Random Python notes

29 Apr 2020

• = and == are operators. = is used to asign a value to a variable. == is used to check if two values are equal. The result of the == operator is a boolean and this can be used write crisp code.

def is_divisible(x, y):
  return x % y == 0

# A more explicit version of the same function
def is_divisible(x, y):
  if x % y == 0:
    return True
  else:
    return False

• Void functions are functions do not return any value. More precisely their return value is None. A function can also return None if the program execution ends without hitting a return statement. The following function returns None when x = 0. This function is incorrect because it can not find the absolute value of 0.

def absolute(x):
  if x < 0:
    return -x
  if x > 0:
    return x 

• Types of Division: Three types of division can be performed in Python.
  • Conventional Division: Returns a floating point number, always. Operator: /
  • Floor division: Returns the integer part of the division leaving out the decimal. The value returned is of either int or float. Operator: //
  • Modulus operation: Returns the reminder. The value returned is of either int or float type. Operator: %

• User input received via input() function is of type of str, by default. If required, it can be converted into an integer or a float by using the function int() or float(), respectively.

• Global variables can be accessed from any function. They belong to a special scope called __main__. They are not destropyed after the execution of the function.

• Local variables are created when the function is called and disappear after the function is executed. They are not accessible from outside of the function in which they are first used.

• Parameters and Arguments: Parameters are the variables names used in the function definition. Arguments are the actual values passed on to the parameters while executing the function.

In the example giveb below, x is the parameter, and 2 is the argument passed on to it.

def foo(x):
  return x**2

print(foo(x=6))

• if-else statement If more than one condition is true, then only the first true branch is run. The following code returns x is divisible by 9 even though the the last condition is also true. This is because, once a condition is met, the remaining conditions are not checked.

x = 999

if (x % 2 == 0):
  print("x is even")
elif (x % 9 == 0):
  print("x is divisible by 9")
elif (x % 3 == 0):
  print("x is divisible by 3")

• Recursive Function: A function that calls itself. Example:

  def countdown(n):
  if n <= 0:
    print("Blastofff")
  else:
    print(n)
    countdown(n-1)
  

• Infinite Recursion This happens when it becomes impossible to hit the return statement. In the example given below, for n = 2.5 (or any number with decimal part), the program doesn’t meet the base case n == 0 and recurses forever. Apparently, the factorials of such numbers can be computed using Gamma functions, and I don’t know how.

def factorial(n):
  if n == 0:
    return 1
  else:
    recurse = factorial(n-1)
    result = n * recurse
    return result 

• isinstance() function takes two arguments: a value and a type and tells us if the given value is of the given type or not. The result of isinstance() is a boolean, meaning that the function returns either True or False.

isinstance(4, int) #returns True
isinstance(4.0, int) #returns False
isinstance('4.0', float) #returns False

The infinite resursion seen in the factorial(n) function can be handled better by using the isinstance() function.

def factorial(n):
  if not isinstance(n, int):
    print("Factorial is defined only for integers")
    return None
  elif n < 0:
    print("Factorial is not defined for negative integers")
    return None
  elif n == 0:
    return 1
  else:
    recurse = factorial(n-1)
    result = n * recurse
    return result

• Return stament exits the function.
  1. It need not always be at the end of the function
  2. It need not be followed by a variable name.

     def print_n(text, n):
       if n <= 0:
        return
       else:
        print(text)
        print(text, n-1)
     

  3. A function can have more than one return statement.

     def compare(x, y):
       if x < y:
        return 1
       if x == y:
        return 0
       else:
        return -1
     

• Boolean expression: An expression that is either true or false. Example: 5 == 5 returns True, 5 == 6 returns False.

• True and False are special values that belong to the type bool. They are not strings. Try type(True) and type(False)

• int() takes any value and returns an integer.
  • If a decimal number is passed as an argument, the decimal part is chopped off and only the integer part is retained.
  • If a whole number is passed as a string, then the number will be converted into an integer.
  • If a decimal number is passed as a string, an error message appears.
  • If a word is passed, an error message appears.

  Examples:

  • int(3.14) returns 3
  • int("3") returns 3
  • int("3.14") returns an error message
  • int("Hi") returns an error message

• float() takes any value and returns a decimal number.
  • If an integer is passed as an argument, the decimal part will be added.
  • If a number (both integer and decimal) is passed as a string, then the corresponding floating point number is returned.
  • If a word is passed, an error message is returned.

  Examples:

  • float(3) returns 3.0
  • float("3") returns 3.0
  • float("3.14") returns 3.14
  • float("Hi") returns an error message

• Split a number into integer and decimal parts

from math import modf

d, i = modf(123.456) #modf() returns a tuple
print(d) # Decimal part
print(i) # Integer part

• Printing an enumerated list of array elements

  for i in range(len(indicators)):
    print(str(i+1)+". "+indicators[i])
  

  The trick is to convert the number (i+1) into a string so that it can be concatenated with the text present in the array indicators.

Reference

1. Allen Downey. Think Python. How to Think Like a Computer Scientist. Green Tea Press, Needham, Massachusetts