Transcript Switch, Do..While, For statements

Programming in C++
Dale/Weems/Headington
Chapter 9
Additional Control Structures
(Switch, Do..While, For statements)
1
Switch Statement
Is a selection control structure for multi-way branching.
SYNTAX
switch ( IntegralExpression )
{
case Constant1 :
Statement(s);
case Constant2 :
Statement(s);
// optional
// optional
.
.
.
default :
Statement(s);
}
// optional
// optional
2
float
char
weightInPounds = 165.8 ;
weightUnit ;
. . .
// user enters letter for desired weightUnit
switch ( weightUnit )
{
case ‘P’:
case ‘p’:
cout << weightInPounds << “pounds”<< endl ;
break ;
case ‘O’:
case ‘o’:
cout << 16.0 * weightInPounds << “ounces”<< endl ;
break ;
case ‘K’:
case ‘k’:
cout << weightInPounds / 2.2 << “kilos”<< endl ;
break ;
case ‘G’:
case ‘g’:
cout << 454.0 * weightInPounds << “grams”<< endl ;
break ;
default :
cout << “That unit is not handled!”<< endl ;
break ;
}
Switch Statement
l
The value of IntegralExpression (of char,
short, int, long or enum type ) determines
which branch is executed.
l
Case labels are constant ( possibly named )
integral expressions. Several case labels
can precede a statement.
4
Control in Switch Statement
l
Control branches to the statement following the case
label that matches the value of IntegralExpression.
Control proceeds through all remaining statements,
including the default, unless redirected with break.
l
If no case label matches the value of
IntegralExpression, control branches to the default
label, if present. Otherwise control passes to the
statement following the entire switch statement.
l
Forgetting to use break can cause logical errors
because after a branch is taken, control proceeds
sequentially until either break or the end of the
switch statement occurs.
5
Do-While Statement
Is a looping control structure in which the loop
condition is tested after each iteration of the
loop.
SYNTAX
do
{
Statement
} while ( Expression ) ;
Loop body statement can be a single statement or a block.
6
Function Using Do-While
void GetYesOrNo ( /* out */ char& response )
// Inputs a character from the user
// Postcondition:
//
response has been input
&& response == ‘y’ or ‘n’
{
do
{
cin >> response ;
// skips leading whitespace
if ( ( response != ‘y’) && ( response != ‘n’) )
cout << “Please type y or n : ?”;
} while ( ( response != ‘y’) && ( response != ‘n’) ) ;
}
7
Do-While Loop vs. While Loop
l
l
l
POST-TEST loop
(exit-condition)
The looping condition
is tested after
executing the loop
body.
Loop body is always
executed at least
once.
l
l
l
PRE-TEST loop
(entry-condition)
The looping condition
is tested before
executing the loop
body.
Loop body may not
be executed at all.
8
Do-While Loop
DO
Statement
WHILE
Expression
TRUE
FALSE
When the expression is tested and found to be false,
the loop is exited and control passes to the
9
statement that follows the do-while statement.
A Count-Controlled Loop
SYNTAX
for ( initialization ; test expression ; update )
{
0 or more statements to repeat
}
10
The for loop contains
an initialization
an expression to test for
continuing
an update to execute after
each iteration of the body
11
Example of Repetition
int num;
for ( num = 1 ; num <= 3 ; num++ )
{
cout << num << “Potato” << endl;
}
12
The output was:
1Potato
2Potato
3Potato
13
What happens?
l
l
l
l
l
l
l
l
l
l
l
l
num is initialized to 1
expression num <= 3 is evaluated as true
body is executed displaying 1Potato
update executed gives num value 2
expression num <= 3 is evaluated as true
body is executed displaying 2Potato
update executed gives num value 3
expression num <= 3 is evaluated as true
body is executed displaying 3Potato
update executed gives num value 4
expression num <= 3 is evaluated as false
so loop ends
14
Count-controlled loop
int count ;
for ( count = 4 ; count > 0 ; count-- )
{
cout << count << endl;
}
cout << “Done” << endl;
OUTPUT:
4
3
2
1
Done
15
What is output?
int count;
for ( count = 0 ; count < 10 ; count++ )
{
cout << * ;
}
16
OUTPUT
**********
NOTE: the 10 asterisks are all on one line. Why?
17
What output from this loop?
int count;
for (count = 0; count < 10; count++) ;
{
cout << * ;
}
18
OUTPUT
l
l
l
l
l
No output from the for loop! Why?
The ; right after the ( ) means that the
body statement is a null statement
In general, the Body of the for loop is whatever
statement immediately follows the ( )
That statement can be a single statement, a
block, or a null statement.
Actually, the code outputs one * after the loop
completes its counting to 10.
19
Several Statements in Body Block
const int MONTHS = 12 ;
int
count ;
float bill ;
float sum = 0.0 ;
for (count = 1; count <= MONTHS; count++ )
{
cout << “Enter bill: “;
cin >> bill ;
sum = sum + bill ;
}
cout << “our total bill is : “ << sum << endl ;
20
Break Statement
l
Break statement can be used with Switch or
any of the 3 looping structures.
l
It causes an immediate exit from the Switch,
While, Do-While, or For statement in which it
appears.
l
If the break is inside nested structures,
control exits only the innermost structure
containing it.
21
Continue Statement
l
Is valid only within loops.
l
Terminates the current loop iteration, but
not the entire loop.
l
In a For or While, continue causes the rest
of the body statement to be skipped. In a
For statement, the update is done.
l
In a Do-While, the exit condition is tested,
and if true, the next loop iteration is begun.
22
Imagine using . . .
l
a character, a length, and a width to draw a
box. For example,
l
using the values 4, and 6 would display
&&&&&&
&&&&&&
&&&&&&
&&&&&&
23
Write prototype for void function
called DrawBox ( ) with 3 parameters.
The first is type char, the other 2 are type int.
void DrawBox( char, int , int );
NOTE: Some C++ books include identifiers in
prototypes. Any valid C++ identifiers, as long
as each is different, can be used.
void DrawBox( char letter, int Num1, int Num2);
24
void DrawBox(char What, int Down, int Across)
// 3 formal parameters
{
int row, col;
// 2 local variables
for ( row = 0; row < Down; row++ )
{
for (col = 0; col < Across; col++ )
{
cout << What;
}
cout << endl;
}
return;
}
THE DRIVER PROGRAM
#include <iostream.h>
void DrawBox (char, int, int);
// prototype
int main (void)
{
char letter = ‘A’;
DrawBox(letter, 4, 2*3); // actual parameters
DrawBox(‘B’, 9, 3);
// appear in call
return 0;
}
26
Write a function using prototype
void DisplayTable ( int ) ;
// prototype
The function displays a specified multiplication
table. For example, the call DisplayTable(6)
displays this table:
1 x6 = 6
2 x 6 = 12
3 x 6 = 18
.
.
.
12 x 6 = 72
27