Deitel - Python, How To Program.pdf

pythonhtp1_03.fm Page 73 Saturday, December 8, 2001 9:34 AM
Chapter 3 Control Structures 73
These single-entry/single-exit control structures make it easy to build programs. The
control structures are attached to one another by connecting the exit point of one control
structure to the entry point of the next. This is similar to the way a child stacks building
blocks; hence, the term control-structure stacking. Control-structure nesting also connects
control structures; we discuss this technique later in the chapter.
Software Engineering Observation 3.2
Any Python program can be constructed from six different types of control structures (sequence,
if, if/else, if/elif/else, while and for) combined in two ways (controlstructure
stacking and control-structure nesting). 3.2
3.5 if Selection Structure
Selection structures choose among alternative courses of action. For example, suppose that
the passing grade on an examination is 60. Then the pseudocode statement
If student’s grade is greater than or equal to 60
Print “Passed”
determines whether the condition “student’s grade is greater than or equal to 60” is true or
false. If the condition is true, then “Passed” is printed, and the next pseudocode statement
in order is “performed.” (Remember that pseudocode is not a real programming language.)
If the condition is false, the print statement is ignored, and the next pseudocode statement
is performed. Note that the second line of this selection structure is indented. Such indentation
is optional (for pseudocode), but it is highly recommended because indentation emphasizes
the inherent hierarchy of structured programs. When we convert pseudocode into
Python code, indentation is required.
The preceding pseudocode if statement may be written in Python as
if grade >= 60:
print "Passed"
Notice that the Python code corresponds closely to the pseudocode. This similarity is the
reason that pseudocode is a useful program development tool. The statement in the body of
the if structure outputs the character string "Passed".
The flowchart of Fig. 3.3 illustrates the single-selection if structure and the diamond
symbol. The decision symbol contains an expression, such as a condition, that can be either
true or false. The diamond has two flowlines emerging from it: One indicates the direction
to follow when the expression in the symbol is true; the other indicates the direction to
follow when the expression is false. We learned, in Chapter 2, Introduction to Python Programming,
that decisions can be based on conditions containing relational or equality operators.
Actually, a decision can be based on any expression. For instance, if an expression
evaluates to zero, it is treated as false, and if an expression evaluates to nonzero, it is treated
as true.
Note that the if structure is a single-entry/single-exit structure. We will soon learn
that the flowcharts for the remaining control structures also contain (besides small circle
symbols and flowlines) rectangle symbols that indicate the actions to be performed and diamond
symbols that indicate decisions to be made. This type of flowchart emphasizes the
action/decision model of programming.