how to create 1*101 of some integer array?

Hi,

Very basic que: I would like to have 1*101 of -4 array. I have to solve an integral numerically

int_0^x -4 dx;

My code is like:

x= linspace(0,1,0.01);
A = zeros(1,101);
..
cumtrapz(x,-4*A);

But the command prompt shows all zeros for cumtrapz which is certainly not.

If I just use :

cumtrapz(x,-4);

it says matrix dimensions must agree. How can I create this array?

Answer 1

[Mohammad Abouali]

Yo have:

A = zeros(1,101);

and then later you have

cumtrapz(x,-4*A);

all elements of A are zero so -4*A is going to be still zero.

change A=zeros(1,101) to A=ones(1,101)

Also as imageAnalyst said, the third argument for linspace is not the spacing so you need to change the code to something like this

x=linspace(0,1,101);
A=ones(1,101);
cumtrapz(x,-4*A);

By the way, are you sure you want to use cumtrapz()? don't you want the output of trapz()?

[Meva]

As far as I know, trapz is for say

int_0^1

cumtrapz is working for

int_0^x

And the last element of cumtrapz is giving trapz.

So I guess cumtrapz is still the right choice. Please correct me if I am wrong.

[Mohammad Abouali]

Both trapz(x,y) and cumtrapz(x,y) integrate between x and y for the entire domain.

Trapz gives you one number. But cumtrapz gives all the values from start to end.

So if x=xStart:dx:xEnd then trapz gives int_xStart^xEnd but cumtrapz will give an array the first one is for int_xStart^{xStart+dx) the second one is for int_xStart^{xStart+2dx} and the n-th number is for int_xStart^{xStart+n*dx}.

the last number of cumtrapz and trapz should be the same.

Both can be useful. just make sure which one is it that you want.

Answer 2

[Image Analyst]

The third argument of linspace is the number of elements, not the increment between elements. Try this:

numberOfElements = 101;  % Whatever you want
x= linspace(0, 1, numberOfElements);
A = zeros(1, numberOfElements);