3.4 Control Statements

Usually, statements in a program execute in the order in which they’re written. This is called sequential execution. Various Python statements enable you to specify that the next statement to execute may be other than the next one in sequence. This is called transfer of control and is achieved with Python control statements.

Forms of Control

In the 1960s, extensive use of control transfers was causing difficulty in software development. Blame was pointed at the goto statement. This statement allowed you to transfer control to one of many possible destinations in a program. Bohm and Jacopini’s research¹ demonstrated that programs could be written without goto statements. The notion of structured programming became almost synonymous with “goto elimination.” Python does not have a goto statement. Structured programs are clearer, easier to debug and change, and more likely to be bug-free.

1. Bohm, C., and G. Jacopini, “Flow Diagrams, Turing Machines, and Languages with Only Two Formation Rules,” Communications of the ACM, Vol. 9, No. 5, May 1966, pp. 336–371.

Bohm and Jacopini demonstrated that all programs could be written using three forms of control—namely, sequential execution, the selection statement and the repetition statement. Sequential execution is simple. Python statements execute one after the other “in sequence,” unless directed otherwise.

Flowcharts

A flowchart is a graphical representation of an algorithm or a part of one. You draw flowcharts using rectangles, diamonds, rounded rectangles and small circles that you connect by arrows called flowlines. Like pseudocode, flowcharts are useful for developing and representing algorithms. They clearly show how forms of control operate. Consider the following flowchart segment, which shows sequential execution:

3.4-1 Full Alternative Text

We use the rectangle (or action) symbol to indicate any action, such as a calculation or an input/output operation. The flowlines show the order in which the actions execute. First, the grade is added to the total, then 1 is added to the counter. We show the Python code next to each action symbol for comparison purposes. This code is not part of the flowchart.

In a flowchart for a complete algorithm, the first symbol is a rounded rectangle containing the word “Begin.” The last symbol is a rounded rectangle containing the word “End.” In a flowchart for only a part of an algorithm, we omit the rounded rectangles, instead using small circles called connector symbols. The most important symbol is the decision (or diamond) symbol, which indicates that a decision is to be made, such as in an if statement. We begin using decision symbols in the next section.

Selection Statements

Python provides three types of selection statements that execute code based on a condition—an expression that evaluates to either True or False:

• The if statement performs an action if a condition is True or skips the action if the condition is False.

• The if…else statement performs an action if a condition is True or performs a differentaction if the condition is False.

• The if…elif…else statement performs one of many different actions, depending on the truth or falsity of several conditions.

Anywhere a single action can be placed, a group of actions can be placed.

The if statement is called a single-selection statement because it selects or ignores a single action (or group of actions). The if…else statement is called a double-selection statement because it selects between two different actions (or groups of actions). The if…elif…else statement is called a multiple-selection statement because it selects one of many different actions (or groups of actions).

Repetition Statements

Python provides two repetition statements—while and for:

• The while statement repeats an action (or a group of actions) as long as a condition remains True.

• The for statement repeats an action (or a group of actions) for every item in a sequence of items.

Keywords

The words if, elif, else, while, for, True and False are keywords that Python reserves to implement its features, such as control statements. Using a keyword as an identifier such as a variable name is a syntax error. The following table lists Python’s keywords.

Python keywords

and	as	assert	async	await	break	class
continue	def	del	elif	else	except	False
finally	for	from	global	if	import	in
is	lambda	None	nonlocal	not	or	pass
raise	return	True	try	while	with	yield

Control Statements Summary

You form each Python program by combining as many control statements of each type as you need for the algorithm the program implements. With Single-entry/single-exit (one way in/one way out) control statements, the exit point of one connects to the entry point of the next. This is similar to the way a child stacks building blocks—hence, the term control-statement stacking. Control-statement nesting also connects control statements—we’ll see how later in the chapter.

You can construct any Python program from only six different forms of control (sequential execution, and the if, if…else, if…elif…else, while and for statements). You combine these in only two ways (control-statement stacking and control-statement nesting). This is the essence of simplicity.

Self Check

1. (Fill-In) You can write all programs using three forms of control—___________ , ___________ and ___________ .
   Answer: sequential execution, selection statements, repetition statements.

2. (Fill-In) A(n) ___________ is a graphical representation of an algorithm.
   Answer: flowchart.