Continue on With Switch Cases Matlab

Program Control

This section covers the following topics:

• Using break, continue, and return
• Using switch Versus if
• MATLAB case Evaluates Strings
• Multiple Conditions in a case Statement
• Implicit Break in switch/case
• Variable Scope in a switch
• Catching Errors with try/catch
• Nested try/catch Blocks
• Forcing an Early Return from a Function

Using break, continue, and return

It's easy to confuse the break, continue, and return functions as they are similar in some ways. Make sure you use these functions appropriately.

Function	Where to Use It	Description
break	for or while loops	Exits the loop in which it appears. In nested loops, control passes to the next outer loop.
continue	for or while loops	Skips any remaining statements in the current loop. Control passes to next iteration of the same loop.
return	Anywhere	Immediately exits the function in which it appears. Control passes to the caller of the function.

Using switch Versus if

It is possible, but usually not advantageous, to implement switch/case statements using if/elseif instead. See pros and cons in the table.

switch/case Statements	if/elseif Statements
Easier to read	Can be difficult to read
Can compare strings of different lengths	You need strcmp to compare strings of different lengths
Test for equality only	Test for equality or inequality

MATLAB case Evaluates Strings

A useful difference between switch/case statements in MATLAB and C is that you can specify string values in MATLAB case statements, which you cannot do in C.

• switch(method)    case 'linear'       disp('Method is linear')    case 'cubic'       disp('Method is cubic') end          

Multiple Conditions in a case Statement

You can test against more than one condition with switch. The first case below tests for either a linear or bilinear method by using a cell array in the case statement.

• switch(method)    case {'linear', 'bilinear'}       disp('Method is linear or bilinear')    case (<and so on>) end          

Implicit Break in switch/case

In C, if you don't end each case with a break statement, code execution falls through to the following case. In MATLAB, case statements do not fall through; only one case may execute. Using break within a case statement is not only unnecessary, it is also invalid and generates a warning.

In this example, if result is 52, only the first disp statement executes, even though the second is also a valid match:

• switch(result)    case 52       disp('result is 52')    case {52, 78}       disp('result is 52 or 78') end          

Variable Scope in a switch

Since MATLAB executes only one case of any switch statement, variables defined within one case are not known in the other cases of that switch statement. The same holds true for if/ifelse statements.

In these examples, you get an error when choice equals 2, because x is undefined.

•                                           -- SWITCH/CASE --                    -- IF/ELSEIF --                          switch choice    case 1                          if choice == 1       x = -pi:0.01:pi;                x = -pi:0.01:pi;    case 2                          elseif choice == 2       plot(x, sin(x));                plot(x, sin(x)); end                                end          

Catching Errors with try/catch

When you have statements in your code that could possibly generate unwanted results, put those statements into a try/catch block that will catch any errors and handle them appropriately.

The example below shows a try/catch block within a function that multiplies two matrices. If a statement in the try segment of the block fails, control passes to the catch segment. In this case, the catch statements check the error message that was issued (returned by lasterr) and respond appropriately.

• try    X = A * B catch    errmsg = lasterr;    if(strfind(errmsg, 'Inner matrix dimensions'))       disp('** Wrong dimensions for matrix multiply') end          

For more information: See Checking for Errors with try-catch in the MATLAB "Programming and Data Types" documentation

Nested try/catch Blocks

You can also nest try/catch blocks, as shown here. You can use this to attempt to recover from an error caught in the first try section:

• try    statement1                    % Try to execute statement1 catch    try       statement2                 % Attempt to recover from error    catch       disp 'Operation failed'    % Handle the error    end end          

Forcing an Early Return from a Function

To force an early return from a function, place a return statement in the function at the point where you want to exit. For example,

• if <done>    return end          

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Source: http://www.ece.northwestern.edu/local-apps/matlabhelp/techdoc/matlab_prog/ch8_pr13.html