PyLecture4 -Python Basics2-

 Write following python script with your text
editor.
 Then, run ‘python your_script.py’
 If you got ‘Hello World’, go on to the next
slide.
def main():
print ‘Hello World’
if __name__ == ‘__main__’:
main()

 Lists – compound data type
l = [1,2,3,5] # defines a list contains 1, 2, 3, 5
print l[0] # getting a value
l[0] = 6 # setting a value
l.append(7) # adding to list
print l # will print ‘[6, 2, 3, 5, 7]’
l.extend([8, 9, 10]) # connecting another list
print l # will print ‘[6, 2, 3, 5, 7, 8, 9, 10]’

print range(3) # stops at 2
print range(1, 3) # starts at 1 and stops at 2
print range(0, 5, 2) # start: 1 stop: 4 and steps by 2

 Create following lists with range function.
1. [0, 1, 2, 3, 4]
2. [3, 4, 5, 6, 7]
3. [3, 6, 9, 12, 15, 18]
4. [2, 4, 6, 8, 12, 16, 20] hint: use + operator

 Answer:
1. range(5)
2. range(3, 8)
3. range(3, 21, 3)
4. range(2, 10, 2) + range(12, 24, 4)

 Dictionaries – compound data type
 Found in other languages as “map”,
“associative memories”, or “associative arrays”
 Lists vs Dictionaries
› You can use only integer number as index on lists
Like a[0], a[-1]
› You can use integer numbers and strings as key on
dictionaries(if its value exists)
Like d[0], d[‘foo’], d[‘bar’]

 Creating a dictionary
tel = {‘John’:8000, ‘Jane’:8001, ‘Joe’:8002}
 Adding key and value
tel[‘Joan’] = 8003
 Getting value from key
tel[‘Jane’]
 Setting value from key
tel[‘Joe’] = 0004
 Removing value from key
del tel[‘John’]
 Getting key list of a dictionary
tel.keys()

 Crate a dictionary from two lists
names = [‘John’, ‘Jane’, ‘Joe’]
tel = [8000, 8001, 8002]
tel = dict(zip(names, tel))

 Create a dictionary that
› Keys are [0, 1, …, 10]
› Values are (key) * 2
› i.e. d[0]=0, d[1]=2, d[2]=4, …, d[10]=20

 Answer:
k = range(11)
v = range(0, 11*2, 2)
d = dict(zip(k, v))

 Logical Operator
 if statements
 for statements

 Compares two values
print 1 < 2
print 1 > 2
print 1 == 2
print 1 <= 2
print 1 >= 2
 And, Or, and Not
print 1 < 2 and 1 == 2
print 1 < 2 or 1 > 2
print not 1 == 2

x = int(raw_input(‘value of x: ’))
if x < 0:
print(‘x is negative’) # must indent
elif x > 0:
print(‘x is positive’)
else:
print(‘x is not positive neither negative’)
print(‘i.e. x is zero’)

For integer variable named ‘x’,
› Print ‘fizz’ if x can be divided by 3
› Print ‘buzz’ if x can be divided by 5
(e.g. if x is 10, print ‘buzz’, x is 15, print ‘fizz buzz’)
(hint: use % operator(e.g. 10 % 3 = 1, 14 % 5 = 4))

x = int(raw_input('x value:'))
if x % 3 == 0:
print 'fizz'
if x % 5 == 0:
print 'buzz'

n = int(raw_input(‘list length: ’))
l = []
for i in range(n): # for each number in (0…n-1)
l.append(raw_input(‘%dth word: ’ % i))
for w in l: # for each word in your list
print w

Create a list only contains
› Multiple of 3 or 5 (3, 5, 6, 9…)
› Between 1 and 20
i.e. [3, 5, 6, 9, 10, 12, 15, 18, 20]

l = []
for i in range(1, 21):
if i % 3 == 0 or i % 5 == 0:
l.append(i)
print l

 Defining a function
def add(a, b):
return a + b
 Calling the function
print add(10, 2)

 Recursive call
def factorial(n):
if n <= 0:
return 1
else:
return n * factorial(n-1)

 Find the 10-th Fibonacci sequence
› Fibonacci sequence is
 0, 1, 1, 2, 3, 5, 8, …
 The next number is found by adding up the two
numbers before it(e.g. 5th number 3 = 2 + 1).

def fib(n):
if n <= 1:
return 0
elif n == 2:
return 1
else:
return fib(n-1) + fib(n-2)
print fib(10)

PyLecture4 -Python Basics2-

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