how to create 1*101 of some integer array?

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Meva
Meva 2015-4-5
评论： Mohammad Abouali 2015-4-6
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?

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Mohammad Abouali
Mohammad Abouali 2015-4-5
编辑：Mohammad Abouali 2015-4-5
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()?
  2 个评论
Meva
Meva 2015-4-6
编辑：Meva 2015-4-6
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
Mohammad Abouali 2015-4-6
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.

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Image Analyst
Image Analyst 2015-4-5
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);

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