Runtime evaluation: Difference between revisions

=={{header|MATLAB}}==

The eval and evalin functions handles any kind of code. It can handle multi-line code, although it needs the lines to be separated by the newline character. It can even allow you to program at runtime, as illustrated in the last example in the code and output below. Errors can occur when mixing eval statements with regular code, especially "compile-time" errors if the code appears to be missing key elements (ending brackets or end statements, etc). Some of these are also demonstrated.

<lang MATLAB>function testEval

fprintf('Expressions:\n')

x = eval('5+10^2')

eval('y = (x-100).*[1 2 3]')

eval('z = strcat(''my'', '' string'')')

try

w eval(' = 45')

catch

fprintf('Runtime error: interpretation of w is a function\n\n')

end

% eval('v') = 5

% Invalid at compile-time as MATLAB interprets as using eval as a variable

fprintf('Other Statements:\n')

nl = sprintf('\n');

eval(['for k = 1:20' nl ...

'fprintf(''%.3f\n'', k)' nl ...

'if k == 3' nl ...

'break' nl ...

'end' nl ...

'end'])

true == eval('1')

try

true eval(' == 1')

catch

fprintf('Runtime error: interpretation of == 1 is of input to true\n\n')

end

fprintf('Programming on the fly:\n')

userIn = true;

codeBlock = '';

while userIn

userIn = input('Enter next line of code: ', 's');

codeBlock = [codeBlock nl userIn];

end

eval(codeBlock)

end</lang>

{{out}}

<pre>Expressions:

x =

105

y =

5 10 15

z =

my string

Runtime error: interpretation of w is a function

Other Statements:

1.000

2.000

3.000

ans =

1

Runtime error: interpretation of == 1 is of input to true

Programming on the fly:

Enter next line of code: fprintf('Goodbye, World!\n')

Enter next line of code: str = 'Ice and Fire';

Enter next line of code: words = textscan(str, '%s');

Enter next line of code: fprintf('%s ', words{1}{end:-1:1})

Enter next line of code:

Goodbye, World!

Fire and Ice</pre>