Copyright 2004-2005 - Curt Hill The C/C++ switch Statement A multi-path decision statement.

Copyright 2004-2005 - Curt Hill

The C/C++ switch Statement

A multi-path decision statement


Flow of Control in C/C++• Recall the flow statements in C and

C++:– Decisions

• If• Switch Case

– Loops• for• while• do• Break

• Here we consider the switch


Why?• There are times when we would

use the following type of nested ifif(a == 2) x = 5;else if(a == 4) { x = 12 * y;… }else if(a == 8) {…}else if(a == 12) {…}else cout << “Error”;


General Form of This• Each if has a similar condition

– One variable compared with a constant– Comparison is always equality

• The Then statement does something• The Else statement does another if• There is only one variable and one

comparison in all the ifs• Generally such a construction needs

a switch-case not an if


If and Switch• An if is a decision of one path out

of two– A bool has only two values

• A switch is a decision of one path out of many

• A switch is based on an int, char, enumeration – Which may have many values


General Form of Switch

switch (integral expression) { case con1: many statements break;

case con2: many statements break;

case con3: many statements break;…}


Notes• switch, case and break are reserved

words• The parenthesized expression is

evaluated to obtain a value– This value chooses the path– The type of the value must be an integer,

character or enumeration– May not be floating point, string or any

object

• The case indicates a path– Must be followed by a constant– The constant value following the case must

match the type of the expression– It may be a defined, literal or constant

declaration


Exampleswitch (credit_hours / 32) {

case 0:

standing = “freshman”;

break;

case 1:

standing = “sophomore”;

break;

case 2:

standing = “junior”;

break;

case 3:

standing = “senior”;

break;

}


More Notes• There is only one set of braces

which surrounds all the cases• The path starts with a case and

ends with a break• Multiple cases allow for multiple

ways to identify a path• The cases do not have to be in any

particular order• Duplicate cases are not allowed

– Compiler actually checks for this


Multiple case exampleswitch (grade) {

case ‘a’:

case ‘A’:

points = 4 * credit_hours;

break;

case ‘b’:

case ‘B’:

points = 3 * credit_hours;

break;

…

}


Missing Path

• What happens if the switch expression does not match any case label?

• Equivalent to the correct case followed by a break– It does nothing

• The default label allows a catch all case


Example Revisited

AnsiString standing;switch (credit_hours / 32) { case 0: standing = “freshman”; break;… case 3: standing = “senior”; break;default: standing = “unknown”; cout << “Error”;

}


default

• default is a reserved word• The default label acts like a case label

with out case or specifying a value• It catches any value not explicitly

mentioned in a case• It is customarily last, but does not have

to be there• Leaving it out is the same as doing

nothing for all the unmentioned cases


The Break Statement

• The break has the effect of leaving a switch or loop

• The last path does not need a break• A return will also work since it

leaves the entire function/method not just the switch

• It is not required, but leaving it out makes for interesting flow


Leaving out a break

• If a break is left out, execution will drop into the next part

• Leaving out the last break is not a problem since there is no next part

• Consider the following example


Omitted break exampleswitch (val) { case 5: x += 5; case 12: x += 8; y = 2; break; case 6: x += 8; y = 4; break;}

If val is 5, x will have 13 added to it and y will be set to 2.

All the effect of val == 12 is also executed for x == 5.


Omitted break notes

• This feature, inherited from C, is both good and bad

• It is somewhat good in that it allows an economy of code– Very few switches actually use this

feature

• It is usually bad because we seldom want it but the compiler never gives an error when a break is left out


Equivalence of if and switch

• We have already seen an if doing what a switch can do

• The reverse is also true:switch(b>c){ case true: …. break; case false: …. break;}

• Always use what is more natural


Common mistakes

• We cannot use strings or other objects for the case expression or labels

• We cannot use floating point values (float or double) for the expression or case labels– Recall the problems with floating

point equality


Uses• switch is much less used than an if

– Just not as much need for it

• Often used to decode an integer or enumeration into a string– Month number into a name

• Used to select different actions for these as well

• Often used for a menu in console programs


Readability• Indenting things is again the rule• My choice:

– Indent the case – Indent the code past the case

• Example:switch (item) { case 1: duh = duh2; … break case 2: … }

Enumerations and Switch

• We frequently use switch to input, output and decode enumerations

• Lets look at enumeration examples• Suppose:enum colortype {ctRed, ctGreen, ctBlue, ctWhite, ctBlack} color;


Display• For displayable output use a switch:• String s = “None”;switch (color) { case ctRed: s = “Red”; break; case ctGreen: s = “Green”; break;… } // end of switch


Copyright © 2005-2007 Curt Hill

Input

• Several approaches• Read in integer and use switch

case to make into enumeration• Read in string and use nested ifs to

create enumeration

Input with integer• In a console program:cout << “Enter color:”;int val;cin >> val;switch (val){ case 0: color = ctRed; break; case 1: …

• The same may be done using text input from windows


Decoding• Decoding is using an enumeration

to perform different actions• Consider:switch (action){ case aExit: Close(); break; case aAbout: Application->MessageBox(……


Finally• Switch is the one of many decision• Switch is the least used decision

– Many more ifs than switches– Less frequently used than most loops

• Use it when the pattern is seenif (a==1) …else if (a==3)…else if (a==2) …

Copyright © 2005-2007 Curt Hill

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Copyright 2004-2005 - Curt Hill The C/C++ switch Statement A multi-path decision statement.

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