Binary Numbers
8 bits |
1 byte |
16 bits |
word |
32 bits |
double word |
always at least 1 byte |
add one to first number in sequence to make negative, add zeros to get full byte |
Machine Learning
-program adjusts itself automatically to fit data, end result is a program trained to achieve a given task |
Supervised learning-given examples of input and desired outputs, predict outputs on future unseen inputs(classification, regression, time series) |
Unsupervised learning-creates a new representation of the input |
Reinforcement learning-learning action to maximize payoff |
Types of supervised learning tasks |
1. classification- predict which predefined set of classes and example belongs to |
2. regression - predict a real value |
2. probability estimation - estimate probability of an event |
Sensitivity = fraction of positive examples predicted to be positive |
TP/(TP+FN) |
Specificity= proportion of negative examples predicted negative |
TN/(FP+TN) |
False-positive rate(FPR)= negatives predicted to be positive |
FP/(FP+TN) |
Sets
Set |
stores an unordered set of objects, CANT BE INDEXED, no duplicates, only immutable objects contained |
myset = set([]) |
creates a set from a list |
len(myset) |
length of the set |
if x in myset: |
evaluates boolean condition |
for element in myset: |
iterate through set |
for set1 = A and set2 = B, A&B = intersection |
elements in common to A and B |
A | B or A.union(B) |
union of two sets |
A-B |
difference of sets, elements in A that are not in B |
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all these rules can be applied to multiple sets |
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Integer Sequences
range(start, end, step) |
(start default is zero, step default 1) |
range(5) |
0 1 2 3 4 |
range(3,8) |
3 4 5 6 7 |
range(len(seq)) |
sequence of index of values in seq |
range(2,12,3) |
2 5 8 11 |
Functions
def functionname( arguments): |
defines a function of given name with given arguments |
return |
only returns a certain value or string generated by the function, doesn't print, exits at this value |
list = [[0 for i in range(ncols)] for j in range(nrowns)] |
creates a two dimensional list of nrows and ncols filled with zeros |
Operations on Lists
lst = [] |
creates empty list |
lst.append(val) |
adds item to list at end |
lst.extend(seq) |
add sequence of items at end |
lst.insert(idx, val) |
inserts item at index |
lst.remove(val) |
remove first item in the list with value val |
lst.pop(idx) |
remove and return item at index |
lst.sort()/lst.reverse() |
sort/reverse list in place |
lst.min()/.max() |
finds the min/max |
matplotlib.pyplot
.plot(x,y, color) |
plots the x values to x values in a certain color |
.show() |
shows the graph |
.ylabel(name) |
names y axis |
.xlabel(name) |
names x axis |
.savefig( figname) |
saves image under figname |
OOP
class |
defines attributes(information we want to keep together) and methods( operations want to be able to perform on that data) |
class some_class_name: |
defines a class of some_class_name |
def __int__(self, other attributes): |
defines and initializes attributes |
object.method() |
calls method on an object of that class |
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Bugs
Sytax errors |
violation of a writing rule |
Exceptions(runtime) |
syntax is fine, some other thing is occasionally flawed:ZeroDivisionError(can't divide by zero), NameError(can't access the variable), IndexError(When the index you are attempting to access does not exist), TypeError( operation on non-compatible type) |
Logical (semantic) errors |
code runs, but doesn't do what its supposed to |
try: |
does something that may cause an exception |
except <SomeExceptionType>: |
do something to handle the exception |
raise[ exception object ] |
raises a certain, defined type of exception |
try, except, else |
try something, that could cause the exception, if its fine, go to else |
finally: |
adds statement that happens no matter what |
for a, b in zip(list 1, list 2): |
iterates over elements of two lists in parallel, yields a tuple with both iterations |
File Input/Output
f = open(myfile, 'x') |
opens the file, x=r for reading only, x=w for writing only, x=a for appending, x=b for file in binary format, x=wr+ reading and writing and so on |
.read(size) |
reads the entire file, returns a single string, size is optional number of characters |
.readlines(size) |
reads all lines and returns them as a list of strings |
.rstrip() |
returns a string with the end-of-line character(\n) removed from the end of the string |
.readline() |
reads single line from file, returns empty string if end of file |
.close() |
closes file |
gzip.open() |
interface to read/write compressed files, .gz extension |
Operations on Dictionaries
dict = {} |
sets up an empty dictionary |
dict[key] = value |
sets value of said at that key to value |
dict[key] |
calls the value at that key |
del dict[key] |
deletes a key from dict |
dict.clear() |
clears the dict of all keys |
dict.update(dict2) |
can update/add associations from another |
dict.keys()/dict.items()/dict.values() |
looks at either all keys, values, or all items in the dict |
dict.pop(key) |
removes element associated to that key |
dict.popitem() |
removes key, value pair and returns it as a tuple |
dict.get(key) |
returns value at that key |
dict.setdefault(key) |
either returns value of the key or creates the given key if not previously existent |
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