Introduction to Computer Science and Programming Using Python(2)

 
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Week two: Simple Programs

  1. Slice strings using[start:stop:step]

    s = 'abcdefgh'
s[::-1] #evaluates to 'hgfedcba'
s[3:6] #evaluates to 'def'
s[-1] #evaluates to 'h'

  2. Floating Point Arithmetic: Issues and Limitations

    No matter how many base 2 digits you’re willing to use, the decimal value 0.1 cannot be represented exactly as a base 2 fraction. In base 2, 1/10 is the infinitely repeating fraction

    0.0001100110011001100110011001100110011001100110011...

    t’s important to realize that this is, in a real sense, an illusion: you’re simply rounding the display of the true machine value.

    One illusion may beget another. For example, since 0.1 is not exactly 1/10, summing three values of 0.1 may not yield exactly 0.3, either:

    >>>

    >>> .1 + .1 + .1 == .3
False

    Also, since the 0.1 cannot get any closer to the exact value of 1/10 and 0.3 cannot get any closer to the exact value of 3/10, then pre-rounding with round() function cannot help:

    >>>

    >>> round(.1, 1) + round(.1, 1) + round(.1, 1) == round(.3, 1)
False

    Though the numbers cannot be made closer to their intended exact values, the round()function can be useful for post-rounding so that results with inexact values become comparable to one another:

    >>>

    >>> round(.1 + .1 + .1, 10) == round(.3, 10)
True

    References: https://docs.python.org/3/tutorial/floatingpoint.html

  3. Scope Example

    1. def f(y):
    x = 1
    x += 1
    print(x)
      
x = 5
f(x) # f(x) = 2
print(x) # 5
      

    2. def g(y):
    print(x)
    print(x+1)
      
x = 5
g(x) # Print(5), Print(6)
print(x) # 5
      

    3. def h(y):
    x = x + 1
      
x = 5
h(x) #x = x + 1
UnboundLocalError: local variable 'x' referenced before assignment
      
print(x)
      
# When you do x = 'abc' it simply binds 'abc' to the name x and treat x as a local variable within the local scope, no looking up needed.
      
#But when you do x = x + 1 like the h(y) example in the video, the right hand side is evaluated first, it starts looking for x in the current scope, and it finds a binding of x in the current scope which is x = x + 1, so x is a local variable that is referenced before it is bound, hence the error.
      
#When you call print(x)in the g(y) example in the video, it looks for x in the current scope, nothing is found, so it looks to x from the global scope and prints it. No rule is violated.
      
#When you call print(x)in the example of f() above, it looks for x in the current scope, it actually finds the binding x = 'abc', so x is a local variable that is referenced before it is bound, hence the error.

  4. Aspects of LANGUAGES

    numbers,strings,simple operations.>

    • syntax

      • English:”Cat dog boy” > not syntactically valid

        ​ “Cat hugs boy” > syntactically valid

      • Programming language: “hi” * 5 > not syntactically valid

        3.2 * 5 > syntactically valid

  5. where things go wrong

    • syntactic errors
    • static semantic errors
    • no semantic errors but different meaning than what programmer intended

  6. Objects

    • Programs manipulate data objects

    • Objects have a ==type== that defines the kinds of things programs can do to them.

    • objects are

      scalar (cannot be subdivided) 标量（有数值，无方向）

      non-scalar(have internal structure that can be accessed)

    • Scalar objects

      • int — integers整数型:5
      • float — real numbers浮点型: 3.27
      • bool— BOOLEAN VALUES布尔型: True and False
      • NoneType — special and has one value: None

  7. ‘for’ loop

    >>> for n in range(5):
	print(n)
   
   	
0
1
2
3
4

  8. Iteration & Recursion

    • a way to design solutions to problems by divide-and-conquer or decrease-and-conquer.

    • in programming, goal is to NOT have infinite recursion

      • must have 1 or more base cases that are easy to solve
      • must solve the same problem on some other input with the goal of simplifying the larger problem input.
    • Iteration
      def mult_iter(a,b):
    result = 0
    while b > 0:
        result += a
        b = b -1
    return result
    • Recursion
      def mult(a,b):
    if b <= 1:
        return a
    else:
        return a + mult(a,b-1)

  9. Modules

a module is a .py file containing a collection Python definitions and statements

pi = 3.14159
def area(radius):
    return pi*(radius**2)
def circumference(radius):
    return 2*pi*radius

import circle
pi = 3

print(pi)
print(circle.pi)
print(circle.area(3))

print(circle.circumference(3))

from circle import *
print(pi)
print(area(3))

  1. Files

Python provides an operating-system independent means to access fies, using a file handle

nameHandle = open('kids', 'w') #'kids' is the file name; 'w' means writing.

Files Write Example

nameHandle = open('kids.doc','w')
for i in range(2):
    name = input ('Enter name:')
    nameHandle.write(name + '\\')
nameHandle.close()

Files Read Example

nameHandle = open('kids.py', 'r')
for line in nameHandle:
    print(line)
nameHandle.close()