How to define a vector that its elements are created in a For Loop?

Question

I want to define a vector,X, with five elements:

syms a
X = zeros(1,5)
X(1) = 1;
for k=1:4
    X(k+1)=X(k)+a^2;
end

Actually I need to have the vector X, that its elements should be based on variable a. But I face an error in Matlab, when I write the code above:

The following error occurred converting from sym to double:
Error using symengine (line 58)
DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use VPA.

Error in Code2 (line 5)
X(k+1)=X(k)*a^2;

How to fix this?

Answer 1

You are mixing between symbolic variables to doubles, Doubles holds a value, you can use the loop that you wrote only if a is a double and already holds a value (the value can be the input of a function) . for example :

function ret=testFunc(a)
X = zeros(1,5)
X(1) = 1;
for k=1:4
    X(k+1)=X(k)+a^2;
end
ret=X
end

if you want to work with syms (for other symbolic analysis) you can define x as a symbol too, for example :

syms a x
x(1)=1;
for i=2:5
x(i)=x(i-1)+a.^2;
end

now, x is a function of a, and if you print x you will get :

[ 1, a^2 + 1, 2*a^2 + 1, 3*a^2 + 1, 4*a^2 + 1]

to evaluate the value of x, you will still need to input a value for a.

Matlab help suggest subs function for replacing a in wanted value as follows:

y = subs(x,a,4)

in that point y is still a symbol and you will need to make it a double by using

double(y)