# Error using sym/subsindex

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Viktoriia Buliuk on 7 Aug 2020
Commented: Viktoriia Buliuk on 11 Aug 2020
Hello!
I got the following mistake:
Error using sym/subsindex (line 836)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym
expression.
Error in examp (line 22)
Q(i,j) = (1/(a(i)^(1/2)))* ((t_max - t_min)./(size(t)-1))*symsum(h(k)*wavelet((tau(k)-ttt(j))/a(i)),k, 1, size(h)-1);
The code is following:
type Ez_dry.csv;
Test = importdata('Ez_dry.csv');
t = Test(:, 1);
h = Test(:, 2);
t_max = max(t);
t_min = min(t);
wav = @(tau)(2/(3.^(1/2)*pi^(1/4))*exp(-tau.^2/2).*(1 - tau.^2));
ttt = t_min:0.1*10^-10:t_max;
a = 0.3*10^-10:0.6*10^-11:10^-9;
for i = 1:length(a)
for j = 1:length(ttt)
syms k
Q(i,j) = (1/(a(i)^(1/2)))*((t_max - t_min)./(size(t)-1))*symsum(h(k)*wav((tau(k)-ttt(j))/a(i)),k, 1, size(h)-1);
end
end
S = real(Q);
figure(2);
contour (ttt, 1./a, S, 500)
Could you, please, help me to find a solution. Thank you in advance!

Stijn Haenen on 7 Aug 2020
There is no symbolic variable as input for symsum. You do h(k), but what does this mean, k had not a value yet so you cannot take the k th element of h. And you didn't define tau so tau(k) does not exist. And you should use length(h) instead of size, i think.
If you have a function of h expressed in k you can solve your problem, i dont know if you can without a function for h.
this line works, but h is excluded from it:
sym_sum_ans=(1/(a(i)^(1/2)))*((t_max - t_min)./(size(t)-1))*symsum(wav((k-ttt(j))/a(i)), 1, length(h)-1);
num_ans=vpa(sym_sum_ans)
Viktoriia Buliuk on 11 Aug 2020
Thanks a lot!
I decided to use a loop instead of symsum.

Walter Roberson on 8 Aug 2020
symbolic variables can never be used as indices. You cannot symsum or symprod and use the control variable as an index.
Instead you need to produce the definite vector of individual values and sum() or prod() that.

#### 1 Comment

Viktoriia Buliuk on 11 Aug 2020
Thank you very much!

KSSV on 7 Aug 2020
Initialize the cell first.
Q =cell(length(a),length(ttt)) ; % initialize
USe this in the loop.
Q{i,j} = %% your formula with syms

Viktoriia Buliuk on 7 Aug 2020
Thanks a lot!
With this eddition I got a new error:
Undefined function or variable 'lenght'.
KSSV on 7 Aug 2020
Check the spelling.....it should be length
Viktoriia Buliuk on 7 Aug 2020
Thank you!
Unfortunately, I've got the same error:
Error using sym/subsindex (line 836)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.