CS110_L2_Notes.py

Python3 Editor

# About variable names:
# Variable names can only contain letters, numbers, and underscores
# Variable names CANNOT start with a digit
# No spaces are allowed in variable names. Use underscore instead. Example: first_name

# In Python3 Editor, there are 4 main colors used to highlight the code:
# yellow = reserved keyword or number literal
# blue = built-in function, type, or True/False literal
# green = string
# orange = library names and user-defined variables/functions

# Your variable names should be ORANGE. If the variable name is any other color, change it.

#---------------------------------------------------
#    NONE
#---------------------------------------------------
# if you do not know what value to assign a variable but want to declare the variable anyway, set its value to None
# this means the variable has no type (or is NoneType) and no operations can be done on it
a = None

#---------------------------------------------------------------------------
#    NUMBER TYPES: Integer and Float
#---------------------------------------------------------------------------

# integers = int type, whole numbers positive, negative, 0
# integer numbers written plainly in the code are considered "integer literals"
a = -12
b = 104
c = 0
# with print statements, can use the comma to separate things being printed in the same statement
# this allows you to have a string and an integer in the same statement (instead of using a plus sign)
# the comma will also add a space before the next part that follows it prints
print("a =", a)
print("b =", b)
print("c =", c)

# floats = float type, numbers with decimals, can be positive or negative
# floating-point numbers written plainly in the code are considered "float literals"
d = 0.5
e = -1.67
print("d =", d)
print("e =", e)

# number operators
f = a + b    # addition
g = a - b    # subtraction
h = a * b    # multiplication
i = a / b    # division
j = a // b   # floor division (drops everything after the decimal, like rounding down)
k = b % 5    # mod - returns the remainder after calculating 13/5
l = 2 ** 3   # exponent - returns the value of 2 ^ 3, a ** b = a^b 
print(a, "+", b, "=", f)
print(a, "-", b, "=", g)
print(a, "*", b, "=", h)
print(a, "/", b, "=", i)
print(a, "//", b, "=", j)
print(b, "% 5 =", k)
print("2**3 =", l)

# there are also complex operators that are the same as Java, but there are no increment/decrement operators
a += 5    # same as: a = a + 5
print("a += 5 =", a)
a -= 10   # same as: a = a - 10
print("a -= 10 =", a)
a *= 2    # same as: a = a * 2
print("a *= 2 =", a)
a /= 3    # same as: a = a / 3
print("a /= 3 =", a)
a //= 3   # same as: a = a // 3
print("a //= 3 =", a)
a %= 3    # same as: a = a % 3
print("a %= 3 =", a)

# can have a larger math expression on the right side of these operators as well
a = 3
a += (10 + 5)/2
print("a =", a)
# this is calculated with the following steps:
# 1: (10 + 5) = 15
# 2: 15/2 = 7.5
# 3: a + 7.5 = 3 + 7.5 = 10.5
# 4: store 10.5 back in variable a

# NOTE: all integer operators can be applied to floats since both are Number types
print()

#-------------------------------------------------------------------------------
#        BOOLEANS
#-------------------------------------------------------------------------------

# these are simple variables that can contain only True/False
# The only 2 Boolean literal values are True and False
isRaining = True
isSunny = False
print("isRaining =", isRaining)
print("isSunny =", isSunny)
print()

# we'll go over these in more depth when we discuss conditions

#-------------------------------------------------------------------------------
#        STRINGS
#-------------------------------------------------------------------------------

# Strings are lists of characters contained within quotes. Can use single-quotes ' or double-quotes ".
# any string declared within quotes directly in the code is called a "String literal".
s = "Hello"

# there are 2 main operators for strings
s2 = s + "World"    # concatenation: adds the string on the right to the end of the string on the left
print(s2)
# Notice when the print statement above runs, it prints out "HelloWorld" with no space? This is because it concatenates the
# string literals exactly as written. Since neither string contains a space that would separate the words, there is no space
# in the output.

# s2 = s + 5    # this line will cause an error to occur since integers cannot be concatenated to strings

s3 = s * 5        # concatenates the value of the string s to the end of itself 5 times
                  # this operator is commutative. s * 5 = 5 * s
print(s3)

# Both of these operators have compound assignment operators with them.
s += "World"    # will concatenate "World" to the end of "Hello"
s *= 5          # will concatenate the current value of s to itself 5 times

# same as integers, can do multiple operations on the right before the operation on the left occurs
s = "Hello"
s += " World" + "!!!"
print(s)
s *= 1 + 2    # first calculates that the right side is 3, then multiplies s by 3
print(s)
print()

#------------------------------------------------------------------------------------------------
# String indices
#------------------------------------------------------------------------------------------------

# Again, strings are lists of characters. Each character has an index.
# The first character in the string has an index of 0.
# The length of the string is the number of characters in the string.
s = "Hello World"
print("There are", len(s), "characters in the string.")

# You can access a specific character in the string using its index. 
# Write the name of the string variable, followed by the index in square brackets
print(s[0])    # s[0] = 'H'
print(s[6])    # s[6] = 'W'
# Negative indices start counting at the end. The last character is at index -1.
print(s[-1])    # s[-1] = 'd'
print(s[-3])    # s[-3] = 'r'

# if you try to access an index that is greater than or equal to the length of the string, you will get an IndexError
# print(s[15])    # len(s) = 11 and 15 > 11, so this will cause an IndexError

# Use the index() function to find the index of a specific character or set of characters.
# It will return the index of the FIRST occurrence of the character/substring in the string.
print(s.index('o'))    # prints out 4, since the letter 'o' is first found at index 4 and ignores the one found at index 7
print(s.index('lo'))   # prints out 3, since 'lo' starts at index 3

# be careful using this, since it will cause a ValueError when that character or substring is NOT in the string
# print(s.index('z'))

# Also using the indices, you can create substrings.
# s[start: end]
#    s = string variable name
#    start = index of first character in substring
#    end = index of last character in substring + 1
# NOTE: the character at the end index IS NOT INCLUDED. In the example below, if it was included, the last character would be 'o'.
print(s[3: 7]) # prints out 'lo W'.

# You can leave out the start value if you want it to start at 0.
print(s[:3])    # s[:3] = 'Hel'
# Leaving out the end value will end it at the last character of the string
print(s[7:])    # s[7:] = 'orld'
# You can also use negative indices here as well.

# REVERSE A STRING
# You can reverse a string by simply writing:
print(s[::-1])
# This will reverse the entire string. The order of this slice is
# s[start:end:skip]
# skip can be any value, positive or negative.
# What prints out when you run this?
print(s[1:10:2])
print()

#-----------------------------------------------------------------------------
#        String Functions
#-----------------------------------------------------------------------------

s = "Hello"
s_upper = s.upper()    # sets all characters to uppercase letters, s_upper = "HELLO"
s_lower = s.lower()    # sets all characters to lowercase letters, s_lower = "hello"

inp = input("Enter something: ")
print("Is alpha?", inp.isalpha())    # returns True if all characters in string are letters
print("Is digit?", inp.isdigit())    # returns True if all characters in string are digits
print("Is alnum?", inp.isalnum())    # returns True if all characters in string are letters OR digits
print("Is space?", inp.isspace())    # returns True if all characters in string are spaces
print("Is upper?", inp.isupper())    # returns True if all characters in string are uppercase
print("Is lower?", inp.islower())    # returns True if all characters in string are lowercase

s2 = "   Howdy all   "
print(s2)
print(s2.strip())

s3 = "5, 4, 7, 12, 6"
print(s3)
print(s3.split(", "))
print()

#----------------------------------------------------------------------------------
#        INPUT
#----------------------------------------------------------------------------------

x = input("Enter a number: ")
print(x, type(x))

# Remember that input() ALWAYS returns a string. If you want the value entered to be an int, you need to convert it to an int.
# Use the int() function to convert it
x = int(x)
print(x, type(x))

# If you want it to be a float, you need to use the float() function to convert it
x = float(x)
print(x, type(x))
print()
# NOTE: if there is a non-numerical character in the string and you attempt to change it to an integer, it will cause an ERROR.

#----------------------------------------------------------------------------------
#         Formatting strings
#----------------------------------------------------------------------------------

name = "Bob"
name2 = "Mary"
age = 34
num_kids = 2
pi = 3.1415926535

# Formatting using the deprecated method
# %s = string
# %d = integer
# %f = float
#    can specify number of characters to include. Example %.2f will print out only 2 digits after the decimal
print("Hi %s! My name is %s!" % (name, name2))
print("I am %d years old. I have %03d children." % (age, num_kids))
print("Pi with the first 5 values after the decimal point are %.5f." % (pi))
print()

# Can also format with the .format() function. The number in the curly brackets is the index of the value in the list in format's parentheses
print("Hi {0}! My name is {1}! I am {2} years old. I have {3} children.".format(name, name2, age, num_kids))
# can still specify things like number of digits after decimal
print("Pi with 5 digits after the decimal is {0:.5f}.".format(pi))

( around 211 lines Python3 code )

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Mentor Sabrina

Created on 2020-10-19T17:20:58Z

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