PROGRAMMING LANGUAGES

STATEMENT-LEVEL CONTROL
STRUCTURES

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 Introduction

 A control structure is a control statement and the statements whose

execution it controls
– Selection Statements
– Iterative Statements

 There is only one design issue that is relevant to all of the selection and
iteration control statements:
– Should a control structure have multiple entries?

Statement-Level Control Structures

In programming, "multiple entries" in control structures typically refers to the ability of a control structure (like
loops or conditional statements) to handle multiple conditions or cases.

Pros
1. Flexibility: Allowing multiple entries can make a control structure more flexible, enabling it to handle various
conditions or states without needing to restructure the code significantly.
2. Efficiency: In some cases, multiple entries can lead to more efficient code execution by allowing the program to
bypass unnecessary checks or operations.

Cons of Multiple Entries

1. Complexity: Multiple entry points can lead to increased complexity in understanding the flow of
control. It may become harder for developers to follow the logic, especially in larger codebases.
2. Readability: Code readability can suffer when there are multiple ways to enter a control structure. This
can make it harder for new developers to understand the code quickly.

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 Selection Statement

 A selection statement provides the means of choosing between two or

more paths of execution.

 Selection statement fall into two general categories:

– Two-way selection
– Multiple-way selection

Statement-Level Control Structures

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 Two-way Selection Statements

 The general form of a two-way selector is as follows:

if control_expression
then clause
else clause

 Design issues
– What is the form and type of the control expression?
– How are the then and else clauses specified?
– How should the meaning of nested selectors be specified?

Statement-Level Control Structures

What is the form and type of the control expression?

•Form: The control expression is the condition you check to decide which block of code to executeFor example, x > 10 is a
control expression that checks if x is greater than 10.
•Type: The control expression must be of a boolean type, meaning it should evaluate to either true or false. In programming
languages, this is enforced so that you can only use expressions that can logically be true or false.
How are the then and else clauses specified?
•Then Clause: This is the block of code that runs if the control expression evaluates to true. You can write a single statement
or multiple statements. If you have multiple statements, they are usually grouped together.
•Else Clause: This is the block of code that runs if the control expression evaluates to false. Like the then clause, it can also be
a single statement or a block of statements.
•Specification: In many programming languages, you use curly braces {} to group multiple lines of code, or you use
indentation (like in Python) to show which lines belong to the then or else clauses.
How should the meaning of nested selectors be specified?
•Nested Selectors: This refers to having an if statement inside another if statement. It allows you to check additional
conditions based on the outcome of the first condition.
•Clarity: When you nest if statements, it’s important to keep the code clear and easy to read. You should use proper
indentation or braces to show which if belongs to which block of code.
•Evaluation Order: The outer if is evaluated first. If it’s true, the code inside it runs, which may include another if statement. If
the outer if is false, the else part runs, which can also contain another if.

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 The control expression

 Control expressions are specified in parenthesis if the then reserved word

is not used to introduce the then clause, as in the C-based languages

 In C89, which did not have a Boolean data type, arithmetic expressions
were used as control expressions
int a = 10, b = 20;
int x = 5;
if (a < b) { // This evaluates to true (1) because
if (x) { // This checks if x is non-zero (true)
10 is less than 20
printf("x is true\n");
printf("a is less than b\n");
} else {
} else {
printf("x is false\n");
printf("a is not less than b\n");
}
}
 In contemporary languages, such as Java and C#, only Boolean expressions
can be used for control expressions

Statement-Level Control Structures

In C89, any non-zero integer value is considered true, while 0 is considered false. This means that you can use any
arithmetic expression that evaluates to an integer as a control expression in if, while, and other control statements.

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 Clause Form

 In most contemporary languages, the then and else clauses either appear
as single statements or compound statements.
 C-based languages use braces to form compound statements.
 One exception is Perl, in which all then and else clauses must be
compound statements, even if they contain single statements
 In Python and Ruby, clauses are statement sequences
 Python uses indentation to define clauses

if x > y :
x=y
print " x was greater than y"

 All statements equally indented are included in the compound statement.

Notice that rather than then, a colon is used to introduce the then clause
in the Python

Statement-Level Control Structures

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 Nesting Selectors

 In Java and contemporary languages, the static semantics of the language

specify that the else clause is always paired with the nearest unpaired then
clause
If properly coded:
If not properly coded: if (sum == 0)
if (sum == 0) if (count == 0)
if (count == 0) result = 0;
result = 0; else
else result = 1;
result = 1;

 A rule, rather than a syntactic entity, is used to provide the disambiguation

 So, in the example, the else clause would be the alternative to the second
then clause

Statement-Level Control Structures

In Java and many other C-like programming languages, this is a well-known situation often referred to as the
"dangling else" problem.

The static semantics of the language resolve this ambiguity by pairing each else with the nearest preceding
unmatched if—even if that is not what the programmer intended visually.

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 Nesting Selectors

 To force the alternative semantics in Java, a different

syntactic form is required, in which the inner if is put in
a compound, as in

if (sum == 0) {
if (count == 0)
result = 0;
}
else
result = 1;

Statement-Level Control Structures

In Java, to force the else to associate with the outer if rather than the nearest inner if, you must use explicit
block structure using curly braces {}.

This syntactic change is necessary to override the default static semantics that pair an else with the nearest
unmatched if.

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 Nesting Selectors

 – C, C++, and C# have the same problem as Java with

selection statement nesting
 – Ruby, statement sequences as clauses:

if sum == 0 then
if count == 0 then
result = 0
else
result = 1
end
end

Statement-Level Control Structures

C, C++, Java, C#: Dangling Else Problem

All of these C-style languages share the same ambiguity due to their grammar and semantics, where:
•An else always pairs with the closest unmatched if.
•To avoid confusion, braces {} are used to explicitly define blocks and control else pairing.
•Ruby avoids the dangling else issue by using end to close each block, which clearly defines scope for each if.

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 Nesting Selectors

 Python, all statements uses indentation to define

clauses

if sum == 0 :
if count == 0 :
result = 0
else :
result = 1

Statement-Level Control Structures

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 Multiple Selection Constructs

 The multiple selection construct allows the selection of one

of any number of statements or statement groups.

 Design Issues
– What is the form and type of the control expression?
– How are the selectable segments specified?
– Is execution flow through the structure restricted to
include just a single selectable segment?
– How are case values specified?
– What is done about unrepresented expression values?

Statement-Level Control Structures

What is the form and type of the control expression?

•This refers to what kinds of expressions can be evaluated to decide which branch to take

How are the selectable segments specified?

•This defines the structure of the branches. C/C++/Java: case value: followed by statements.

Is execution flow restricted to a single selectable segment?

•In some languages, control can fall through to subsequent branches if not explicitly stopped.

How are case values specified?

•The format and rules for assigning values to branches:

What is done about unrepresented expression values?

•What happens if no case matches?

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 Multiple Selection Constructs

C, C++, Java, and JavaScript switch

switch (expression) {
case constant_expression1 : statement1;
...
case constant_expressionn : statementn;
[default: statementn+1]
}

Statement-Level Control Structures

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 Multiple Selection Constructs

 C# switch
 – C# switch statement differs from C-based in that C# has static semantic rule disallows
the implicit execution of more than one segment
 – The rule is that every selectable segment must end with an explicit unconditional branch
statement either a break, which transfers control out of the switch construct, or a goto,
which can transfer control to on of the selectable segments. C# switch statement example:

switch (value) {
case -1: Negatives++;
break;
case 0: Positives++;
goto case 1;
case 1: Positives ++;
default: Console.WriteLine(“Error in switch \n”);
}

Statement-Level Control Structures

No Fall-Through
•In C#, only one case is run, unless you tell it to go to another.
•Unlike C or C++, it won't accidentally run the next case unless you use goto.

Each Case Must End Clearly

•You must end each case with something like:
• break (stop the switch)
• goto case (jump to another case)
• return (exit the method)
• throw (throw an error)
Default Case
•If no case matches, C# will run the default: block — just like an "else" in an if.

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 Multiple Selection Constructs

 Multiple Selection Using if

– Multiple Selectors can appear as direct extensions to two-way
selectors, using else-if clauses
– Ex, Python selector statement (note that else-if is spelled elif in
Python):

if count < 10 :
bag1 = True
elif count < 100 :
bag2 = True
elif count < 1000 :
bag3 = True

Statement-Level Control Structures

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 Multiple Selection Constructs

 which is equivalent to the following:

if count < 10 :
bag1 = True
else :
if Count < 100 :
bag2 = True
else :
if Count < 1000 :
bag3 = True

The elsif version is the more readable of the two.

Statement-Level Control Structures

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 Multiple Selection Constructs

 The Python example can be written as a Ruby case

case
when count < 10 then bag1 = true
when count < 100 then bag2 = true
when count < 1000 then bag3 = true
end

 Notice that this example is not easily simulated with a switch-case

statement, because each selectable statement is chosen on the basis of a
Boolean expression

Statement-Level Control Structures

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 Iterative Statements

 An iterative statement is one that cause a statement or collection of

statements to be executed zero, one, or more times

 The repeated execution of a statement or compound statement is

accomplished either by iteration or recursion

 An iterative statement is often called loop

 Iteration is the very essence of the power of computer

 The repeated execution of a statement is often accomplished in a

functional language by recursion rather than by iteration

Statement-Level Control Structures

Iteration
•Uses loops like while, for, or do-while.
•Common in imperative languages like C, Java, Python.
•Repeats a block of code until a condition is false.

Recursion
•A function calls itself to solve a problem.
•Often used in functional languages like Haskell, Scheme.

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 Iterative Statements

 General design issues for iteration control statements:

– How is iteration controlled?
– Where should the control mechanism appear in the loop statement?
 The primary possibilities for iteration control are logical, counting, or a
combination of the two
 The main choices for the location of the control mechanism are the top of
the loop or the bottom of the loop
 The body of a loop is the collection of statements whose execution is
controlled by the iteration statement
 The term pretest means that the loop completion occurs before the loop
body is executed
 The term posttest means that the loop completion occurs after the loop
body is executed
 The iteration statement and the associated loop body together form an
iteration statement

Statement-Level Control Structures

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 Counter-Controlled Loops

 A counting iterative control statement has a variable,

called the loop variable, in which the count value is
maintained

 It also includes means of specifying the initial and

terminal values of the loop variable, and the difference
between sequential loop variable values, called the
stepsize.

 The initial, terminal and stepsize are called the loop

parameters.

Statement-Level Control Structures

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 Counter-Controlled Loops

 The for statement of the C-based languages

for ([expr_1] ; [expr_2] ; [expr_3])

loop body

– The loop body can be a single statement, a

compound statement, or a null statement

for (count = 1; count <= 10; count++)

Statement-Level Control Structures

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 Counter-Controlled Loops

– All of the expressions of C’s for are optional

– If the second expression is absent, it is an

infinite loop

– If the first and third expressions are absent, no

assumptions are made

Statement-Level Control Structures

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 Counter-Controlled Loops

– C’s for is more flexible than the counting loop statements of Fortran and Ada, because
each of the expressions can comprise multiple statements, which in turn allow multiple loop
variables that can be of any type

– Consider the following for statement:

for (count1 = 0, count2 = 1.0;

count1 <= 10 && count2 <= 100.0;
sum = ++count1 + count2, count2 *= 2.5)
;

Statement-Level Control Structures

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 Counter-Controlled Loops

– The for statement of C99 and C++ differs from earlier version of C in two ways:
• It can use an arithmetic expression or a Boolean expression for loop control
• The first expression can include variable definitions (scope is from the definition to
the end of the loop body), for example

for (int count = 0; count <= 10; count++) { . . . }

– The for statement of Java and C# is like that of C++, except that the loop control
expression is restricted to Boolean

Statement-Level Control Structures

The for statement in C99 and C++ includes improvements over earlier C versions that make it more flexible and
safer.

In Java and C#, the loop control (condition) must be Boolean.

You cannot use an integer expression like in older versions of C.

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 Counter-Controlled Loops

 The for statement of Python

– The general form of Python’s for is:

for loop_variable in object:

- loop body names = ["Mike","Ana","Jun"] #<-LIST
[else: for name in names:
- else clause] print(name)

– The object is often range, which is either a list of values in brackets ([2,
4, 6]), or a call to the range function (range(5), which returns 0, 1, 2, 3, 4)
– The loop variable takes on the values specified in the given range, one
for each iteration
– The else clause, which is optional, is executed if the loop terminates
normally

Statement-Level Control Structures

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 Counter-Controlled Loops

– Consider the following example:

for count in [2, 4, 6] :

print count

produces

2
4
6

Statement-Level Control Structures

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 Thank you

to be continued…

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 Activity – group
• Design Your Own Loop Construct
– When designing a new loop, you should define:
• Initialization: How do you set up the loop to start? (2 points)
• Condition: What condition determines when the loop stops? (2 points)
• Iteration: How does the loop move forward? This could be updating variables or
changing states. (2 points)
– Define the Syntax: (4 point)
• Decide how your loop should look in terms of syntax.
Set Up Loop Initialization (5 points):
Define how the loop will start. For example: You may want to initialize variables or objects that
will be used inside the loop.
Define the Loop Condition (5 points):
Determine what will stop the loop. Is it based on a boolean condition? A counter reaching a
certain value?
Specify Iteration Behavior (5 points):
Define how the loop will "move" forward. Does it increment a variable, process data, or check a
different condition after each iteration?
Decide on Loop Termination (5 points):
Create rules for when the loop will stop. If you're designing a repeat-until loop, the condition is
checked after the body of the loop. For a while loop, it's checked before executing the body.

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