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Control Structures. CS103
Sometimes it is necessary to alter the sequential
Computer
flow of control in a program. Programs
This is achieved by control flow statements. Simple Programs
Numbers
Apart from the normal sequential flow, there are
Strings
two principal nonsequential control structures: Functions
conditional flow and repetition flow. Graphics
Decisions
Loops
statement-1
test? test?
statement-2
statement statement
statement-n
The if Statement. CS103
Computer
S YNTAX An if statement has the form Programs
Simple Programs
if <condition>:
Numbers
<block>
Strings
where <condition> is an expression that
Functions
evaluates to either True or False, and <block> is
Graphics
a sequence of one or more statements, indented Decisions
under the if header. Loops
S EMANTICS
1 <condition> is evaluated.
2 If True, the statements in <block> are executed
(and control passes on to the next statement).
3 If False, the <block> is skipped and control
passes immediately to the next statement.
2-Way Decisions: The if-else Statement. CS103
S YNTAX An if-else statement has the form Computer
if <condition> : Programs
Simple Programs
<block 1>
Numbers
else :
Strings
<block 2> Functions
S EMANTICS Evaluate <condition>. If True Graphics
execute <block 1> under the if. If False execute Decisions
Loops
<block 2> under the else. In any case, control
then passes to the next statement.
test?
statement 1 statement 2
Multi-Way Decisions: if-elif-else. CS103
S YNTAX An if-elif-else statement has the form
Computer
if <condition 1> : Programs
<block 1> Simple Programs
elif <condition 2> : Numbers
<block 2> Strings
Functions
...
Graphics
elif <condition n> :
Decisions
<block n> Loops
else :
<block 0>
test1? test2? ... testn?
statement1 statement2 statementn statement0
...
Relational Expressions. CS103
Computer
A relational expression has the form Programs
<expr1> <relop> <expr2> Simple Programs
where <expr1> and <expr2> are (arithmetical) Numbers
expressions and the relational operator <relop> is Strings
Functions
one of the following:
Graphics
Decisions
< a<b less than Loops
<= a b less than or equal
== a=b equal
>= a b greater than or equal
> a>b greater than
!= a=b not equal
in a∈b membership test
is object identity
Boolean Expressions. CS103
Python provides a data type bool and the literals Computer
Programs
True and False for truth values. Simple Programs
S YNTAX The boolean operators Numbers
<expr1> and <expr2> Strings
<expr1> or <expr2> Functions
Graphics
not <expr>
Decisions
are used to build boolean expressions out of
Loops
expressions which evaluate to True or False.
S EMANTICS The value of a boolean expression is
defined in terms of truth tables:
P Q P and Q P or Q
P not P True True True True
True False True False False True
False True False True False True
False False False False
Boolean Algebra. CS103
Commutative Laws.
Computer
P and Q == Q and P Programs
P or Q == Q or P Simple Programs
Numbers
Laws of 0 and 1.
Strings
P and False == False (p · 0 = 0) Functions
P and True == P (p · 1 = p) Graphics
P or False == P (p + 0 = p) Decisions
P or True == True (p + 1 = 1 ???) Loops
Double Negation.
not (not P) == P (−(−p) = p)
De Morgan’s Laws.
not (P or Q) == (not P) and (not Q)
not (P and Q) == (not P) or (not Q)
Distributive Laws.
P or (Q and R) == (P or Q) and (P or R)
P and (Q or R) == (P and Q) or (P and R)
Exception Handling: try-except. CS103
Computer
S YNTAX A try-except statement has the form Programs
try : Simple Programs
Numbers
<block>
Strings
except <error>: Functions
<handler> Graphics
where <block> and <handler> are sequences of Decisions
one or more statements and <error> is an error Loops
type.
A try statement can have several except clauses.
S EMANTICS Attempt to execute the statements in
<block>. If no error occurs, control passes to the
next statement. Otherwise find a matching error
type <error> and execute the statements
<handler> of its except clause.
Summary: Decision Structures. CS103
Decision structures . . . Computer
Programs
. . . are control structures that allow a program to Simple Programs
execute different sets of instructions under different Numbers
circumstances. Strings
Functions
. . . are implemented in Python with if statements.
Graphics
. . . are based on the evaluation of conditions. Decisions
. . . can be nested. Loops
Conditions can involve relational operators and
boolean operators.
The value of a boolean expression is either True
or False.
Python has a boolean data type bool .
Python also provides a try-except statement for
exception handling.
