# There is an error in exp function but it doesn't tell what the error is?

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AYUSH KUMAR on 19 Mar 2019
Commented: Walter Roberson on 20 Mar 2019
I have made a function which contains the following lines
D = Widths(n)/Precision ;
potl = Pots (n) + (V_a * q) - (xposl * Slope);
for l=1:Precision
potr = (potl - (D*Slope)) ;
if (mod(n,2)==1)
a = (sqrt(2*me_Al*(potl - Energy))/h_bar) ;
M(1,1)=exp(-a*D);
M(1,2)= 0 ;
M(2,1)= 0 ;
M(2,2)= exp(a*D) ;
else
I am using the function in this script
E=0:0.01:0.6;
T_E = rtd_te (0, E, 1);
semilogy(E,T_E);
But when I run the script it gives the error
Error in rtd_te (line 45)
M(1,1)=exp(-a*D);
Error in Test2 (line 2)
T_E = rtd_te (0, E, 1);
I have not initialised or defined the matrix M anywhere else.
Can anybody tell me why is the error coming and how I can rectify it??
##### 5 CommentsShowHide 4 older comments
Walter Roberson on 20 Mar 2019
In your experience what is the difference between variables and matrices such that size(a) and size(D) somehow become meaningless concepts of no valid interest to people attempting to help you debug your program .

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### Answers (2)

Jos (10584) on 20 Mar 2019
Use
dbstop if error
and inspect the contents of the variables! You'll see what is wrong :-)
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Steven Lord on 20 Mar 2019
M(1, 1) refers to one element in M.
I suspect one of a and DivW is not a scalar (I'm guessing one of them is 1-by-61, the size of the E vector you created and passed into your rtd_te function as the second input argument.)
If my suspicion is correct that means exp(-a*DivW) will not be a scalar, it will likely be a 1-by-61 vector. Just like you can't fit a dozen eggs into one cup of an egg carton (at least not without breaking some eggs) you can't fit a dozen (or 61, or even just 2) elements from exp(-a*DivW) into that one element in M.
##### 1 CommentShowHide None
Walter Roberson on 20 Mar 2019
You can easily fit a dozen eggs into one standard cup of a egg carton if the eggs are louse eggs.
(Sorry, I couldn't resist nit-picking.)

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