Day 11

Computational Physics (PHYS 202)
Cal Poly, Spring 2015
Brian E. Granger

In class

Go over midterm
- Discuss solutions
- Grade distribution
Coding tips and help
- Difference between return and print.
- Transforming data between different container types.
- How to count things.
- Comments on performance.

Fetch today's material:

  nbgrader fetch phys202-2015 day11
  nbgrader fetch phys202-2015 assignment08

Coding tips and help

Different between `return` and `print`

A function that has no return statement will always return None:



In [ ]:

    
def f(x):
    print(x**2)



In [ ]:

    
a = f(2)



In [ ]:

    
print(a)

If you want a function to do anything useful, you have to return something:



In [4]:

    
def g(x):
    return x**2



In [5]:

    
b = g(2)



In [6]:

    
print(b)

Transforming container types

String can be turned into lists and tuples:



In [11]:

    
import random
alpha = 'abcdefghijklmnopqrstuvwxyz'



In [21]:

    
l = [random.choice(alpha) for i in range(10)]
l









    Out[21]:





['l', 'r', 's', 'o', 'c', 'w', 'a', 'x', 'i', 'u']



In [22]:

    
s = ''.join(l)
s









    Out[22]:





'lrsocwaxiu'



In [25]:

    
for c in s:
    print(c)









    



l
r
s
o
c
w
a
x
i
u



In [27]:

    
[c.upper() for c in s]









    Out[27]:





['L', 'R', 'S', 'O', 'C', 'W', 'A', 'X', 'I', 'U']



In [23]:

    
list(s)









    Out[23]:





['l', 'r', 's', 'o', 'c', 'w', 'a', 'x', 'i', 'u']



In [24]:

    
tuple(s)









    Out[24]:





('l', 'r', 's', 'o', 'c', 'w', 'a', 'x', 'i', 'u')



In [29]:

    
digits = str(1001023039)



In [31]:

    
list(digits)









    Out[31]:





['1', '0', '0', '1', '0', '2', '3', '0', '3', '9']



In [32]:

    
[int(d) for d in digits]









    Out[32]:





[1, 0, 0, 1, 0, 2, 3, 0, 3, 9]

How to count things



In [38]:

    
def random_string(n):
    return ''.join([random.choice(alpha) for i in range(n)])



In [39]:

    
random_string(100)









    Out[39]:





'ckggkxljgihymvcfwnpksgkpdoarugriwbtftuyhjxrotjmxambqrpsjsbbferfdcyvqzjyosxihsyrqdlprkidfljkanwtltgga'



In [46]:

    
def count0(seq):
    counts = {}
    for s in seq:
        counts[s] = seq.count(s)
    return counts



In [65]:

    
rs = random_string(10)
count0(rs), rs









    Out[65]:





({'c': 1, 'd': 1, 'e': 1, 'k': 2, 'n': 1, 'q': 1, 's': 1, 'v': 1, 'w': 1},
 'cwnsqkkedv')



In [52]:

    
%timeit count0(random_string(10000))









    



10 loops, best of 3: 118 ms per loop



In [72]:

    
def count1(seq):
    counts = {}
    for s in seq:
        if s in counts:
            counts[s] += 1
        else:
            counts[s] = 1
    return counts



In [73]:

    
rs = random_string(10)
count1(rs), rs









    Out[73]:





({'b': 1, 'c': 2, 'f': 1, 'l': 1, 'n': 1, 'q': 1, 'v': 1, 'y': 1, 'z': 1},
 'qlcfvcnbyz')



In [74]:

    
%timeit count1(random_string(10000))









    



100 loops, best of 3: 12.3 ms per loop



In [75]:

    
from collections import defaultdict

def count2(seq):
    counts = defaultdict(int)
    for s in seq:
        counts[s] += 1
    return counts



In [76]:

    
rs = random_string(10)
count2(rs), rs









    Out[76]:





(defaultdict(<class 'int'>, {'t': 1, 'j': 1, 'r': 1, 'n': 1, 'v': 3, 'y': 1, 'e': 1, 'p': 1}),
 'pytvenjrvv')



In [77]:

    
%timeit count2(random_string(10000))









    



100 loops, best of 3: 11.9 ms per loop



In [78]:

    
from collections import Counter



In [84]:

    
def count3(seq):
    return dict(Counter(seq))



In [85]:

    
rs = random_string(10)
count3(rs), rs









    Out[85]:





({'b': 1,
  'c': 1,
  'f': 1,
  'h': 1,
  'i': 1,
  'k': 1,
  'p': 1,
  's': 1,
  'v': 1,
  'y': 1},
 'fvhkypbisc')



In [86]:

    
%timeit count3(random_string(10000))









    



100 loops, best of 3: 11.6 ms per loop

Out of class

Complete assignment08 by Wednesday (Day 12).