matlab避免用eval的方法

Alternatives to the eval Function

Why Avoid the eval Function?

Although the 

eval

 function is very powerful and flexible, it not always the best solution to a programming
problem. Code that calls 

eval

 is often less efficient and more difficult to read and debug than code
that uses other functions or language constructs. For example:

MATLAB® compiles code the first time you run it to enhance performance for future runs. However, because code in an 

eval

 statement
can change at run time, it is not compiled.

Code within an 

eval

 statement can unexpectedly create or assign to a variable already in the current
workspace, overwriting existing data.

Concatenating strings within an 

eval

 statement is often difficult to read. Other language constructs
can simplify the syntax in your code.

For many common uses of 

eval

, there are preferred alternate approaches, as shown in the following examples.

Variables with Sequential Names

A frequent use of the 

eval

 function is to create sets of variables such as 

A1

, 

A2

, 

...

, 

An

,
but this approach does not use the array processing power of MATLAB and is not recommended. The preferred method is to store related data in a single array. If the data sets are of different types or sizes, use a structure or cell array.

For example, create a cell array that contains 10 elements, where each element is a numeric array:

numArrays = 10;
A = cell(numArrays,1);
for n = 1:numArrays
A{n} = magic(n);
end

Access the data in the cell array by indexing with curly braces. For example, display the fifth element of 

A

:

A{5}

ans =
17    24     1     8    15
23     5     7    14    16
4     6    13    20    22
10    12    19    21     3
11    18    25     2     9

The assignment statement 

A{n} = magic(n)

 is more elegant and efficient than this call to 

eval

:

eval(['A', int2str(n),' = magic(n)'])     % Not recommended

For more information, see:

Create a Cell Array

Create a Structure Array

Files with Sequential Names

Related data files often have a common root name with an integer index, such as 

myfile1.mat

 through 

myfileN.mat

.
A common (but not recommended) use of the

eval

 function is to construct and pass each file name to
a function using command syntax, such as

eval(['save myfile',int2str(n),'.mat'])     % Not recommended

The best practice is to use function syntax, which allows you to pass variables as inputs. For example:

currentFile = 'myfile1.mat';
save(currentFile)

You can construct file names within a loop using the 

sprintf

 function
(which is usually more efficient than 

int2str

), and then call the 

save

 function
without 

eval

. This code creates 10 files in the current folder:

numFiles = 10;
for n = 1:numFiles
randomData = rand(n);
currentFile = sprintf('myfile%d.mat',n);
save(currentFile,'randomData')
end

For more information, see:

Command vs. Function Syntax

Import or Export a Sequence of Files

Function Names in Variables

A common use of 

eval

 is to execute a function when the name of the function is in a variable string.
There are two ways to evaluate functions from variables that are more efficient than using 

eval

:

Create function handles with the 

@

 symbol or with the 

str2func

 function.
For example, run a function from a list stored in a cell array:

examples = {@odedemo,@sunspots,@fitdemo};
n = input('Select an example (1, 2, or 3): ');
examples{n}()

Use the 

feval

 function.
For example, call a plot function (such as 

plot

, 

bar

,
or 

pie

) with data that you specify at run time:

plotFunction = input('Specify a plotting function: ','s');
data = input('Enter data to plot: ');
feval(plotFunction,data)

Field Names in Variables

Access data in a structure with a variable field name by enclosing the expression for the field in parentheses. For example:

myData.height = [67, 72, 58];
myData.weight = [140, 205, 90];

fieldName = input('Select data (height or weight): ','s');
dataToUse = myData.(fieldName);

If you enter 

weight

 at the input prompt, then you can find the minimum 

weight

 value
with the following command.

min(dataToUse)

ans =
90

For an additional example, see Generate Field
Names from Variables.

Error Handling

The preferred method for error handling in MATLAB is to use a 

try,
catch

 statement. For example:

try
B = A;
catch exception
disp('A is undefined')
end

If your workspace does not contain variable 

A

, then this code returns:

A is undefined

Previous versions of the documentation for the 

eval

 function include the syntax 

eval(expression,catch_expr)

.
If evaluating the 

expression

 input returns an error, then 

eval

 evaluates 

catch_expr

.
However, an explicit 

try/catch

 is significantly clearer than an implicit catch in an 

eval

 statement.
Using the implicit catch is not recommended.