ch. 2: numbers and strings

programming w numbers and strings

variables

a named storage location in a computer program

define a variable by telling the compiler:

• what name you will use to refer to it

• initial value

it’s like the parking space in a garage

the assignment statement

• use assignment operator (=) to place a value into a variable

• assigning a value to an existing variable replaces the previously stored value

• the “=” sign is NOT used for comparison

3 primitive data types

• whole number (7) integer

• number with a fraction part (7.0) float

• sequence “Bob” string

string literal are specified by enclosing a sequence of character within a matching pair of either single or double quotes

the data type in Python is associated with the value, not the variable:

cansPerPack = 6 # int

canVolume = 12.0 # float

Python infers the data type of a variable by the value it holds

• a variable can be assigned different values at different places in a program

naming variables

1. must start with a letter or the underscore (_) character

2. cannot use other symbols and spaces are not permitted

3. separate words with ‘camelCase’, ‘PascalCase’, or ‘snake_case’ notation

4. don’t use ‘reserved’ Python keywords

snake_case

• pros: concise when it consists of a few words

• cons: redundant when it gets longer

push_something_to_first_queue, pop_what, get_whatever…

PascalCase

• pros: seams neat

GetItem, SetItem, Convert, …

• cons: barely used

camelCase

• pros: widely used int he programmer community

• cons: looks ugly when a few methods are n-worded

push, reserve, beginBuilding, …

constants

a variable whose value should not be changed after it’s assigned an initial value

python will let you change the value of a constant

totalvolume = bottles * 2.0 ← aviod magic numbers, a number in the code that seems arbitrary and has no context/meaning

totalvolume = bottles * BOTTLE_VOLUME

python comments

use comments at the beginning of each program and to clarify details of the code

basic arithmetic operations

+ → addition

- → subtraction

* → multiplication

/ → division

% → modulus operator to get remainder in integer division ( ex. 5%2, result: 1)

** → exponent

// → integer division/floor division (ex. 5//2 III -5//2, results: 2 III -3)

line continuation

the backslash (\)

calling functions

a function is a collection of programming instructions that carry out a particular task

print() function can display information, but there are other function that requires arguments

calling function that return a value

most function return a value, when a function completes its task, it passes a value back to the point where the function was called

type conversion functions

use int() and float() to convert between integer and floating-point values

you lose the fractional part of the floating-point value (no rounding occurs) when you use int()

functions in python libraries: library

a collection of code, written and compiled by someone else, that’s ready for you to ise in your program

standard library

a library that’s considered part of the language and must be included with any Python system

provide standardized solutions for many problems that occur in everyday programming

modules

python’s standard library is organized into__

functions defined in a module other than the built-in module must be explicity imported into your program before they can be used

built-in functions

a small set of functions that are defined as a part of the python language that can be used directly

can be used w/o importing any modules

strings

• sequence of characters

• in python, string literals are specified by enclosing a sequence of characters witghin a matching pair of either single or double quotes

string length

number of characters in a string is

a string of length 0, empty string. contains no characters and is written as ““ or ‘‘

string concatenation “+”

concatenation: you can ‘add’ one String onto the end of another

using “+” to concatenate string is an example of operator overloading. the “+” operator performs different functions of variables of different types

string repetition “*”

produce a string that is the result of repeating a string multiple time, also overloaded

ex. In [20]: dashes = “-” * 50

unicode charaters

• encode every character into an integer number

• defines over 100,000 characters

• designed to be able to encode text essentially all written languages

conversion between unicode and character

the ord() function returns the number used to represent a given character

the chr() function returns the character associated with a given code

round function()

round(number, ndigits=None)

round a number to a given precision in decimal digits

return value is an integer if ndigits is omitted or None. otherwise the return value has the same type as the number

methods (in object-oriented programming)

• a collection of programming instructions ot carry out a specific task

  • like a function but only applied to an object of the type for which is was defined

  • are specific to a type of object

• apple the upper () method to any string

  • ex.

name = “John Smith

uppercaseName = name.upper() #Sets uppercaseName to “JOHN SMITH”

input

read a user input from the console, input() function

name = input (“Please enter your name: “)

numerical input

the user’s input will always be returned by input() as a string

• this is wrong:

age = input (“Please enter age: “)

ageNextYear = age +1

• to use the input as number, convert it using int() or float()

age = int (input(“Please enter age: “))

• the above is equivalent to doing it two steps (getting the input and then converting it to a number):

ageString = input (“Please enter age:) #String input

age = int(ageString) # Converted to int

print output

using String format() method with positional arguments

String format() with keyword arguments

ways of defining placeholders

can be identified using named indexes { price }, numbered indexes { 0 }, or even empty placeholders { }

formatted output

• outputting floating point values can look strange

• to control the output appearance of numeric variables, use formatted output such as:

ln [18]: print (“Price per liter: {:.2f}”.format (price))

Price per liter: 1.22

format specifier

‘:10.2f’

<type> Subcomponent in a format specifier

using python f-string

the string has the f prefix and uses {} to evaluate variables

find(substring)

returns the index of the first occurrence of a substring, or -1 if it is not found.

count(substring)

counts how many times a substring appears in the string

startswith(prefix)

returns True if the string begins with the specified text.

endswith(suffix)

returns True if the string ends with the specified text.