity. - Value: A value such as 4 or "abcdef". - Type: The type of the object, such as integer or string. - Identity: A unique identifier that describes the object (i.e. its address in the principal memory). a=4 print(a) # Output: 4 print(type(a)) # Output: <class ‘int’> print(id(a)) # Output: 140728879073924

Floating-point numbers • Floating-point numbers are real number, like 98.6 or 0.0001. • The built-in function float() is used for these numbers, which have decimals. #P1 calculate hours to cover the distance miles = float(input('Enter a distance in miles: ')) hours_to_fly = miles / 500.0 #500 miles/hour is the speed to fly hours_to_drive = miles / 60.0 #60 miles/hour is the speed to drive print(miles, 'miles would take:') print(hours_to_fly, 'hours to fly') print(hours_to_drive, 'hours to drive’) Input/Output: Enter a distance in miles: 450 450.0 miles would take: 0.9 hours to fly 7.5 hours to drive

Scientific notation • Scientific notation is useful for representing floating-point numbers that are much greater than or much less than 0, such as 6.02x1023 • 6.02x1023 is written as 6.02e23 • Float-type objects can be stored on a 32 bits (or 4 bytes) computer if their values are between 2.3x10-308 and 1.8x10308 • Outside this interval the float-type object produces underflow/overflow. • Ex. print(2**1024) will produce an overflow error (‘2**1024’ means 2 to the power 1024). Note that print() accepts mathematical expressions, which are executed before the display of the result.

Formatting floating-point output • Some floating-point numbers have many digits after the decimal point. Ex: Irrational numbers such as the value of Pi = 3.14159265359… • Python has the method format() which allows the reduction of the number of decimals #P2 pi_v = 3.141592653589793 print('Default output of Pi:', pi_v) print('Pi reduced to 4 digits after the decimal:', end=' ') print('{:.4f}'.format(pi_v)) Output: Default output of Pi: 3.141592653589793 Pi reduced to 4 digits after the decimal: 3.1416

Arithmetic Expressions • An expression is a combination of items, like variables, operators, and parentheses, that evaluates to a value. For example: 2 * (x + 1). • A common place where expressions are used is on the right side of an assignment statement, as in y = 2 * (x + 1). • The arithmetic operators are: +, -, *, /, %, ** • % is the modulo operator producing the remainder of an int division • ** is the exponent, or power operator • In a long expression the following precedence rules will apply: - operations inside parentheses are executed first - unary minus and the exponent are executed next (ex. x* -2, 2 * x**2) - next are *,/,% in the left-to-right order - last are +, - in the left-to-right order

Examples • total_cost = down_payment + payment_per_month * num_months • 24 % 10 is 4. Reason: 24 / 10 is 2 with remainder 4. • 50 % 50 is 0. Reason: 50 / 50 is 1 with remainder 0. • 1 % 2 is 1. Reason: 1 / 2 is 0 with remainder 1. • 20 / 10 is 2.0 (Division with ‘/’ produces a float) • 50 / 50 is 1.0 • 5 / 10 is 0.5 • 20//10 is 2 (‘//’ is the floored division operator; produces an int) • print(12 - 1 / 2 * 3 % 4 + 10 / 2 * 4) produces 30.5

Example 1 - money The program shown below shows the calculation of the number of $5 bills for a certain integer amount. What is the output if the input is 19? #P3 amount_to_change = int(input()) num_fives = amount_to_change // 5 num_ones = amount_to_change % 5 print('Change for $', amount_to_change) print(num_fives, 'five dollar bill(s) and', num_ones, 'one dollar coin(s)’) Ans: Change for $ 19 3 five dollar bill(s) and 4 one dollar coin(s)

Example 2 - time The program shown below extracts the number of hours from an input of minutes. What will be the output for 275 ? #P4 minutes = int(input('Enter minutes:')) hours = minutes // 60 minutes_remaining = minutes % 60 print(minutes, 'minutes is', end=' ') print(hours, 'hours and', end=' ') print(minutes_remaining, 'minutes.n', end=' ‘) Input/Output Enter minutes: 275 275 minutes is 4 hours and 35 minutes

Digits in an integer The last digit in 23 is obtained by: 23%10 The last 3 digits in 1234 are obtained by: 1234%1000 The first digit in 23 is obtained by: 23//10 The first digit in 1234 is obtained by: 1234//1000

Example 3 - Telephone number #P5 nr = int(input('Enter a 10 digit integer:')) tmp = nr // 10000 #tmp will have the value 103458 mid = tmp % 1000 #mid will have the value 458 first = nr // 10000000 #first will have the value 103 last = nr % 10000 #last will have the value 8231 print('(',first,') ',mid,'-',last) #notice the brackets and - Input/Output: Enter a 10 digit integer: 1034588231 ( 103 ) 458 - 8231

Using modules • Programmers often write code in more than just a single script file. Collections of logically related code can be stored in separate files, and then imported for use into a script that requires that code. • A module is a file containing Python code that can be used by other modules or scripts. • A module is made available for use via the import statement. • Once a module is imported, any object defined in that module can be accessed using dot notation. • Python comes with a number of predefined modules, such as math and random.

Using the random module • Python has a randrange() function as part of the module random. This function creates a random integer between the function parameters. import random print(random.randrange(1,10)) # the dot notation Output: 6 # a random number between 1 and 10 • An alternative is: from random import randrange print(randrange(1,10)) Output: 8 # another random number between 1 and 10

Equation Solutions using the math module The solutions for the equation ax2 + bx + c = 0 are: 𝑥1 = −𝑏 + 𝑑 2𝑎 and 𝑥2 = −𝑏 − 𝑑 2𝑎 , where d = b2 - 4ac Write a Python program which enters from the keyboard the values of a, b, c (as integers) and then calculates and shows x1 and x2. #P6 import math a = int(input('Enter the value of a:')) b = int(input('Enter the value of b:')) c = int(input('Enter the value of c:')) d = b*b - 4*a*c; x1 = (-b + math.sqrt(d))/2/a x2 = (-b – math.sqrt(d))/2/a print('x1=', x1) print('x2=', x2)

Using the math module Given three floating-point numbers x, y, and z, output x to the power of z, x to the power of (y to the power of z), the absolute value of (x minus y), and the square root of (x to the power of z). #P7 import math x=float(input()) # enter 5.0 y=float(input()) # enter 1.5 z=float(input()) # enter 3.2 v1=x**z v2=x**(y**z) v3=math.fabs(x-y) v4=math.sqrt(x**z) print('{:.2f} {:.2f} {:.2f} {:.2f}'.format(v1,v2,v3,v4)) Output: 172.47 361.66 3.50 13.13

Using the random module • Write a Python program which rolls two dice. This program should use the module random. #P8 import random a = random.randrange(1,6) b = random.randrange(1,6) print(‘the sum is ‘, a+b) Possible Output: the sum is 10

Using Constants in Python • Constants are variables which have values that cannot be changed during the execution of the program • In other programming languages such as C constants are written in capital letters. In Java one uses the reserved word final in front of the constant to ensure that its value cannot be changed. • Python does not have a way to enforce that the value of a constant is not changed. A good method to signal that certain variables are constants is to create a constants module.

Creating and using the constants module The time of a free falling object from a certain height is given by the following formula: time = (2 ∗ height / g), where g is the gravitational constant. In this program let us compare the times for falls on Earth and Mars. constants.py # user defined module # Gravitational constants for various planets earth_g = 9.81 # m/s**2 mars_g = 3.71 ======================================================== fall_time.py #P9 # Find seconds to drop from a height on some planets. import constants import math # module available in Python height = int(input('Height in meters: ')) # Meters from planet print('Earth:', math.sqrt(2 * height / constants.earth_g), 'seconds') print('Mars:', math.sqrt(2 * height / constants.mars_g), 'seconds’) # notice the dot notation for functions and constants

Class exercise - Distance between two points The distance between point (x1, y1) and point (x2, y2) is given by the formula: (x2 − x1)∗∗2 + (y2 − y1)∗∗2 . All these variables have floating - point numbers. Write a Python program which reads the coordinates of the 2 points and shows the value of the distance, shown as a floating – point number with 3 decimals. For example if the 4 numbers entered are 1.1, 2.2, 4.4 and 5.5 The output should be: Points distance: 4.667

Class exercise - Solution import math x1 = float(input('Enter x1:')) y1 = float(input('Enter y1:')) x2 = float(input('Enter x2:')) y2 = float(input('Enter y2:')) point_dist = math.sqrt((x2 - x1)**2 + (y2 - y1)**2) print('Points distance: {:.3f}'.format(point_dist))

introduction to python programming course 2

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