# How to create a multidimensional array of fixed dimensions?

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Mariana on 8 May 2020
Commented: Mariana on 14 May 2020
I would like to create an array in 4 dimensions. Each dimension has a fixed size.
x =0 to 200 -->step 1
y =-25 to 25 --> step 0.5
z = 0 to 180 --> step 1
h = -10 to 10 --> step 0.1
The idea behind is to save one value on a specifc position.
For example if I have an input array [30,-0.5,100,2 ] = 21
At that location I want to save the value.
At a specific amount I would like to read the values inside this array.
Hope you can help me! :)

Walter Roberson on 9 May 2020
xmin = 0; xmax = 200; xincr = 1;
ymin = -25; ymax = 25; yincr = 0.5;
zmin = 0; zmax = 180; zincr = 1;
hmin = -10; hmax = 10; hincr = 0.1;
xvec = xmin : xincr : xmax;
yvec = ymin : yincr : ymax;
zvec = zmin : zincr : zmax;
hvec = hmin : hincr : hmax;
x2xidx = @(xval) round((xval - xmin)/xincr) + 1;
y2yidx = @(yval) round((yval - ymin)/yincr) + 1;
z2zidx = @(zval) round((zval - zmin)/zincr) + 1;
h2hidx = @(hval) round((hval - hmin)/hincr) + 1;
nx = length(xvec);
ny = length(yvec);
nz = length(zvec);
nh = length(hvec);
M = zeros(nx, ny, nz, nh);
%example of use
M(x2xidx(17), y2yidx(-6.5), z2zidx(93), h2hidx(-4) ) = 1;
[XIDX, YIDX, ZIDX, HIDX] = ind2sub(size(M), find(M));
disp([XIDX, YIDX, ZIDX, HIDX])
disp([xvec(XIDX), yvec(YIDX), zvec(ZIDX), hvec(HIDX)])
You can store by index or you can use the helper functions to convert numeric value to index.
Mariana on 14 May 2020
What if I have a variable step per vector. How will you suggest to find the index?
rmin = 0; rmax = 90; rincr = 0.5;
r2min = 91; r2max = 180; r2incr = 1;
r3min =181.5; r3max = 250 ; r3incr = 1.5;
rvec = rmin : rincr : rmax;
r2vec = r2min : r2incr : r2max;
r3vec = r3min : r3incr : r3max;
relx = single([rvec,r2vec,r3vec]);

Steven Lord on 8 May 2020
There's no such thing as the -25th column in an array in MATLAB, nor is there such a thing as the 1.5th column.
You can make an array with separate coordinates that you can use like indices (though as usual, be careful when performing exact equality comparisons with == on floating-point numbers. What I wrote below is safe because all the numbers in x and y can be represented exactly in double precision.)
x = (-5:5).';
y = (-3:0.5:3);
z = x.^2 + y.^3;
z(x == -4, y == 1.5) % (-4)^2+(1.5)^3
If you tell us a little more detail about what you're trying to do with this array we may be able to offer alternate suggestions.
Mariana on 9 May 2020
This is the kind of the idea that I would like to do. Just to save a value at a specific space of this array. The array has 8 dimensions or 8 indices. How would you suggest to build and add values to this storage array?

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