lesson04.py

# Welcome to the fourth lesson in the Yesselman Group's Python series
# Topics covered: For and While loops
########################################################################################################################
# Part I: The For Loop
# So far, you've learned how to assign data to variables and manipulate these individual variables(conditionally if desired). 
# Likewise, you know how to use lists to store large amounts of data, but how can you manipulate large amounts of data?
# The answer: For Loops. In python, Loops are used to repeat blocks of code a set number of times, with the For loop being
# the most common. Python For Loops are used to access the elements of an "iterable" object (like a list, for instance). Here,
# an iterable is just something that has divisible parts. Consider the below example:

num_list = [1,2,4,6,7,65,5]

# for number in num_list:
    # print(number)

# What does the above code do? How would you change it so that the numbers only print if the number is less than 5? 
# The For loop has the following format and similar indentation rules to if loops:

# for [item] in [iterable_object]:
#     [code to execute]

# Where [item] is a temporary variable that is defined only within the loop, [code to execute] is a block of code indented
# one tab in and [iterable_object] is list-like object. You will see more iterable objects over time, but for now, a good way 
# to test if something is iterable is whether or not you can you [] on it. Calling back to out previous lesson, this means that 
# strings are iterables, since they are "lists" of one letter strings themselves. Given this, what will the following do?

my_string = "this_is_a_string"

# for letter in my_string:
#   print(letter)

# A useful tool for doing something a set number of times is the range() function. The range takes an integer and makes a 
# list from 0 to n-1 i.e. range(4) => [0,1,2,3]. Consider the following code that prints "Hello World" 100 times:

for i in range(100):
  print("Hello World")

# Keep in mind that if the iterable object contains iterable objects, they won't be iterated over automatically. What will happen 
# when you run the code on lines 44/45?
matrix = [
  [1,2,3,4],
  [5,6,7,8]
]
#for row in matrix:
#  print(row)

########################################################################################################################
# Part II: The While Loop
# Unlike the For loop, which iterates over a list or similar object, a While loop executes a block of code as long as a boolean
# statement is true. It has the following structure:
#
# While [Boolean]:
#   [code to execute]
#
# Below is a rewrite of the above "Hello world" For loop. Uncomment it and see what happens.
# hw_count = 0
# while hw_count < 100:
#   print("Hello World")
#   hw_count += 1 # what happens if this is deleted?
#
# often times, you can achieve a repitive process with a While or For Loop, but it is useful to know how to do both. A unique
# ability of the While loop is to do "infinite" looping. Consider the following code. Will it ever stop?
#
# while True:
#   print("Will this loop stop?")
#
# Infinite looping may not seem useful, but uncomment and run the following example:
#
# while True:
#   my_num = input("enter a number")
#   print(my_num)
#
# This new function, input(), lets the user give information to the program with an option message. Does the above syntax
# make sense to you? Why or why not? Either way, this strategy of infinite looping is used in many applications where real
# time events happen, like video games or webpages. 
########################################################################################################################
# Part III: Combining Loops and If statements
# When used together, Loops and If statements give you the ability to rapidly manipulate data in a conditional manner. 
# Consider the following example:

test_values = [1,2,3,4,5,6,7,8,9,10]

# for value in test_values:
#     if value%2 == 0:
#         print(value, " is even")
#     else:
#         print(value, " is odd")
# Note the combination of a variable and a string with the "," in print. Print allows formatted output (which we will cover
# more later) and it is very useful for debugging and making better program flow. With this knowledge, the answer to Lesson2.py's 
# challenge problem likely makes a lot more sense:

rna_sequence = "GCUAGCCUAAAAUCUTUAA"
allowed_bases = ["A","C","G","U"]

# for nucleotide in rna_sequence:
#     if nucleotide not in allowed_bases:
#       print(nucleotide, " not an allowed base")
    
# In addition to analyzing data, for loops can be used to generate data. The following makes a "checkerboard" string:

checker_board = ""
for row in range(5):
    for col in range(5):
       if (row*5 + col)%2 == 0:
          checker_board += "X"
       else:
          checker_board += " "
    checker_board += "\n"
# print(checker_board)
# Somtimes, you also want to break out of a loop when some condition is met. This is done with the "break" keyword, as shown
# below:

rna_sequence = "GGACGCUTA"
allowed_bps = ["A", "C", "G", "U"]

for bp in rna_sequence:
  if bp not in allowed_bps:
    print(bp," is not an allowed bp")
    break

########################################################################################################################
# Homework
# (T/F) Loops are good for dealing with iterable objects like lists or strings.
# (T/F) range(N) produces a list from 0 to N
# Write a for loop that prints all integers from 1 to 100 

# Write a While loop that replicates the previous problem's output
            
# Bug Busters
# David is writing some code to analyze some bar codes. His bar codes are strings that are supposed to be length 5. Because
# he is going to need to change bad barcodes, he would like the code to stop executing when a bad barcode comes up. Why 
# doesn't the code work? What changes need to be made.
bar_codes = ["12345","54543","666789"]

#for code in bar_codes:
#    if len(code) != 5:
#            print(code," is a bad barcode")
            
# Challenge problem: Write code that takes a user input and creates a multiplication table and prints it to the console.
# Example: 
# User input = 4
# Output
# 1 2 3 4
# 2 4 6 8
# 3 6 9 12
# 4 8 12 16