EVAL Evaluate a String

Section: FreeMat Functions

Usage

The eval function evaluates a string. The general syntax for its use is 
   eval(s)

where s is the string to evaluate. If s is an expression (instead of a set of statements), you can assign the output of the eval call to one or more variables, via 

   x = eval(s)
   [x,y,z] = eval(s)

Another form of eval allows you to specify an expression or set of statements to execute if an error occurs. In this form, the syntax for eval is 

   eval(try_clause,catch_clause),

or with return values, 

   x = eval(try_clause,catch_clause)
   [x,y,z] = eval(try_clause,catch_clause)

These later forms are useful for specifying defaults. Note that both the try_clause and catch_clause must be expressions, as the equivalent code is 

  try
    [x,y,z] = try_clause
  catch
    [x,y,z] = catch_clause
  end

so that the assignment must make sense in both cases.

Example

Here are some examples of eval being used. 
--> eval('a = 32')

a = 
 32 

--> b = eval('a')

b = 
 32

The primary use of the eval statement is to enable construction of expressions at run time. 

--> s = ['b = a' ' + 2']

s = 
b = a + 2
--> eval(s)

b = 
 34

Here we demonstrate the use of the catch-clause to provide a default value 

--> a = 32

a = 
 32 

--> b = eval('a','1')

b = 
 32 

--> b = eval('z','a+1')

b = 
 33

Note that in the second case, b takes the value of 33, indicating that the evaluation of the first expression failed (because z is not defined).