Control structure

CONTROL STRUCTURE in C

Flow of Control• Unless specified otherwise, the order of statement

execution through a function is linear: one statement after another in sequence

• Some programming statements allow us to:

– decide whether or not to execute a particular statement– execute a statement over and over, repetitively

• These decisions are based on boolean expressions (or conditions) that evaluate to true or false

• The order of statement execution is called the flow of control

Conditional Statements• A conditional statement lets us choose which statement

will be executed next

• Therefore they are sometimes called selection statements

• Conditional statements give us the power to make basic decisions

• The C conditional statements are the:

– if statement– if-else statement– switch statement

Logic of an if statement

conditionevaluated

statement

truefalse

The if Statement• The if statement has the following syntax:

if ( condition ) statement;

if is a Creserved word

The condition must be aboolean expression. It mustevaluate to either true or false.

If the condition is true, the statement is executed.If it is false, the statement is skipped.

Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

The if-else Statement• An else clause can be added to an if

statement to make an if-else statementif ( condition ) statement1;else statement2;

• If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

• One or the other will be executed, but not both

Boolean Expressions• A condition often uses one of C's equality operators or relational

operators, which all return boolean results:

== equal to!= not equal to< less than> greater than<= less than or equal to>= greater than or equal to

• Note the difference between the equality operator (==) and the assignment operator (=)

Boolean Expressions in C• C does not have a boolean data type.

• Therefore, C compares the values of variables and expressions against 0 (zero) to determine if they are true or false.

• If the value is 0 then the result is implicitly assumed to be false.

• If the value is different from 0 then the result is implicitly assumed to be true.

• C++ and Java have boolean data types.

Block Statements

• Several statements can be grouped together into a block statement delimited by braces

• A block statement can be used wherever a statement is called for in the C syntax rules

if (total > MAX){ printf ("Error!!\n"); errorCount++;}

Block Statements• In an if-else statement, the if portion, or the else portion, or both, could be block statements

if (total > MAX){ printf("Error!!"); errorCount++;}else{ printf ("Total: %d“, total); current = total*2;}

Nested if Statements• The statement executed as a result of an if statement or else clause could be another if statement

• These are called nested if statements

• An else clause is matched to the last unmatched if (no matter what the indentation implies)

• Braces can be used to specify the if statement to which an else clause belongs

The switch Statement

• The switch statement provides another way to decide which statement to execute next

• The switch statement evaluates an expression, then attempts to match the result to one of several possible cases

• Each case contains a value and a list of statements

• The flow of control transfers to statement associated with the first case value that matches

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The switch Statement• Often a break statement is used as the last statement in

each case's statement list

• A break statement causes control to transfer to the end of the switch statement

• If a break statement is not used, the flow of control will continue into the next case

• Sometimes this may be appropriate, but often we want to execute only the statements associated with one case


switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break; default: otherCount++; break;}

• An example of a switch statement:


• A switch statement can have an optional default case

• The default case has no associated value and simply uses the reserved word default

• If the default case is present, control will transfer to it if no other case value matches

• If there is no default case, and no other value matches, control falls through to the statement after the switch


• The expression of a switch statement must result in an integral type, meaning an integer (byte, short, int,) or a char

• It cannot be a floating point value (float or double)

• The implicit test condition in a switch statement is equality

• You cannot perform relational checks with a switch statement

The switch Statement• The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

Repetition in Programs• In most software, the statements in the

program may need to repeat for many times.– e.g., calculate the value of n!.– If n = 10000, it’s not elegant to write the code as

1*2*3*…*10000.• LoopLoop is a control structure that repeats a group

of steps in a program.– Loop bodyLoop body stands for the repeated statements.

• There are three C loop control statements:– whilewhile, forfor, and do-whiledo-while.

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Flow Diagram of Loop Choice Process

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e.g., calculate the value of n!

e.g., read the content in a file

Comparison of Loop Choices (1/2)

Kind When to Use C StructureCounting loop We know how many loop

repetitions will be needed in advance.

while, for

Sentinel-controlled loop

Input of a list of data ended by a special value

while, for

Endfile-controlled loop

Input of a list of data from a data file

while, for

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Comparison of Loop Choices (2/2)

Kind When to Use C StructureInput validation loop

Repeated interactive input of a value until a desired value is entered.

do-while

General conditional loop

Repeated processing of data until a desired condition is met.

while, for

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The while Statement in C• The syntax of while statement in C:while (loop repetition conditionloop repetition condition)

statement• Loop repetition conditionLoop repetition condition is the condition

which controls the loop.• The statement is repeated as long as the loop

repetition condition is truetrue.• A loop is called an infinite loopinfinite loop if the loop

repetition condition is always true.5-23

An Example of a while Loop

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Statement

Loop repetition condition

Loop control variableLoop control variable is the variable whose value controls loop repetition.

In this example, count_emp is the loop control variable.

Flowchart for a while Loop

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Statement

The for Statement in C• The syntax of for statement in C:for (initialization expressioninitialization expression;

loop repetition conditionloop repetition condition; update expressionupdate expression) statement

• The initialization expressioninitialization expression set the initial value of the loop control variable.

• The loop repetition conditionloop repetition condition test the value of the loop control variable.

• The update expressionupdate expression update the loop control variable.

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An Example of the for Loop

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Initialization Expression

Update Expression

count_emp is set to 0 initially.

count_emp should not exceed the value of number_emp.

count_emp is increased by one after each iteration.

Increment and Decrement Operators

• The statements of increment and decrement are commonly used in the for loop.

• The increment (i.e., ++++) or decrement (i.e., ----) operators are the frequently used operators which take only one operand.

• The increment/decrement operators increase or decrease the value of the single operand. – e.g., for (int i = 0; i < 100; i++i++){ … }– The variable i increase one after each iteration.

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Comparison of Prefix and Postfix Increments

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The value of the expression (that uses the ++/-- operators) depends on the position of the operator.

The value of j is

increased

The value of j is not increased

The do-while Statement in C

• The syntax of do-while statement in C:do

statementwhile (loop repetition conditionloop repetition condition);

• The statement is first executed.• If the loop repetition conditionloop repetition condition is true, the

statement is repeated.• Otherwise, the loop is exited.

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An Example of the do-while Loop

/* Find even number input */do{

printf(“Enter a value: ”);scanf(“%d”, &num);

}while (num % 2 !=0)

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This loop will repeat if the user inputs odd number.

Control structure

Engineering