Notes of Py for informatics

Chapter 2 Var, exp and statements

2.1 Type Conversion

sval = '123'type(sval)ival = int(sval)PRint ival+1
2.2 User Input
nam = raw_input('Who are you?') # the screen shows 'Who are you?', and the input value is saved in namprint 'Welcome', nam# if we want to read a number from the user. We must convert it from a string to a number using a type conversion functioninp = raw_input('Europe Floor?')print 'American Floor:', int(inp)+1
Chapter 3 Conditional execution
3.1 One Way Decision & Indentation
if x == 6:   print 'is 6'   print 'is still 6'Turn off tabs: If you mix tabs and spaces, you may get 'indentation errors'.
3.2 Two Way Decisions
if x > 2:   print 'Bigger'else:   print 'Smaller'
3.3 Muti-way
if x < 2:   print 'small'elif x < 10:     #elif -> else if   print 'medium'else:   print 'large'print 'all done'3.4 try / except structure
You surround a dangerous section of code with try and except.
If the code in the try works - the except is skipped.
If the code in the except works - the try is skipped.
astr = 'Hello Bob'try:    istr = int(astr)  # if the input is 'qwe', then the 'try' can not be executedexcept:    istr = -1comment: have a try. It's totally different from the if/else. It has a network. 'a blow up' stuff.example:
rawstr = raw_input('Enter a number:')try:    ival =int(rawstr)except:    ival = -1if ival > 0:    print 'Nice work'else:    print 'Not a number'3.5 if not
Chapter 4 Functions
two types of functions:
build-in functions
functions that we define ourselves and then use
no arguments:
def print_lyrics():    print "I sleep all night and I work all day"
arguments:
def greet(lang):    if lang == 'es':       print 'Hola'....
return value:
def greet(lang):    if lang == 'es':       return 'Hola'....print greet('es')
multiple parameters / Arguments:
def addtwo(a,b):    added = a + b    return addedvoid function
Chapter 5 Loops and Iteration
5.1 breaking out of a loop
while True:    line = raw_input('>')    if line == 'done':        break    print lineprint 'Done!'break: end the current loop and jump to the statement immediately following the loop
5.2 continue
the continue statement ends the current iteration andjumps to the top of the loop and starts the next iteration
5.3 Indefinte Loops & definite Loops
Indefinte Loops: While loops are called "indefinite loops" because they keep going until a logical condition becomesFalse.  while
Defintie Loops: finite list of things to do. For
for i in [5,4,3,2,1]:    print i print 'Blastoff!'
friends = ['Joe','Sally','Mike']for friend in friends:    print 'Happy New Year:', friendprint 'Done!'
5.4 Loop Idioms
smallest = None...if smallest is None: # trigger the first iteration  #if value is None/True/False is ok # if value is 4     ..elif value < smallest:    smallest = valueChapter 6 Strings
both quotes: 'Hello' and "Hello" are ok
'+' means concatenate
6.1 Look inside strings
fruit = 'banana'letter = fruit[1]print letter
output:
a
you will get a python error if you attempt to indexbeyond the end of a string
len(fruit)
# approach1
index = 0while index < len(fruit):    ...    index = index + 1
# approach2
for letter in fruit:     ...# I love the 'in'
6.2 Slicing Strings: colon Operator
s = 'Monty Python'print s[0:4]  #starts from1, up to but not including 4
output:
Mont
print s[:2]print s[8:]print s[:]
output:
MothonMonty Python
6.3 Using in as an operator
print 'nan' in fruit
Output:
True
Word < 'banana'word == 'banana'#comparison6.4 String Library
xxx.function()
These functions do not modify the original string, instead theyreturn a new string that has been altered.  =
greet = 'Hello Bob'zap = greet.lower()print zap
output:
hello bob
print dir(greet)['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '_formatter_field_name_split', '_formatter_parser', 'capitalize', 'center', 'count', 'decode', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'index', 'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']
Find
pos = fruit.find('na')print posoutput:
2
pos = fruit.find('a',pos+1) #starts from the position of the next characterfind() finds the first occurance of the substring
if the substring is not found, find() returns-1
Replace
nstr = greet.replace('Bob','Jane')print nstrHello JaneStripping Whitespace
Sometimes we want to take a string and remove whitespace at the beginning and/or end
lstrip() #left
rstrip() #right
strip() #all
greet = '     Hello Bob'print greet.strip()     Hello BobHello Bob
Prefixes
print greet.startswith(' ')True