#!/usr/bin/env python
# coding: utf-8

# # Lecture 2B—Logic flow

# In this notebook, we learn to control the logic flow. We will use `if`, `while`, `for` to create condition and iteration.

# ## Table of Content
# 
# - [if-condition](#if-condition)
# - [while-loop](#while-loop)
# - [Exercise: while-loop](#Exercise:-while-loop)
# - [for-loop](#for-loop)
# - [Exercise: for-loop](#Exercise:-for-loop)
# - [Range(start, end, step)](#range(start,-end,-step))

# ## if-condition

# In[1]:


score = 61

if score >= 60:
    print("Pass")
else:
    print("Fail")


# Let’s take a deeper look of the above code block.
# 
# ![Explaining indentation](indentation.png)
# 
# Whenever we have logic flow that have code only execute under certain condition, we add a `:` and **indent** the following block.
# 
# The **indentation** means the code _belongs to_ the condition. In this example, the `print("Pass")` only execute when the condition `score >=60` meets. We can tell from the code that the line of code has **4 leading spaces**.
# 
# The `print("Fail")` only execute when the condition `score >=60` fails. We can tell the line of code has **4 leading spaces** under `else:` block.

# In the following example, we take the input and compare the input. Remember that the input value is always string. We need to type cast the input value into `int` by using `int()` function.

# In[2]:


age = input("What is your age? ")

if int(age) >= 18:
    print("You may drink, a little bit.")
else:
    print("Please don’t drink.")
    
print("Good bye and have a nice day.")


# ## Putting logic into function

# Sometimes, the conditional checking repeats for the whole data set. We can define function to reuse the same code. For example, the following code defines a function the _return_ either "Pass" or "Fail" based on the given _score_ value.

# Function is a block of code that takes **parameters** and **return** calculation result. This block of code is executed only when other line of code calls it.

# In[3]:


def is_student_pass(score):
    if score >= 60:
        return "Pass"
    else:
        return "Fail"


# Note the **indentation** in the above code. The whole `if` block has at least **4 leading spaces** to indiciate that block of code is under the `def is_student_pass(score):` function. The two `return` statements has 4 further leading spaces to indicate that they are under the `if score>=60:` and `else:` statement.
# 
# ![Explaining indentation](indentation2.png)

# In[4]:


result_a = is_student_pass(59)
print(result_a)


# In[5]:


result_b = is_student_pass(61)
print(result_b)


# You can think of function as a block of coded pre-defined and stored under a name. It is like variable for a list of "procedures". Every time we call the function, it executes the pre-defined lines of code and **return** the calculated result.

# ## while-loop vs. for-loop
# 
# While-loop is usually used for iteration that we don’t know the total count.
# 
# For-loop is when we know the iteration count. For example, we already have the list of items. Or we already know how many times to repeat.
# 

# ## for-loop

# In[8]:


fruits = ['Apple', 'Banana', 'Orange']

for fruit in fruits:
    print(fruit)


# Let’s bring back our `is_student_pass` function we defined. We can use a for loop to get a list of Pass/Fail result.

# In[9]:


def is_student_pass(score):
    if score >= 60:
        return "Pass"
    else:
        return "Fail"

scores = [80, 59, 62, 70, 99, 40, 51]

for score in scores:
    result = is_student_pass(score)
    print(f"{score}: {result}")


# ## Exercise: for-loop
# 
# In this exercise, we bring back the book lists and loop books to print each details.

# In[10]:


books = [
    {'title': 'Python Tricks', 'category': 'Programming', 'price': 240},
    {'title': 'Python Crash Course', 'category': 'Programming', 'price': 200},
    {'title': 'Getting Real', 'category': 'Startup', 'price': 200}
]


# Try to loop the books and print the title and their price. We don’t need to print the category in this exercise.

# In[11]:


for book in books:
    # Your code here
    title = None
    price = None
    print(None)


# |Expected result|
# |---|
# |Python Tricks: \$240|
# |Python Crash Course: \$200|
# |Getting Real: \$200|

# ## range(start, end, step)
# 
# Sometimes, we don’t have the list to iterate. But we know how many steps we want to loop. In this case, we can use `range`.

# In[12]:


# print("0-9")
for i in range(10):
    print(i)


# In[13]:


# print("1-10")
for i in range(1,11):
    print(i)


# In[14]:


# print("1,3,5,7,9")
for i in range(1,10,2):
    print(i)


# In[15]:


# print("2,4,6,8,10")
for i in range(2,11,2):
    print(i)


# ## Code example: while-loop
# 
# By combining list and while loop, we can keep appending the value to the `tasks` list until the input is "q". The `break` command exits the while-loop.

# In[6]:


tasks = []

while True:
    value = input("Please input a to-do task, or 'q' to quit. ")

    if value == 'q':
        break
        
    tasks.append(value)
    
print(tasks)


# You can tell from the indentation that line 4—9 under the `while True:` loop. Line 7 is under the `if value=='q':` loop.
# 
# Keep in mind that we need to be careful when using `while True:`. the condition is always true so the loop will run forever until we meet the `break` command. If the logic we designed has flaw, the loop may never end.

# ## Exercise: while-loop
# 
# In this exercise, we would like to create a guest list by modifying `while` loop example given above.

# In[7]:


guests = []

# Your code here
while None:
    value = input("Your text here ")
    if value == 'q':
        break
    
    None # append the value to guests
    
print(guests)


# ## Summary
# 
# In this lesson, we learned the basic logic flow. Specifically:
# 
# - if-condition
# - loop with or without known iteration count
# - while-loop
# - for-loop
# - range

# In[ ]: