6.2.1 – Identify program components

Values and assignments

A program needs names for the values it works with and commands that set or change those values.

Look at this valid Python:

LIMIT = 6                 # line 1
total = 0                 # line 2
total = total + 2         # line 3
attempts = 1              # line 4

Line 2 initialises total: it is the first assignment that gives the variable a usable starting value. Line 3 is a later assignment. Python first evaluates the expression on the right, using the old value of total, and then stores the result using the identifier on the left.

Initialisation is therefore a kind of assignment, but not every assignment is an initialisation. The symbol = performs assignment; it does not ask whether two values are equal. Python uses == for an equality comparison.

The four lines also form a command sequence: instructions are encountered in order from top to bottom. A sequence inside a selection, loop or subprogram still follows its local order, even though another construct may skip or repeat the whole block.

Sequence and selection

Sequence gives a program an order. Selection changes which sequence is followed: a condition is evaluated, and one path is selected.

ticket = 4                # line 1
ticket = ticket + 1       # line 2
if ticket >= 5:           # line 3
    message = "Next level"  # line 4
else:                     # line 5
    message = "Keep going"  # line 6
print(message)            # line 7

Trace the sequence statically. Line 1 makes ticket equal to 4; line 2 updates it to 5. The condition at line 3 is then true, so line 4 executes and line 6 is skipped. After the selection finishes, line 7 is next, giving the predicted output Next level.

The indented commands show the alternatives belonging to the selection. A common mistake is to call only the if header selection; the structural component is the whole decision, lines 3-6 here.

Loops and data structures

A data structure groups related items so a program can treat them as one organised value. In the PLS, a string is a sequence of characters, while Python lists are used to represent arrays and records.

Square brackets show that both the array and record use a Python list. Their contents and purpose distinguish the role; not every list should automatically be called an array.

Loops make a command sequence happen more than once. Pearson's PLS gives two useful labels for inspection:

• Condition-controlled repetition: a while loop executes its block while a condition is true.
• Count-controlled repetition: a for loop with range repeats for a planned sequence of counts.
• Iteration over data: a for loop visits each item of a structure directly.

All are loops. The labels distinguish what controls the next pass; for alone does not prove that the purpose is visiting a stored data structure.

scores = [2, 4, 1]        # line 1
index = 0                 # line 2
while index < 2:          # line 3
    print(scores[index])  # line 4
    index = index + 1     # line 5
for score in scores:      # line 6
    print(score)          # line 7

The while block is repetition. It prints only the items at index 0 and index 1, so its predicted output is 2 then 4. The for block is iteration over the data structure and visits all three items, so it then produces 2, 4, 1.