If, Elif and Else

The first conditional statement we will consider is the if statement. This is also one of the oldest statements used in high level languages.

1. I have opened VSC and created a new directory C:\Python\Exercise_03

2. I have created a file, my_if.py

3. Over the next few minutes, I am going to build a code template I can use in any project.

4. In Python, control flow is determined by colons (:) and indentation. Indentation must be perfect, or you will generate errors when you try to run the program. Way back, people used spaces (how many?) and tabs interchangeably.

The style guide PEP8 tells us to use four spaces for indentation.

The syntax is

if condition1:
    # Execute code
elif condition2:
    # Execute code
else:
    # Execute other code

If a condition1 is met, code is executed until the end of the if indented block.

If the first condition is not met, condition2 is checked, if it is, code is executed until the end of the elif indented block. You may have many elif conditions, but once one is met, its code block will be executed and the programme will move beyond the if, elif, else code block.

If no condition is met, the else code block is executed. I pulled an example from a program I wrote c. 2015 to decode some Smart Grid data. Note that I put enough comments in that even now, I can figure out what I did and why.

# First byte AA flags start of sequence
# Now test the second byte to identify the frame type
if SecondByte == 0x00:                                   
    # AA0x Data frame
    self.FrameType = 'Data'      
elif SecondByte == 0x10:                                
    # AA1x Header frame
    self.FrameType = 'Header'      
elif SecondByte == 0x20:                                
    # AA2x Configuration Frame1
    self.FrameType = 'ConfigurationFrame1'                
else:                                                                    
    # Corrupt data?
    self.FrameType = 'Unknown'

Take note of the indentation, the location of colons and the conditions. 

Exercise

Try the following and save as my_if.py

a = True
b = True
c = True

if a:
    print("a was true")
elif b:
    print("b was true")
elif c:
    print("c was true")
else:
    print("None of our boolean variables were true")

Run this code, then:

1. Set a = False and run, examine the output

2. Set b = False and run, examine the output

3. Set c = False and run, examine the output

Notice the limitations, once the first test is met, the if/elif/else code block terminates.

I wrote the following code snippet to check more complicated Boolean conditions. Play around with the numbers and conditions so you can understand what I did.

if (3<2) and (5<9):
    print("Yup!")
else:
    print("Nope!")