[C012] Python-Basic tutorial (III)

Chapter 5 conditions, loops, and other statements

More information about print and import

>>> print 'Age:',42Age: 42>>> print 'Age;';42Age;42>>> name = 'Gumby'>>> salutation = 'Mr'>>> greeting = 'Hello,'>>> print greeting,salutation,nameHello, Mr Gumby

>>> import math as foobar>>> foobar.sqrt(4)2.0>>> from math import sqrt as foobar>>> foobar(4)2.0

Assign value magic

>>> x,y,z = 1,2,3>>> print x,y,z1 2 3>>> x,y = y,x>>> print x,y,z2 1 3>>> values = 1,2,3>>> values(1, 2, 3)>>> x,y,z = values>>> x1>>> y2>>> >>> scoundrel = {'name':'Robin','girlfriend':'Marion'}>>> key,value = scoundrel.popitem()>>> key'girlfriend'>>> value'Marion'

Chained assignment

>>> x = y = 1>>> x1>>> y1>>> 

Incremental assignment

>>> x = 2>>> x += 1>>> x3>>> x *= 4>>> x12>>> fnord = 'foo'>>> fnord += 'bar'>>> fnord'foobar'>>> fnord *=2>>> fnord'foobarfoobar'>>> 

Statement block: ease of play

Condition and Condition Statement

>>> TrueTrue>>> FalseFalse>>> True == 1True>>> False == 0True>>> True + False + 4344>>> >>> bool('I think, therefore I am')True>>> bool(42)True>>> bool('')False>>> bool("")False>>> bool(0)False>>> bool([])False>>> bool({})False>>> bool(False)False>>> bool(None)False>>> 

Conditional execution and if statements (if, elif, else)

name = raw_input('What is your name?')if name.endswith('Gumby'): print 'Hello, Mr.Gumby'

num = input('Enter a number:')if num > 0:    print 'The number is positive'elif num < 0:    print 'The number is negative'else:    print 'The number is zero'

Nested code block

name = raw_input('What is your name?')if name.endswith('Gumby'):    if name.startswith('Mr'):        print 'Hello, Mr.Gumby'    elif name.startswith('Mrs'):        print 'Hello, Mrs.Gumby'    else:        print 'Hello, Gumby'else:    print 'Hello, stranger'

More complex conditions

Comparison Operators

>>> 'foo' == "foo"True>>> 'foo' == 'bar'False>>> x = y = [1,2,3]>>> z = [1,2,3]>>> x == yTrue>>> x is yTrue>>> x is zFalse>>> x = z>>> x ==zTrue>>> >>> x = [1,2,3]>>> y = [2,4]>>> x is not yTrue>>> del x[2]>>> x[1, 2]>>> y[1] = 1>>> y.reverse()>>> y[1, 2]>>> x == yTrue>>> x is yFalse

name = raw_input('What is your name?')if 's' in name:    print 'Your name contains the letter "s".'else:    print 'Your name doesn\'t contain the letter "s".'

>>> "alpha" < "beta"True>>> 'FnOrD'.lower() == 'Fnord'.lower()True>>> [1,2] < [2,1]True>>> [2,[1,4]] < [2,[1,5]]True

number = input('Enter a number between 1 and 10:')if number <= 10 and number >= 1:    print 'Great'else:    print 'Wrong'

Assertions

Loop

While Loop

x = 1while x <= 1000:    print x    x += 1

name = ''while not name:    name = raw_input('Please enter your name?')print 'Hello, %s!' % name

For Loop

words = ['this','is','an','ex','parrot']for word in words:    print word

numbers = [0,1,2,3,4,5,6,7,8,9]for number in numbers:    print number

>>> range(0,10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>>> range(10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>>> 

d = {'x':1,'y':2,'z':3}for key in d:    print key, 'corresponds to', d[key]    

y corresponds to 2x corresponds to 1z corresponds to 3

Some iteration tools

names = ['anne','beth','george','damon']ages = [12,45,32,102]for i in range(len(names)):    print names[i],'is',ages[i],'year old'

Flip and sort Iteration

>>> sorted([4,3,6,8,3])[3, 3, 4, 6, 8]>>> sorted('Hello, world!')[' ', '!', ',', 'H', 'd', 'e', 'l', 'l', 'l', 'o', 'o', 'r', 'w']

Skip Loop

Break

from math import sqrtfor n in range(99,0,-1):    root = sqrt(n)    if root == int(root):        print n        break

>>> range(0,10,2)[0, 2, 4, 6, 8]>>> 

Continue

While True/break

word = 'dummy'while word:    word = raw_input('Please enter a word:')    print 'The word was ' + word

word = raw_input('Please enter a word:')while word:    print 'The word was ' + word    word = raw_input('Please enter a word:')

while True:    word = raw_input('Please enter a word:')    if not word:break    print 'The word was ' + word

from math import sqrtfor n in range(92,99,1):    root = sqrt(n)    if root == int(root):        print n        break    else:        print "Didn't find it!",n

Didn't find it! 92Didn't find it! 93Didn't find it! 94Didn't find it! 95Didn't find it! 96Didn't find it! 97Didn't find it! 98

from math import sqrtfor n in range(99,89,-1):    root = sqrt(n)    if root == int(root):        print n        break    else:        print "Didn't find it!",n

Didn't find it! 99Didn't find it! 98Didn't find it! 97Didn't find it! 96Didn't find it! 95Didn't find it! 94Didn't find it! 93Didn't find it! 92Didn't find it! 91Didn't find it! 90

The range function does not include the following boundary ~
List derivation-lightweight Loop

>>> [x*x for x in range(10)][0, 1, 4, 9, 16, 25, 36, 49, 64, 81]>>> [x*x for x in range(10) if x % 3 == 0][0, 9, 36, 81]>>> [(x,y) for x in range(3) for y in range(3)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

result = []for x in range(3):    for y in range(3):        result.append((x,y))print result

>>> girls = ['alice','bernice','clarice']>>> boys = ['chris','arnold','bob']>>> [b+'+'+g for b in boys for g in girls if b[0] == g[0]]['chris+clarice', 'arnold+alice', 'bob+bernice']

Comparison (Python judges the statement relationship based on Indentation)

result = []for x in range(3):    for y in range(3):        result.append((x,y))print result

[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

result = []for x in range(3):    for y in range(3):        result.append((x,y))        print result

[(0, 0)][(0, 0), (0, 1)][(0, 0), (0, 1), (0, 2)][(0, 0), (0, 1), (0, 2), (1, 0)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1)][(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

 

Chapter 6 Abstraction

fibs = [0,1]num = input('How many Fibonacci numbers do you want?')for i in range(num-2):    fibs.append(fibs[-2] + fibs[-1])print fibs

How many Fibonacci numbers do you want?10[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Function

def hello(name):    return 'Hello,'+name+'!'

>>> print hello('world')Hello,world!>>> print hello('Gumby')Hello,Gumby!

def fibs(num):    result = [0,1]    for i in range(num-2):        result.append(result[-2]+result[-1])    return result

>>> fibs(10)[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]>>> fibs(10)[2]1

Record Function

def square(x):    'Calculates the square of the number x.'    return x*x

def square(x):    'Calculates the square of the number x.'    return x*x

Functions that are not real functions

def test():    print 'This is printed'    return    print 'This is not'

>>> x = test()This is printed>>> x>>> print xNone>>> 

Parameter magic

>>> def change(n):n = 'Alex'>>> name = 'mr'>>> change(name)>>> name'mr'

>>> def change(n):n[0] = 'Mr. Gumby'>>> names = ['Mrs. Entity','Mrs. Thing']>>> change(names)>>> names['Mr. Gumby', 'Mrs. Thing']>>> >>> names = ['Mrs. Entity','Mrs. Thing']>>> n = names>>> n[0] = 'Mr. Gumby'>>> names['Mr. Gumby', 'Mrs. Thing']>>> 

>>> names = ['Mrs. Entity','Mrs. Thing']>>> n = names>>> n is namesTrue>>> n == namesTrue>>> n = names[:]>>> n is namesFalse>>> n == namesTrue

Is True when pointing to the same object; otherwise, it is False.