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Transcript of 1 Python Chapter 2 © Samuel Marateck, 2010. 2 After you install the compiler, an icon labeled IDLE...
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PythonChapter 2
© Samuel Marateck, 2010
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After you install the compiler, an icon labeled IDLE (Python GUI) will appear on the screen. If you click it, a window will appear on the screen with some copyrightinformation. This window is called the shell.IDLE is an acronym for Integrated DeveLopment Environment which allows you to write, edit and run a program.
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Under that a prompt indicated by >>> will
appear. This prompts you to type some
Instruction. For instance,
>>> 3*4
This instructs the computer to multiply
3 times 4. The computer responds with
the product, 12. So you see:
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>>>3*4
12
There are two types of numbers we will
be working with, integers and floats.
Let’s discuss integers first.
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An integer has no decimal point. Examples
3, 456, -12. These are called integer literals
We first list symbols that you are familiar
with:
+ (addition), - (subtraction), * (multiplication),
and / (division).
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We first list symbols that you are familiar
with:
+ (addition), - (subtraction), * (multiplication),
and / (division).
The first three produce an integer; but the /
produces a number with a decimal point.
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The / produces a number with a decimal point. So for instance typing 4/2 we get>>> 4/22.0
The number 2.0 in called a floating pointnumber or simply, a float. It is an exampleof a float literal.
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There are three other operations you can use
with integers. Let’s try “//”, integer division:
>>> 7//2
3
Now 7/2 produces 3.5. The // operator
instructs the compiler to truncate the
results, i.e., lops off the part of the number
to the right of the decimal point. So 3.5 becomes 3
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The second operator is %, the remainder
operator.
>>>7%2
1
Think of it this way, after 7//2 is performed,
the reminder is 1.
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The third operator is ** which indicates
exponentiation, so
>>> 2**3
8
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As oppose to other languages , there is no
limit to the number of digits in an integer.
So
>>>2**300
20370359763344860862684456884093781
61051468393665936250636140449354381
299763336706183397376
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The operators have a precedence.
the operator with the highest precedence
is ** , then comes *, /, //, and %. These all
have the same precedence. Finally comes
+, -.
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So 3*4 + 5 evaluates as 12+5 or 17.
How do you change the precedence, so that
the addition is done before the
multiplication?
.
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.How do you change the precedence, so that the
addition is done before the multiplication?
Use (). So:
>>> 3*(4+5)
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The operations in the () are always done first.
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Similarly
>>> 3+2**4
19
How do you have the addition done before
the exponentiation?
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To have the addition done before
the exponentiation, use ():
>>>(3+2)**4
625
or 5**4
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Order of Operations
Operator Operation Precedence
() parentheses 0
** exponentiation 1
* multiplication 2
/ division 2
// int division 2
% remainder 2
+ addition 3
- subtraction 3
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The computer scans the expression fromleft to right, first clearing parentheses,second, evaluating exponentiations from left to
right in the order they are encounteredthird, evaluating *, /, //, % from left to right in the order they are encountered,fourth, evaluating +, - from left to right in the order they are encountered
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How would you evaluate (3+3*5)/2*3?
Is it 18/6 i.e, 3 or is it 9*3?
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These are the steps in evaluating the
Expression (3+3*5)/2*3
1.(3 + 15)/2*3
2. 18/2*3
3. 9*3
4. 27
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The same operators that apply to integers
apply to floats. For instance.
In 5.2//2.1 python first uses regular division,
5.2/2.1 getting 2.4761904761904763 and
truncates the answer to 2.
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Let’s do 6.3//2.1. You might expect to get
3.0 but you get 2.0. Why?
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First, let’s calculate 6.3/2.1 using regular
division, the answer is 3.0 .
But floats are only approximations. The
answer that’s stored in memory is 2.999999
but is printed as 3.0. The // operation
truncates the results. So 6.3//2.1 is 2.99999
truncated to 2.0
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So we’d expect 6.4//2.1 to produce 3.0
and it does.
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Integers are stored in the memory exactly;
whereas stored floats are only
approximations.
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There are math functions that can be Imported. To do this, type import math. If you then type
help(math),
the computer will type all themath functions and what they do:>>>import math>>help(math)
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Then if you want to find the √5, you type
>>>math.sqrt(5) and you get
2.2360679774997898
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If you don’t want to type math each time
you use a function, type:
>>>from math import *
Then you can simply write:
>>>sqrt(5)
2.2360679774997898
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If you just want to see the functions in the
math module, type
>>>dir(math)
and you get:
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['__doc__', '__name__', '__package__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'exp', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'hypot', 'isinf', 'isnan', 'ldexp', 'log', 'log10', 'log1p', 'modf', 'pi', 'pow', 'radians', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'trunc']
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If you want to see the meaning of a
function, place the function name in the
() in help. Example:
>>>help(hypot)
This gives:
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>>> help(hypot)
Help on built-in function hypot in module math:
hypot(...)
hypot(x,y)
Return the Euclidean distance, sqrt(x*x + y*y).
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What does
>>> hypot(3,4)
give you?
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>>> hypot(3,4)
5.0
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Strings
A string is a group of one or more characters
sandwiched between “ or ‘. So “123sd” and
‘sde’ are strings. The characters “ and ‘
are called delimiters. If the string starts with
a “, it must end with a “; if it starts with a
‘ it must end with a ‘.
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The + operator is overloaded. This means it
use depends upon context. We have seen
its use with floats and integers. It can also
be used with strings.
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When used with strings, it joins them
creating a new string. So
>>>’abc’ + “123”
‘abc123’
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To get the number of characters in a string
Use the len function.
>>>len(’abc’ + “123”)
6
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Using Variables
A variable is a name associated with a memory location. Examplesone = 12two = 12.5st = ‘123as’
One, two and s are variable names.The type of the literal determines the variabletype. So one is an integer, two a float, and st astring.
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Variable names are case sensitive, so one
and One are two different variables (since
One is capitalized and one is not) and
describe two different memory locations.
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Conversion functions
You can’t concatenate a string with anothertype. So>>> 'anc' + 3
producesTraceback (most recent call last): File "<pyshell#2>", line 1, in <module> 'anc' + 3 TypeError: Can't convert 'int' object to str
implicitly
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The error message:
TypeError: Can't convert 'int' object to str implicitly
means that we have to explicitly make 3 in:'anc' + 3a string. The function str does this:
>>> 'anc' + str(3)'anc3‘
The error is an example of a syntax error since it doesn’t follow the grammaticalrules of Python.
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The following table show the conversion
functions.
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function use
Str() Converts a number to
a string
int() Converts an object to an
integer and truncates.
round() Converts an object to an
integer and rounds.
float Converts an object to a
float.
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Examples:>>>int(‘1234’)1234>>>float(‘1234’)1234.0>>>round(12.56)13>>>str(12.34)’12.34’
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When int() operates on a string, what is embedded between the quotes must bean integer, so>>>int(’1.234’)
because 1.234 is a float, produces:
int('1.234')ValueError: invalid literal for int() with base 10:
'1.234'
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When an integer is used in an expression
containing a float, the result is also a float:
>>12 + 3.0
15.0