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

# # Module 2
# 
# ## Video 4: Program Flow I
# **Python for the Energy Industry**
# 
# This is a Jupyter Notebook. A Notebook is composed of cells of Python code which can be run separately. When a cell runs, each line of Python code is run sequentially, and any outputs are displayed below the cell. 

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# A simple example of running a cell
my_num = 5
print('my_num =',my_num)

my_num = my_num + 2
print('my_num =',my_num)

my_num = my_num * 2
print('my_num =',my_num)


# ### Understanding Indentation
# 
# In Python, indendation is used to control the order in which lines of code are run. Consider the below:
# 
# ```python
# control line with colon:
#     indented code block line
#     indented code block line
#     indented code block line
# 
# code block ends when indent ends
# ```
# 
# The 'control line' determines if, and how many times, the indented code block runs. Once this finishes, the next line of code that is not indented will run. 
# 
# This will make more sense when we see some examples. So let's consider the two main types of control line: conditions, and loops.

# ### Conditionals
# 
# A conditional control line will allow you to run some lines of code, only if a particular statement is true. The following is an example of the `if` statement.

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my_num = 2

if my_num == 2:            # is my_num equal to 2?
    my_num = my_num * 3    # if so, multiply it by 3
    
print('my_num =',my_num)


# Here `==` is used to check whether two things are equal. So in this example, the indented code line which multiplies `my_num` by 3, will only run if `my_num` is equal to 2. Note that `==` is different from `=`, the 'assignment' operator, which sets the value of a variable.
# 
# `==` is an example of a 'comparative' operator. Other common comparative operators in Python are:
# 
# | operator | meaning |
# |---|---|
# | `<` | less than |
# | `>` | greater than |
# | `<=` | less than or equal to |
# | `>=` | greater than or equal to |
# | `!=` | not equal to |

# ### Exercise
# 
# Modify the above code block to:
# 
#         a) Change the initial value of my_num
#         b) Change the operator used in the control line
# 
# Check that the printed output changes as you would expect.

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