Choose my k-term from matrix and defines by itself on for loop

Suppose i have n×1 matrix before, (column vector), namely "x1" (i'm defining this matrix with x1). I can't write my x1 because of it depends on my input. And this matrix could be 3×1, 7×1, until n×1. In this case i won't mention it on my script below

x1
for c=1:10
    for d=0:9
        'a',c,=x1(c,1)
    end
end

My wish is when i have x1 is matrix of 10×1 (Look at my c) my for loop will display

a1=
   2
a2=
   3
.
.
.
a10=
    4

2, 3, 4 are arbitrary numbers depend on my input of matrix x1. That means, i want it defines a variable by itself generating from my "c" (Look at my fourth line)

And that 'a',c, can be used for evaluating another formula.

For example

Px=a1+a2+a3

Is that possible? Thanks. If my question is not clear, please tell me.

EDIT :

Another alternative in my thought is the following: For example, i want to construct summation

My attempt is:

x1
  for c=1:10
      for d=0:9
          P=sum((x1(c,1)))*9^d
      end
  end

But it's Not work and displays weird thing.

As i said before, actually my goal is evaluating the k-element in my matrix that is i take it randomly (not literally "random") And construct it as like this:

Where is the k-element from my matrix, and x is just independent variable in that equation and depend on my input.

I've edit my question. I hope it's clear now and tell me if it's not. Thanks again.

Answer 1

[Image Analyst]

It's not clear. Not sure what x1, a, and c are. It looks like the user enters some number of values for x1, then somehow, some way (unknown to me), a vector "a" is generated from x1 because you say the "a" are "arbitrary numbers depend on my input of matrix x1" so it might possibly have values of [2,3,....10] (for one example of "a" that you gave).

Anyway, if you have a vector "a" and another vector x1, you can construct the terms of the series like

theTerms = a .* x1; % Note: a and x1 are either both row vectors, or both column vectors.

so theTerms(1) is the first a times teh first x1, and theTerms(2) is the second a times the second x1, and so on.

So now you can get P(x) like this

Px = cumsum(theTerms);

where Px(1) is theTerms(1), and

Px(2) = theTerms(1) + theTerms(2), and

Px(3) = theTerms(1) + theTerms(2) + theTerms(3), and so on.

[Fahmy Shandy]

I'm sorry not being clear. a is the element of a column vector x1, and i want randomly choose a for matrix x1, and evaluating it on another formula which is a summation containing a as a constant (notice that a is an element of matrix x1), and x is an independent variable in that is can be input any numbers that i want.

c is the number of element in matrix x1. For example if i have x1 is a matrix of 10×1, then my c is 10.

What i've done so far

    for c=1:n
        for d=0:m
            P(x)=cumsum((x1(c,1))*x^d)
        end
    end

Where n and m is some large number depend on my input.

My P(x) is

And i'm confuse with "cumsum", because my x is a scalar instead of a matrix.

[Image Analyst]

Don't use for loops. And you don't need a second index, 1, on x1.

Just do this

x1 = [2, 3, 4, 5, 10] % Whatever
coefficients = 1 : length(x1)  % 1, 2, 3, 4, etc.
theTerms = coefficients .* x1; % Note: a and x1 are either both row vectors, or both column vectors.
Px = cumsum(theTerms)

You'll see:

x1 =

2 3 4 5 10

coefficients =

1 2 3 4 5

Px =

2 8 20 40 90