Variables, Expressions, and Standard Functions

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# Variables, Expressions, and Standard Functions - PowerPoint PPT Presentation

Variables, Expressions, and Standard Functions. Topics. Basic calculation Expressions, variables, and operator precedence Data types Input / Output Examples. A calculator. We can use a computer as a calculator. Just type expressions into the Python Shell. Python Shell in Wing IDE.

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### Variables, Expressions, and Standard Functions

Topics
• Basic calculation
• Expressions, variables, and operator precedence
• Data types
• Input / Output
• Examples
A calculator
• We can use a computer as a calculator.
• Just type expressions into the Python Shell

Python Shell in Wing IDE

A calculator
• Try this.

Type it intoPython Shell

>>> 10 * 5

50

>>> 1 + 2 + 3 + 4

10

>>> 1+2+3+4

10

>>> 1 * 4 + 5 ** 2

29

Spaces are irrelevant

** is for exponentiation

Expression
• What we have just typed into the Python Shell is called expressions.
• After the shell reads each expression, the shell evaluate it and reports the result.
Easy calculation
• An object moves with the starting speed of 10 m/s with an acceleration of 2 m/s2. After 5 seconds, how far is the object from its starting position?

s = ut + at2/2

10 * 5 + 2 * (5*5) / 2

Operators (1)
• In previous examples, we use many operators such as +, -, or /, to tell Python to perform various computations with the data.
• An operator tells Python what operation to perform with its operands.

operands

10 * 5

operator

Operators (2)
• Operators can be
• Binary operators that work with two operands, e.g., +, -, or *.5 * 3 10 – 2 15*7
• Unaryoperators that work with a single operand, e.g, –.-3 +2 -5 * 7
Operators (2)
• Basic mathematical operators are shown in the table below
Division in Python
• There are two operators related to division

division

modulo – find the remainder

Numbers in Python
• There are two types for numbers in Python: integer(type int) andfloating points (type float)
Integers v.s. Floating-points
• If you write a number without a "dot", it will be treated as an integer.
• Results
• Every mathematical operation between integer and integer returns an integer, except for division.
• Division returns floating-point numbers.
• Any operations with floating-point numbers return floating-point numbers.
3/5*2
• Evaluation is usually done from left to right

((3/5)*2)

( 0.6 *2)

Operator precedence
• But operators have different precedence, e.g., * or/ have higher precedence over + or-.

2+3*6

2+(3*6)

This is just….
• High-school math!
Operator precedence
• Evaluation order is from left-to-right for operators with the same precedence, except **.
Try this (1)

2+5*6/3+(7-2*3)

What is the result?

13.0

Try this(2)

2 ** 2 ** 3

What is the result?

2 ** (2 ** 3)

28 = 256

Reusing values
• A force of 2.5 newton pushes a rock with mass 1 kg to the left. Where is the rock after 1 second, 5 second and 15 second?

(1.0/2.5)*1*1/2

Redundant

(1.0/2.5)*5*5/2

(1.0/2.5)*15*15/2

Variables
• We can use variables to refer to result from previous computation.

a = 1.0/2.5

a*1*1/2

a

0.4

a*5*5/2

a*15*15/2

Variables

a = 1.0/2.5

• A variableis used to refer to various data.
• Use "=" to assign a value to a variable.
• When we refer to that variable, we get the value that the variable is referring to.

0.4

a

Variables can be "modified" (1)

a = 10a * 5b = 3a + ba = 7a + ba = b + 5aa + b

50

13

10

8

11

Variables can be "modified" (4)

x = 10x = x * 2 + 5

25

Working onWing IDE

Shellor Console

After you type commands or expressions, the Python Interpreter evaluates them and prints out the output

Typing programs inWingIDE

Editing area

Any commands here are executed after you hit the "run" button.

A program
• A program is a sequence of commands (or statements)
• a = 10b = a + 5a - bc = 12b = a + cc = a*ba + b + c1 + a - c

Try totype this intoWing IDE

Result
• Because the program does not have statements that output anything.

Empty

Printing
• We can use function

print

to display results

A program
• A program is a sequence of commands (or statements)

Add"print"to display the valueof the required expressions

• a = 10b = a + 5print(a – b)c = 12b = a + cc = a*bprint(a + b + c)print(1 + a – c)
Function calls
• print(10)

Function name

Arguments

What's going on?
• print(a + b * 2)
• The expressions on the parameter list are evaluated before sending the result to the function.
• 20

Assume that

a = 5

b = 10

• 25
• print(25)
A simple calculation program
• We have the following coins
• 5 one-baht coins
• 7 ten-baht coins
• 2 twenty-baht notes
• 3 hundred-baht notes

How muchmoney do wehave ?

sum1 = 1 * 5sum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3sum = sum1+sum5+sum20+sum100print(sum)

A simple program(2)
• Or we can even remove variablesum

sum1 = 1 * 5sum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3print(sum1+sum5+sum20+sum100)

Meaningful names

Comparethese twoprograms

a = 1 * 5b = 10 * 7c = 20 * 2d = 100 * 3e = a + b + c + dprint(e)

sum1 = 1 * 5sum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3sum = sum1+sum5+sum20+sum100print(sum)

Which oneis easier to understand?

Suggestions
• Write programs for people to read
• At the very least, one of the audience is yourself.
• Everything after the # symbol is a comment.

# this program calculates total money# from the amount of each type of coins or# bank notes that you havesum1 = 1 * 5 # value of 1-baht coinssum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3print(sum1+sum5+sum20+sum100)

Strings
• A computer can work with many types of data.
• Astring is another data type which is very important. It is a type for texts or messages.
• Formally, a string is a sequence of characters.

”Hello

”Hello, world”

String constants
• We can either use single or double quotes to specify strings, e.g.,
• ”Hello”
• ’World’
• However, the starting quotes and the ending quotes must match.
• We can also have special characters inside a string. They will start with backslash" \ ".
Examples (1)

print("hello")

hello

print('hello')

hello

print("I'm 9")

I'm 9

print('I'm 9')

ERROR

print('I\'m 9')

I'm 9

print("I\'m 9")

I'm 9

Examples (2)

print("123")

123

print(123)

123

print("12" + "3")

123

print(12 + 3)

15

print("12" + '3')

123

print("12" + 3)

ERROR

A slightly better program

sum1 = 1 * 5sum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3sum = sum1+sum5+sum20+sum100print("The total is",sum)

The total is 415

A slightly even better program

sum1 = 1 * 5sum10 = 10 * 7sum20 = 20 * 2sum100 = 100 * 3sum = sum1+sum5+sum20+sum100print("The total is",sum,"bath.")

The total is 415 bath.

Sidenote: printing and new lines
• We can display data usingfunction print.
• It will always add a new line at the end.
• If we want to avoid the new line, we can add an additional option "end" to print.

print(10)print(20)print(10,end='')print(20)

10201020

This tells print to end this output with an empty string, instead of a new line.

• We can use function input to read data from the user

input

• The function returns a string that the user types into the shell.
Examples in the Python Shell

>>> name = input()

Somchai

>>> print("Hello", name)

Hello Somchai

>>> a = input()

10

>>> b = input()

100

>>> print(a+b)

10100

Remarks
• Consider this statement

print(a+b)

• Since both variables a andb are strings from function input. When we add two strings, we only get the concatenation of them.
?????
• How are we going to do calculations when we can only read strings from the user?
Conversion
• We can use functionint, float, andstr to convert between various data types

int("10")

10

float("10")

10.0

float(10)

10.0

int(10.6)

10

Type conversion

int("10")+10

20

float("10")+10

20.0

float(10)+int(5)

15.0

str(10)+str(5)

105

Conversion between float and int (1)

int(10.2)

10

int(10.9)

10

int(-10.1)

-10

Always return the integers without the fractional parts.

Conversion between float and int (2)

We can also use function round that returns the closest integers.

round(10.2)

10

round(10.9)

11

round(-10.1)

-10

# This program adds two numbersastr = input()a = int(astr)bstr = input()b = int(bstr)print("The result is",a+b)

Nested function calls (1)
• Consider this part of the program
• We use variableastr to refer to an input string. We then convert it to an integer and assign the result to variable a.
• We can avoid using variable astr:

astr = input()a = int(astr)

a = int(input())

Nested function calls (2)
• This is how it works.

a = int(input())

input()

a

"12345"

int

12345

a = 12345

Money calculation (improved)

# This program calculates total amount# of money from numbers of bank notesp1 = int(input())sum1 = 1 * p1p5 = int(input())sum5 = 10 * p5p20 = int(input())sum20 = 20 * p20p100 = int(input())sum100 = 100 * p100sum = sum1+sum5+sum20+sum100print("The total is",sum,"bath.")

A prompt
• We can tell function input to display a prompt before reading input by providing a string argument to input

x = int(input("Enter X: "))print(x+10)

Enter X:

100

110

Thinking corner
• An object, initially sitting still, starts moving with an acceleration of a m/s2 for t seconds.
• Write a program that reads the acceleration and the duration and computes the displacement.
Solution

a = float(input("Enter a: "))t = float(input("Enter t: "))print("Total distance =", a*t*t/2)

Thinking corner

Enter length in inch: 320It is 26 feet, 8 inch.

x = int(input("Enter length in inch"))xf = int(x/12)xi = x – xf * 12print("It is", xf, "feet,",xf,"inch.")

Volume Calculation
• Compute the volume of a cylinder

r

r2 x h

h

• We can use22/7 (which is quite inaccurate).
• We can use a closer estimation, in modulemath.

3.141592653589793

Themath module
• In Python, functions are categorized into various groups, called modules. To use functions from a module, we have to declare that by using the import statement.
• Then all functions can be referred to by prefixing with the module name.

import math

print("Pi is", math.pi)

3.141592653589793

Thinking corner
• Write a program that reads r and h and compute the volume of a cylinder.

r

r2 x h

h

Solution

import mathr = float(input("Enter r: "))h = float(input("Enter h: "))print("Volume =", math.pi*r*r*h)

Another example
• Projection

f

#don't forget toimport math#recall that the angle must be in radianr = t * math.pi/180fy = f*math.sin(r)fx = f * math.cos(r)

import math# radian anglesfy = f*math.sin(t)fx = f * math.cos(t)

t