Control Statements By Anil Dutt

1. CONTROL STATEMENTS
Anil Dutt

2. 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

3. 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

4. Logic of an if statement
condition
evaluated
statement
true
false

5. The if Statement
• The if statement has the following syntax:
if ( condition )
statement;
if is a C
reserved word
The condition must be a
boolean expression. It must
evaluate to either true or false.
If the condition is true, the statement is executed.
If it is false, the statement is skipped.

6. For example:
if (hours > 70)
hours = hours + 100;
printf("Less hours, no bonus!n");
 If hours is less than or equal to 70, its value will remain unchanged and the printf() will
be executed.
 If it exceeds 70, its value will be increased by 100.
if(jobCode == '1')
{
carAllowance = 100.00;
housingAllowance = 500.00;
entertainmentAllowance = 300.00;
}
printf("Not qualified for car, housing and entertainment allowances!");
The three statements enclosed in the curly braces { } will only be executed if jobCode is equal
to '1', else the printf() will be executed.

7. Logic of an if-else statement
condition
evaluated
statement1
true false
statement2

8. The if-else Statement
• An else clause can be added to an if
statement to make an if-else statement
if ( 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

9. For example:
if(myCode == '1')
rate = 7.20;
else
rate = 12.50;
If myCode is equal to '1', the rate is 7.20 else, if myCode
is not equal to '1' the rate is 12.50.
Anil Dutt
Equal/not equal (=) is not a value comparison, but a character comparison!!!

10. 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 (=)

11. 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.

12. 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++;
}

13. 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;
}

14. 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

15.  The if-else constructs can be nested (placed one within another) to any depth.
 General forms: if-if-else and if-else-if.
 The if-if-else constructs has the following form (3 level of depth example),
if(condition_1)
if(condition_2)
if(condition_3)
statement_4;
else
statement_3;
else
statement_2;
else
statement_1;
next_statement;
Anil Dutt

16. 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

17. 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

18. The switch Statement
switch (option)
{
case 'A':
aCount++;
break;
case 'B':
bCount++;
break;
case 'C':
cCount++;
break;
default:
otherCount++;
break;
}
• An example of a switch statement:

19. The 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

20. The switch Statement
• 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

21. 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 ...
}
switch
and
case
are
reserved
words
If expression
matches value2,
control jumps
to here

23. 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.
• Loop is a control structure that repeats a group of steps
in a program.
– Loop body stands for the repeated statements.
• There are three C loop control statements:
– while, for, and do-while.
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24. 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

25. Comparison of Loop Choices (1/2)
Kind When to Use C Structure
Counting 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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26. Comparison of Loop Choices (2/2)
Kind When to Use C Structure
Input 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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27. The while Statement in C
• The syntax of while statement in C:
while (loop repetition condition)
statement
• Loop repetition condition is the condition which controls the
loop.
• The statement is repeated as long as the loop repetition
condition is true.
• A loop is called an infinite loop if the loop repetition condition is
always true.
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28. An Example of a while Loop
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Statement
Loop repetition condition
Loop control variable is the variable whose value controls loop
repetition.
In this example, count_emp is the loop control variable.

29. Flowchart for a while Loop
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Loop repetition condition
Statement

30. Compound Assignment Operators (1/2)
• The loop body usually consists of statements of the
form: variable = variable op expression.
– e.g., count_emp = count_emp + 1;
• C provides compound assignment operators which
enable a more concise notation for this kind of
statements.
– “variable op = expression” is the same to
“variable = variable op expression.”

31. Compound Assignment Operators (2/2)
Simple Assignment Operators Compound Assignment
Operators
count_emp = count_emp +
1;
count_emp += 1;
time = time -1; time -= 1;
product = product *
item;
product *= item;
total = total / number; total /= number;
n = n % (x+1); n %= x+1;
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32. The for Statement in C
• The syntax of for statement in C:
for (initialization expression;
loop repetition condition;
update expression)
statement
• The initialization expression set the initial value of the loop
control variable.
• The loop repetition condition test the value of the loop control
variable.
• The update expression update the loop control variable.
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33. An Example of the for Loop
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Loop repetition condition
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.

34. 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++){ … }
– The variable i increase one after each iteration.
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35. Comparison of Prefix and Postfix Increments
Copyright ©2004 Pearson Addison-Wesley. All
rights reserved.
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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

36. Nested Loops
• Nested loops consist of an outer loop with one or more
inner loops.
• e.g.,
for (i=1;i<=100;i++){
for(j=1;j<=50;j++){
…
}
}
• The above loop will run for 100*50 iterations.
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Inner loop
Outer loop

37. The do-while Statement in C
• The syntax of do-while statement in C:
do
statement
while (loop repetition condition);
• The statement is first executed.
• If the loop repetition condition is true, the statement is
repeated.
• Otherwise, the loop is exited.
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38. 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.