CONTROL STATEMENTS
Lakhbir Singh(Lect.IT)
S.R.S.G.P.C.G. Ludhiana
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
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
conditionconditionconditioncondition evaluatedevaluatedevaluatedevaluated
statementstatementstatementstatement
truetruetruetrue falsefalsefalsefalse
The if Statement • The if statement has the following syntax:
if ( condition ) statement;
if is a Cis a Cis a Cis a C reserved wordreserved wordreserved wordreserved word
The The The The condition must be amust be amust be amust be a boolean expression. It mustboolean expression. It mustboolean expression. It mustboolean expression. It must evaluate to either true or false.evaluate to either true or false.evaluate to either true or false.evaluate to either true or false.
If the If the If the If the condition is true, the is true, the is true, the is true, the statement is executed.is executed.is executed.is executed. If it is false, the If it is false, the If it is false, the If it is false, the statement is skipped.is skipped.is skipped.is skipped.
Logic of an if-else statement
conditionconditionconditioncondition evaluatedevaluatedevaluatedevaluated
statement1statement1statement1statement1
truetruetruetrue falsefalsefalsefalse
statement2statement2statement2statement2
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
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 © 2004 Pearson Addison-Wesley. All rights reserved
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
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:
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
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
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 andandandand case areareareare
reservedreservedreservedreserved wordswordswordswords
If If If If expression matches matches matches matches value2,,,, control jumpscontrol jumpscontrol jumpscontrol jumps to hereto hereto hereto 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.
• 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. 5-19
Flow Diagram of Loop Choice Process
Copyright ©2004 Pearson Addison-Wesley.
All rights reserved. 5-20
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 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
5-21
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
5-22
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.
5-23
An Example of a while Loop
5-24
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.
Flowchart for a while Loop
5-25
Loop repetition condition
Statement
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.”
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; 5-27
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. 5-28
An Example of the for Loop
5-29
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.
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.
5-30
Comparison of Prefix and Postfix
Increments
Copyright ©2004 Pearson Addison-Wesley.
All rights reserved. 5-31
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
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. 5-32
Inner loop
Outer loop
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.
5-33
An Example of the do-while Loop
/* Find even number input */
do{ printf(“Enter a value: ”);
scanf(“%d”, &num);
}while (num % 2 !=0)
5-34
This loop will repeat if the user
inputs odd number.
Top Related