Things That Flow

Code Flows Too

• The flow of code and the steps that impact that flow are called “control flow”.
• In general code flows in the way same as a book (with some exceptions).
  • Each function is completed before considering the next.
• Functions can be nested in other functions - the inner most function is completed first.
• Control structures can be used to change the course of a program.
• Repetition structures can be used to repeat a section of code.

Sequential Structure

• In general code flows like a book reads:
  • Statements (like lines of code) run top to bottom line,
  • Left to right in statement,
  • Each statement being completed before moving to the next.

v = "2"         #stores "2" as a string in 'v'
i = int(v)      #stores v as an integer 'i'
t = type(i)     #stores the type of 'i' as 't'
print(t)        #prints 't'

Nesting Functions and Operations

• We are not limited to a single function or operation per row.
• The previous example could be re-written as:
  • print(type(int("2")))
• Nesting functions can be useful, however care should be taken and it may be easier to separate functions over multiple rows.

Control Structures

• Control structures (also known as decision structures) allow the flow to respond to varying situations.
• A decision is made based on one or more conditions.
• These control structures are very similar to the IF function in Excel and the CASE statement in SQL (but remember that indentation matters in Python).

The Importance of Being Boole(an)

Repetition Structures

• Repetition structures (commonly referred to as “loops”) allow for us to recycle chunks of code to perform the same or similar operation a specified number of times or until a condition changes.
• For loops cycle through a series of iterations, until they reach the end performing each series of statements for each iteration.
  • This can be used to cycle through a list, dictionary or other iterable as well as working through ranges of numbers
• While loops continue until a predetermined condition changes from True to False.
  • This can be useful for testing conditions but comes with a warning:
• Make sure your condition will change at some point or else your loop cannot end.

What is Iterability?

• Iterability is the ability to split an object into discrete items. The item may be ordered or unordered, each item will be extracted, processed ad set aside.

• In general if an object can be split into multiple items it can be iterated (integers and floats are not iterable).

• Iterable objects include:

  • Strings (the word “strings” contains 7 iterable items).
  • Lists eg [1, 2, 3, 4, 4, 4, 5, 6]
  • Tuples eg (12, ‘Red’, ‘Apples’)
  • Sets eg {1, 2, 3, 4, 5, 6}
  • Dict eg {ICB_Code: ‘QSL’, Metric_Code: E.M.10}

Sequential Structure Example

• The following sequential code will create a variable called ‘var’ which is a string, it converts this string to an integer and conducts a series of mathematical operators before printing the result:

var = '22' # set "var" to '22'
var = int(var) # convert "var" to int
var = var  / 2 - 3 # apply math operators
print (var) # print result

Control Structure Example

• This code checks if a variable called ‘provider’ in this list is equal to a selection of values and prints out an associated string.

if provider == ‘ryr’:
  print(‘SUSSEX’)
elif provider == ‘rhu’:
  print(‘HIOW’)
elif provider == ‘rxq’:
  print(‘BOB’)
else:
  print(‘Unknown’)

Control Structure Example

For comparison, this is the equivalent SQL CASE statement.

CASE
  WHEN provider = 'ryr'
    THEN 'Sussex'
  WHEN provider = 'rhu'
    THEN 'HIOW'
  WHEN provider = 'rhu'
    THEN 'BOB'
  ELSE 'Unknown'
  END

Watch Out for Unintended Consequences

Final Thoughts

• Don’t worry about memorising any of this!
• The aim of this session is to give a basic understanding of the logic needed to implement control flow in your program.

Further Reading