Python Basics Flashcards
2 types of functions?
built in functions and custom made
how to create function
define it. def functionname(arguments).
function
function is some stored code we use. we take some input and create output
implicent
when you multipy int and float in implicently becomes float.
converting datatypes
newint = int(oldval)
cant convert string to inv if not number
function
def print_lyrics():
print ("I'm a lumberjack, and I'm okay.”)
print ('I sleep all night and I work all day.’)function argument
An argument is a value we pass into the function as its input when we call the function
We use arguments so we can direct the function to do different kinds of work when we call it at different times
We put the arguments in parentheses after the name of the function
parameter
A parameter is a variable which we use in the function definition. It is a “handle” that allows the code in the function to access the arguments for a particular function invocation.
lang is the parameter.
return
Often a function will take its arguments, do some computation, and return a value to be used as the value of the function call in the calling expression. The return keyword is used for this.
return is
A “fruitful” function is one that produces a result (or return value)
The return statement ends the function execution and “sends back” the result of the function
parameter order and number
We can define more than one parameter in the function definition
We simply add more arguments when we call the function
We match the number and order of arguments and parameters
void
When a function does not return a value, we call it a “void” function
Functions that return values are “fruitful” functions
Void functions are “not fruitful”
what is a list?
A List is a kind of Collection.
A collection allows us to put many values in a single “variable”
A collection is nice because we can carry all many values around in one convenient package.
describe list
List constants are surrounded by square brackets and the elements in the list are separated by commas A list element can be any Python object - even another list A list can be empty . print ([1, 24, 76]) [1, 24, 76] >>> print (['red', 'yellow', 'blue’]) ['red', 'yellow', 'blue'] >>> print(['red', 24, 98.6]) ['red', 24, 98.6] >>> print([ 1, [5, 6], 7]) [1, [5, 6], 7] >>> print([]) []
simple for loop examples
friends = [‘Joseph’, ‘Glenn’, ‘Sally’]
for friend in friends :
print(‘Happy New Year:’, friend)
print(‘Done!’)
z = [‘Joseph’, ‘Glenn’, ‘Sally’]
for x in z:
print(‘Happy New Year:’, x)
print(‘Done!’)
ping element
friends = [ ‘Joseph’, ‘Glenn’, ‘Sally’ ]
»> print(friends[1])
Glenn
strings vs list
Strings are “immutable” - we cannot change the contents of a string - we must make a new string to make any change Lists are “mutable” - we can change an element of a list using the index operator fruit = 'Banana' >>> fruit[0] = ‘b' Traceback lotto[2] = 101 >>> print (lotto) [2, 14, 101, 41, 63]
len() function
The len() function takes a list as a parameter and returns the number of elements in the list Actually len() tells us the number of elements of any set or sequence (such as a string...) >>> greet = 'Hello Bob' >>> print (len(greet)) 9 >>> x = [ 1, 2, 'joe', 99] >>> print (len(x)) 4
range()
The range function returns a list of numbers that range from zero to one less than the parameter
We can construct an index loop using for and an integer iterator
»> print (range(4))
[0, 1, 2, 3]
»> friends = [‘Joseph’, ‘Glenn’, ‘Sally’]
»> print (len(friends))
3
»> print (range(len(friends)))
[0, 1, 2]
»>
2 more loop examples using range/len and for
friends = [‘Joseph’, ‘Glenn’, ‘Sally’]
for friend in friends :
print (‘Happy New Year:’, friend)
for i in range(len(friends)) :
friend = friends[i]
print (‘Happy New Year:’, friend)
add lists togeter
We can create a new list by adding two existing lists together a = [1, 2, 3] >>> b = [4, 5, 6] >>> c = a + b >>> print (c) [1, 2, 3, 4, 5, 6] >>> print (a) [1, 2, 3]
more list functions
List.count(element) List.insert (index, element) List. append(new element) #appends an element to the end of the list List.remove(element) List.sort() del list[index]
show list functions
a = [66.25, 333, 333, 1, 1234.5] # create a list
print a.count(333), a.count(66.25), a.count(‘x’)
a.insert(2, -1) #List.insert(index, element)
a.append(333) # appends an element to the end of the list.
a
a.index(333) #show the first found element’s index
a.remove(333) # removes first found elements of the same value.
a
a.sort()# ascending order. If its list of strings, it will show in alphabetically order.
a
a.pop()#list.pop(pos) pos needs to be a specified position that you want to remove. If no input of pos, then remove the last element.
a
list vs. dictionary
List
- A linear collection of values that stay in order
Dictionary
- A “bag” of values, each with its own label
Dictionaries
Dictionaries are Python’s most powerful data collection
Dictionaries allow us to do fast database-like operations in Python
Dictionaries have different names in different languages
- Associative Arrays - Perl / PHP
- Properties or Map or HashMap - Java
- Property Bag - C# / .Net
describe picture of dictionary statement
Construct dictionary
food={‘ham’: ‘yes’, ‘egg’: ‘yes’}
a = dict(one=1, two=2, three=3) #use dict() function
We can use any types as value but no lists or dictionaries can be used for keys.
Use [key] to index the entries
»>food[‘ham’]
‘yes’
Add new element:
»>food[’spam’]=‘no’
»>food
food = {“ham” : “yes”, “egg” : “yes”, “spam” : “no” }
change value of dictionary
Change the value
»> food = {“ham” : “yes”, “egg” : “yes”, “spam” : “no” }
»> food
{‘egg’: ‘yes’, ‘ham’: ‘yes’, ‘spam’: ‘no’}
»> food[“spam”] = “yes”
»> food
{‘egg’: ‘yes’, ‘ham’: ‘yes’, ‘spam’: ‘yes’}
copy and merge dictionary.
A dictionary can be copied
food={'ham': 'yes', 'egg': 'yes'}
w=food.copy()
Clean
>>> w.clear()
>>> print w
{}
Merging
>>> w={"house":"Haus","cat":"Katze","red":"rot”}
>>> w1 = {"red":"rouge","blau":"bleu"}
>>> w.update(w1)
>>> print w
{'house': 'Haus', 'blau': 'bleu', 'red': 'rouge', 'cat': 'Katze'}dif between list and dict
lst = list()
>>> lst.append(21)
>>> lst.append(183)
>>> print (lst)
[21, 183]
>>> lst[0] = 23
>>> print(lst)
[23, 183]
ddd = dict()
>>> ddd['age'] = 21
>>> ddd['course'] = 182
>>> print(ddd)
{'course': 182, 'age': 21}
>>> ddd['age'] = 23
>>> print(ddd)
{'course': 182, 'age': 23}find something in dict
It is an error to reference a key which is not in the dictionary We can use the in operator to see if a key is in the dictionary >>> ccc = dict() >>> print(ccc['csev']) Traceback (most recent call last): File "", line 1, in KeyError: 'csev' >>> 'csev' in ccc False
gets method
We can use get() and provide a default value of zero when the key is not yet in the dictionary - and then just add one
Counts = dict()
Names = [‘bush’, ‘cwen’, ‘zhen’, ‘raja’, ‘cwen’, ‘raja’]
for name in Names :
Counts[name] = Counts.get(name, 0) + 1
print(counts)
count words.
counts=dict()
A = raw_input(‘Please enter a line of text: ‘)
words = A.split()
print ‘Words:’, words
print ‘Counting…’
for word in words:
counts[word] = counts.get(word,0) + 1
print counts
why numpy?
ndarray: vectorized arithmetic operations and broadcasting capabilities
Standard mathematical functions for fast operations on entire arrays of data. No need to write loops.
Tools for reading/writing array data
Linear algebra, random number generation and so on
how does mapreduce work?
MapReduce works by breaking the processing into two phases: the map phase and
the reduce phase. Each phase has key-value pairs as input and output, the types of
which may be chosen by the programmer. The programmer also specifies two
functions: the map function and the reduce function.
