Structured Programming Details

Background

Since the invention by Von Neumann of the stored program computer, it was recognized that a tremendous potential of computing equipment was the ability to alter its behavior, depending on the input data. Calculating machines had, for some time, been able to perform fixed arithmetic operations on data, but the potential of machines capable of making decisions opened up many new possibililities. Machines that could make decisions were capable of sorting records, tabulating and summarizing data, searching for information, and many more advanced operations that could not even be imagined at the time.

In early programming languages, like Fortran (first invented in 1954) and various low level machine languages, the goto statement allowed the computer to deviate from the sequential execution of the program instructions. The goto statement was recognized to be a very powerful construction, and soon, programs of increasing complexity and power were developed.

However, the increasing complex code became harder and harder to maintain. Dijkstra, in 1966, was one of the first persons to recognize that this run away complexity of programs was due to the overuse of the goto statement (Dijkstra, E. W., "Go To Considered Harmful," Communications of the ACM, March 1966). In fact, it was determined shortly thereafter, that the goto statement is not needed at all. This was the birth of the discipline of Structured Programming.

Python does not have a goto statement.

Structured Programming in Everyday Life

Sequence

Example: Baking Bread

Add flour
Add salt
Add yeast
Mix
Add water
Knead
Let rise
Bake

Repetition

Example: Washing Dishes

stack dishes by sink
fill sink with hot soapy water
while there are more dishes
   get dish from counter
   wash dish
   put dish in drain rack
end
wipe off counter
rinse out sink

Selection

Example: Computing Course Grade

compute course_score
if course_score is greater than or equal to 90
   assign course grade to A
elsif course_score is greater than or equal to 75
   assign course grade to B
elsif course_score is greater than or equal to 60
   assign course grade to C
elsif course_score is greater than or equal to 50
   assign course grade to D
else
   assign course grade to D
end
submit course grade to registrar

Repetition and Selection

Example: Sorting Mail

get mail from mailbox
put mail on table
while more mail to sort
   get piece of mail from table
   if piece is personal
      read it
   elsif piece is magazine
      put in magazine rack
   elsif piece is bill
      pay it
   elsif piece is junk mail
      throw it in wastebasket
   end
end

Structured Programming in Python

Structured programming is a program written with only the three constructions sequence, decision (if..elif statements), and repetition (while or for statements). Important: the body of a Python if, elif, while, or for statement is indicated by indenting four spaces. Python does not use end statements.

Sequence. Lines or blocks of code are written and executed in sequential order.

Example:
```
x = 56
y = 11
z = x + y
print(z)
```
Decision. Execute a block of code (action) if a condition is true. The block of code is executed at most once.
```
if condition:
    action
```
Example:
```
if x % 2 == 0:
    print("The number is even.")
```
Repetition. Repeat a block of code (action) while a condition is true. There is no limit to the number of times that the block can be executed.
```
while condition:
    action
```
Example:
```
n = 1
while n < 100:
    print(n)
    n = n * n
```

Extensions

To make programs easier to read, some additional constructs were added to the basic three original structured programming constructs:

Definite Repetition Execute a block of code once for each element in an array:
```
for element in a range
  action
end
```
Example:
```
for x in range(1, 5):
    print(x, end="")
```
Important: A Python range is inclusive for the lower limit, but exclusive for the upper limit. range(1, 5) actually goes from 1 to 5 - 1 = 4. This range consists of the values [1, 2, 3, 4].

if..elif Statements Execute the first action whose corresponding condition is true. Here is the general form:

if condition1:
    action1

elif condition2:
    action2

elif condition3:
    action3

else
    default_action:

Example:

n = int(input("Enter a positive integer: "))

if n == 1:
    print "one"

elsif n == 2
    print "two"

elsif n == 2
    print "three"

else:
    print "many"