Program Control :: Programming Tips (Programming)

Programming

Program Control

This section covers the following topics:

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.

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.

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-elseif 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 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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