5. Data Structure

5.1 More on Lists

fruits =['orange','apple','pear','banana','kiwi','apple','banana']
fruits.count('apple')
2
print(fruits)
['orange', 'apple', 'pear', 'banana', 'kiwi', 'apple', 'banana']
fruits.index('orange')
0
fruits.index('banana',4)
6
fruits.reverse()
fruits
['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange']
fruits.append('grepe')
fruits
['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange', 'grepe']
fruits.sort()
fruits
['apple', 'apple', 'banana', 'banana', 'grepe', 'kiwi', 'orange', 'pear']
fruits.pop()
'pear'
fruits
['apple', 'apple', 'banana', 'banana', 'grepe', 'kiwi', 'orange']

5.1.1. Using Lists as Stacks

stack = [3,4,5]
stack.append(6)
stack.append(7)
stack
[3, 4, 5, 6, 7]
stack.pop()
7
stack
[3, 4, 5, 6]

5.1.2. Using Lists as Queues

from collections import deque
queue = deque(["Eric","John","Michael"])
queue.append("Terry") 
queue.append("Graham")
queue.popleft()
'Eric'
queue
deque(['John', 'Michael', 'Terry', 'Graham'])

5.1.3. List Comprehensions

squares = []
for x in range(10):
    squares.append(x**2)
    
squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
squares = list(map(lambda x : x**2, range(10)))
squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
squares = list(map(lambda x : x**2, range(11)))
squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
squares = [ x**2 for x in range(11)]
squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
[(x,y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
combs = []
for x in [1,2,3]:
    for y in [3,1,4]:
        if x != y:
            combs.append((x,y))
            
combs
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

from math import pi
[str(round(pi,i)) for i in range(1,6)]
['3.1', '3.14', '3.142', '3.1416', '3.14159']

5.1.4. Nested List Comprehensions

matrix2 = [ [1,2,3,4],
            
            [5,6,7,8],
            [9,10,11,12],
            ]
[[row[i] for row in matrix2] for i in  range(4)]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

transposed = []
for i in range(4):
    transposed.append([row[i] for row in matrix2])
    
transposed
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

5.3 Tuples and Sequences

t = 12344, 543,'hello'
t[0]
12344
t
(12344, 543, 'hello')
# Tuples may be nested:
u = t, (1, 2,3,5)
u
((12344, 543, 'hello'), (1, 2, 3, 5))
# tuples are immutable:
t[0] = 888
Traceback (most recent call last):
  File "<input>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
# but they can contain mutable objects:
v = ([1,2,4],[4,2,1])
v
([1, 2, 4], [4, 2, 1])

6.Modules

# Fiboonaccci numbers module
def fib(n): # write Fibonacci series up to n
    a, b = 0,1
    while b < n:
        print(b, end =' ')
        a, b= b, a + b
    print()

def fib2(n): # return Fibonacci serries up to n
    result = []
    a, b = 0,1
    while b < n:
        result.append(b)
        a, b = b, a+b
    return result

import fibo
fibo.fib(1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 
fibo.fib2(1000)
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987]
fibo.__name__
'fibo'

6.1. More on Modules

fibo.__name__
'fibo'
fibo.fib2(100)
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
from fibo import fib,fib2
fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 
import fibo import *
 File "<input>", line 1
   import fibo import *
                    ^
SyntaxError: invalid syntax
from fibo import *
fib(20)
1 1 2 3 5 8 13 

6.1.1 Executing modules as scripts

if __name__ == "__main__":
    import sys
    fib(int(sys.argv[1]))

6.2 Standard Modules

import sys
sys.path.append('/ufs/guido/lib/python')