MATLAB: Error:Unexpected MATLAB expression.

beginnerMATLAB

Part o f a program I am writing goes as folows:
    if(y2(i)>y1(i)) 
                         ntra=ceil((y2(i)-y1(i))/h;
                         htra=(y2(i)-y1(i))/ntra; 
                         y=[y1(i):htra:y2(i)];
                         x=xtab(i);
                         ytabtra=eval(f);
The definition of f beeing:
temp=input('veuillez encoder la fonction principale f(x,y):','s');
    f= vectorize(temp);
I tested it with the input '3x+y'
When I programed this yesterday, everything worked, but when I tried again today I get:
??? Error: Unexpected MATLAB expression.
Error in ==> principal_1 at 45
                     ytabtra=eval(f);
Could anybody help me please?

Best Answer

Well, what if "f"? You never assigned anything to it, so when it gets to eval(f) it doesn't know what f is. What were you intending? Are you sure that your code assigning f actually was run before you hit the eval statement? Set a breakpoint there to check and make sure f has some string value.
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