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Programming Thinking and Method (2). Zhao Hai 赵海 Department of Computer Science and Engineering Shanghai Jiao Tong University zhaohai @cs.sjtu.edu.cn. Outline. Values and Types Variables Assignment Type Conversion Summary. Values and Types. Numbers

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Programming thinking and method 2

Programming Thinking and Method (2)

Zhao Hai 赵海Department of Computer Science and EngineeringShanghai Jiao Tong [email protected]


Outline

Outline

Values and Types

Variables

Assignment

Type Conversion

Summary


Values and types

Values and Types

Numbers

Integers: 12 0 12987 0123 0X1A2

Type ‘int’

Can’t be larger than 2**31

Octal literals begin with 0 (0981 illegal!)

Octal literals begin with 0O in python 3.x

Hex literals begin with 0X, contain 0-9 and A-F

Floating point: 12.03 1E1 1.54E21

Type ‘float’

Same precision and magnitude as C double


Values and types1

Values and Types

Long integers: 10294L

Type ‘long’

Any magnitude

Python usually handles conversions from ‘int’ to ‘long’

Complex numbers: 1+3J

Type ‘complex’

Python provides a special function called type(…) that tells us the data type of any value. For example,


Values and types2

Values and Types

>>> type(3)

<type 'int'> (python 2.x)

<class 'int'> (python 3.x)

>>> type(1E1)

<type 'float'>

>>> type(10294L)

<type 'long'>

>>> type(1+3J)

<type 'complex'>


Values and types3

Values and Types

String

A string is a sequence of characters.

>>> type("Hello world!")

<type 'str'>

Single quotes or double quotes can be used for string literals.

>>> a = 'Hello world!'

>>> b = "Hello world!"

>>> a == b

True


Values and types4

Values and Types

Produces exactly the same value.

>>> a = "Per's lecture"

>>> print a

Per's lecture

Special characters (escape sequence) in string literals: \n newline, \t tab, others.


Values and types5

Values and Types


Values and types6

Values and Types

Triple quotes useful for large chunks of text in program code.

>>> big = """This is

... a multi-line block

... of text; Python puts

... an end-of-line marker

... after each line. """

>>> big

'This is\na multi-line block\nof text; Python puts\nan end-of-line marker\nafter each line. '


Variables

Variables

Variable Definition

A variable is a name that refers to a value.

Every variable has a type, a size, a value and a location in the computer’s memory.

A state diagram can be used for representing variable’s state including name, type, size and value.


Variables1

Variables

Variable Names and Keywords

Variable names (also called identifier) can be arbitrarily long.

They can contain both letters and numbers, but they have to begin with a letter or underscore character (_) .

The underscore character is often used in names with multiple words, such as my_name.

Although it is legal to use uppercase letters, by convention we don’t. Note that variable names are case-sensitive, so spam, Spam, sPam, and SPAM are all different.


Variables2

Variables

For the most part, programmers are free to choose any name that conforms to the above rules. Good programmers always try to choose names that describe the thing being named (meaningful), such as message, price_of_tea_in_china.

If you give a variable an illegal name, you get a syntax error:

>>> 76trombones = ’big parade’

SyntaxError: invalid syntax


Variables3

Variables

>>> more$ = 1000000

SyntaxError: invalid syntax

>>> class = ’Computer Science 101’

SyntaxError: invalid syntax

Keywords (also called reserved words) define the language’s rules and structure, and they cannot be used as variable names. Python has twenty-nine keywords:


Variables4

Variables

Variable Usage

Variables are created when they are assigned. (Rule 1)

No declaration required. (Rule 2)

The type of the variable is determined by Python. (Rule 3)

For example,

>>> a = 'Hello world!' # Rule 1 and 2

>>> print a

'Hello world!'

>>> type(a) # Rule 3

<type 'str'>


Variables5

Variables

A variable can be reassigned to whatever, whenever. (Rule 4)

For instance,

>>> n = 12

>>> print n

12

>>> type(n)

<type 'int'>

>>> n = 12.0 # Rule 4

>>> type(n)

<type 'float'>


Variables6

Variables

>>> n = 'apa' # Rule 4

>>> print n

'apa'

>>> type(n)

<type 'str'>


Assignment

Assignment

Expressions

Numeric expressions

Operators: special symbols that represent computations like addition and multiplication.

Operands: the values the operator uses.


Assignment1

Assignment

The “/” operator performs true division (floating-point division) and the “//” operator performs floor division (integer division). But you should use a statement “from __future__ import division” to distinguish the above divisions.

For example,

>>> 3 / 4

0

>>> 3 // 4

0

>>> from __future__ import division


Assignment2

Assignment

>>> 3 / 4

0.75

>>> 3 // 4

0

The modulus operator (%) yields the remainder after integer division.

For instance,

>>> 17 % 5

2


Assignment3

Assignment

Order of operations: When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.


Assignment4

Assignment

For example:

y = ( a * ( x ** 2 ) ) + ( b * x ) + c

a = 2; x = 5; b = 3; c = 7.


Assignment5

Assignment


Assignment6

Assignment

Boolean expressions

‘True’ and ‘ False’ are predefined values, actually integers 1 and 0.

Value 0 is considered False, all other values True.

The usual Boolean expression operators: not, and, or.

For example,

>>> True or False

True

>>> not ((True and False) or True)

False


Assignment7

Assignment

>>> True * 12

12

>>>

0 and 1

0

Comparison operators produce Boolean values.


Assignment8

Assignment


Assignment9

Assignment

>>> 1 < 2

True

>>> 1 > 2

False

>>> 1 <= 1

True

>>> 1 != 2

True


Assignment10

Assignment

Statements

A statement is an instruction that the Python interpreter can execute. For example, simple assignment statements:

>>> message = “What’s up, Doc?”

>>> n = 17

>>> pi = 3.14159


Assignment11

Assignment

A basic (simple) assignment statement has this form:

<variable> = <expr>

Here variable is an identifier and expr is an expression.

For example:

>>> myVar = 0

>>> myVar

0

>>> myVar = myVar + 1

>>> myVar

1


Assignment12

Assignment

A simultaneous assignment statement allows us to calculate several values all at the same time:

<var>, <var>, ..., <var> = <expr>, <expr>, ..., <expr>

It tells Python to evaluate all the expressions on the right-hand side and then assign these values to the corresponding variables named on the left-hand side.

For example,

sum, diff = x+y, x-y

Here sum would get the sum of x and y and diff would get the difference.


Assignment13

Assignment

Another interesting example:

If you would like to swap (exchange) the values of two variables, e.g., x and y, maybe you write the following statements:

>>> x = 1

>>> y = 2

>>> x = y

>>> y = x

>>> x

2


Assignment14

Assignment

>>> y

2

What’s wrong? Analysis:

variables x y

initial values 1 2

x = y

now 2 2

y = x

final 2 2


Assignment15

Assignment

Now we can resolve this problem by adopting a simultaneous assignment statement:

>>> x = 1

>>> y = 2

>>> x, y = y, x

>>> x

2

>>> y

1


Assignment16

Assignment

Because the assignment is simultaneous, it avoids wiping out one of the original values.

Assigning input mode: in Python, input is accomplished using an assignment statement combined with a special expression called input. The following template shows the standard form:

<variable> = input(<prompt>)

Here prompt is an expression that serves to prompt the user for input. It is almost always a string literal.

For instance:


Assignment17

Assignment

>>> ans = input("Enter an expression: ")

Enter an expression: 3 + 4 * 5

>>> print ans

23

Simultaneous assignment can also be used to get multiple values from the user in a single input. e.g., a script:

# avg2.py

# A simple program to average two exam scores

# Illustrates use of multiple input


Assignment18

Assignment

def main():

print "This program computes the average of two exam scores."

score1, score2 = input("Enter two scores separated

by a comma: ")

average = (score1 + score2) / 2.0

print "The average of the scores is:", average

main()


Assignment19

Assignment

This program computes the average of two exam scores.

Enter two scores separated by a comma: 86, 92

The average of the scores is: 89.0


Type conversion

Type Conversion

Type conversion converts between data types without changing the value of the variable itself.

Python provides a collection of built-in functions that convert values from one type to another.

For example, the int function takes any value and converts it to an integer:

>>> int("32")

32

>>> int("Hello")


Type conversion1

Type Conversion

Traceback (most recent call last):

File "<interactive input>", line 1, in <module>

ValueError: invalid literal for int() with base 10: 'Hello‘

int can also convert floating-point values to integers, but remember that it truncates the fractional part:

>>> int(3.99999)

3

>>> int(-2.3)

-2


Type conversion2

Type Conversion

The float function converts integers and strings to floating-point numbers:

>>> float(32)

32.0

>>> float("3.14159")

3.1415899999999999

The str function converts to type string:

>>> str(32)

'32'


Type conversion3

Type Conversion

>>> str(3.14149)

'3.14149'

The repr function is a variant of str function

intended for strict, code-like representation of values

str function usually gives nicer-looking representation

>>> repr(32)

'32'

>>> repr(3.14149)

'3.1414900000000001'


Type conversion4

Type Conversion

The function eval interprets a string as a Python expression:

>>> eval('23-12')

11

Note that obj == eval(repr(obj)) is usually satisfied.


Summary

Summary

There are different number types including integer, floating point, long integer, and complex number.

A string is a sequence of characters. Python provides strings as a built-in data type.

Strings can be created using the single-quote (') and double-quote characters (").

Python also supports triple-quoted strings. Triple-quoted strings are useful for programs that output strings with quote characters or large blocks of text.


Programming thinking and method 2

Python offers special characters that perform certain tasks, such as backspace and carriage return.

A special character is formed by combining the backslash (\) character, also called the escape character, with a letter.

A variable is a name that refers to a value, whose consists of letters, digits and underscores (_) and does not begin with a digit.

Every variable has a type, a size, a value and a

Summary


Programming thinking and method 2

location in the computer’s memory.

Python is case sensitive—uppercase and lowercase letters are different, so a1 and A1 are different variables.

Keywords (reserved words) are only used for Python system, and they cannot be used as variable names.

Operators are special symbols that represent computations.

Operands are the values that the operator uses.

Summary


Programming thinking and method 2

If the operands are both integers, the operator performs floor division. If one or both of the operands are floating-point numbers, the operator perform true division.

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.

The usual Boolean expression operators: not, and, or.

Comparison operators produce Boolean values.

Summary


Programming thinking and method 2

A statement is an instruction that the Python interpreter can execute.

A simultaneous assignment statement allows us to calculate several values all at the same time.

Because the assignment is simultaneous, it avoids wiping out one of the original values.

Simultaneous assignment can also be used to get multiple values from the user in a single input.

Summary


Programming thinking and method 2

Type conversion converts between data types without changing the value of the variable itself.

Python provides a collection of built-in functions that convert values from one type to another.

Summary


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