matlab script for f, df and d2f using excel

Hello

I'm a biologystudent trying to write some code. What I wanted to do is taking a function (i've choosen the simple f=a.*t.^3+b.*t.^2+c.*t+d but the original formula is more complex). Next I wanted to derive the first and second derivative (step1). Here the outcome seems like expected.

For step2 an excel table test0 with the values for a,b,c and d was uploaded. Every row has different values for a,b,c and d and there are 10 rows. If I use the following code I get 10 different f values with the first function has the values for a,b,c and d from the first row. The second function has the values for a,b,c and d from the second row untill the tenth row.

The outcome for f seems as expected. However df and d2f in my outcome just give the values of f again ten times.

Can Anyone point out where in my code I made a mistake?

Thanks

My code: ----------------------------------------------------------------------

step1: analytical deriving f, df and d2f

    t=sym('t');
    a=sym('a');
    b=sym('b');
    c=sym('c');
    d=sym('d');
    f=a.*t.^3+b.*t.^2+c.*t+d
    df=diff(f,t,1)
    d2f=diff(f,t,2)

step2: import excelfile to replace variables

    ar=xlsread('test0','A:A');
    br=xlsread('test0','B:B');
    cr=xlsread('test0','C:C');
    dr=xlsread('test0','D:D');

    f=subs(f,{a,b,c,d},{ar,br,cr,dr})
    df=subs(f,{a,b,c,d},{ar,br,cr,dr})
    d2f=subs(f,{a,b,c,d},{ar,br,cr,dr})