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 nam
print '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 function
inp = 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 executed
except:
    istr = -1

comment: 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 = -1

if 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 added

void function

Chapter 5 Loops and Iteration

5.1 breaking out of a loop

while True:
    line = raw_input('>')
    if line == 'done':
        break
    print line
print '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:', friend

print '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 = value

Chapter 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 = 0
while 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:

Mo
thon
Monty Python

6.3 Using in as an operator

print 'nan' in fruit

Output:

True

word < 'banana'
word == 'banana'
#comparison

6.4 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 pos

output:

2

pos = fruit.find('a',pos+1) #starts from the position of the next character

find() finds the first occurance of the substring

if the substring is not found, find() returns-1

Replace

nstr = greet.replace('Bob','Jane')
print nstr
Hello Jane

Stripping 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 Bob
Hello Bob

Prefixes

print greet.startswith(' ')
True