Matlab function output , should be simple

Question

So i have a 1xn matrix x and the for loop calculates the absolute value of differences in consecutive entries of x starting with entry 3.

n=length(x);
for i=3:n
    y=abs(x(i)-x(i-1));

what i need my output to be is a 2 column vector. First column displays x and second column displays zeros for the first two rows followed by the results of the for loop.

x is a loaded vector

this is my function function [ z ] = dome(x)

n=length(x);
z = zeros(n, 2);
for i=3:n;

z(3:n,2)=abs(x(i)-x(i-1));
z(:,1) = x;


end

I'm getting this as output

ans =

1.000000000000000 0 1.500000000000000 0 1.286953767623375 0.000009575517218 1.402540803539578 0.000009575517218 1.345458374023294 0.000009575517218 1.375170252816038 0.000009575517218 1.360094192761733 0.000009575517218 1.367846967592133 0.000009575517218 1.363887003884021 0.000009575517218 1.365916733390040 0.000009575517218 1.364878217193677 0.000009575517218 1.365410061169957 0.000009575517218 1.365137820669213 0.000009575517218 1.365277208524479 0.000009575517218 1.365205850297047 0.000009575517218 1.365242383718839 0.000009575517218 1.365223680225282 0.000009575517218 1.365233255742500 0.000009575517218

marsei · Accepted Answer · 2013-09-20 08:11:34Z

1

You can do it this way. The code uses the matlab's diff function.

x = rand(1,10)   % vector of 1x10

y = [x' [0 ; abs(diff(x))']];
y(1:3,:) = [];

This gives

y =

0.0462    0.2308
0.0971    0.0510
0.8235    0.7263
0.6948    0.1286
0.3171    0.3777
0.9502    0.6331
0.0344    0.9158

If you want to keep the for loop, this code

Y = zeros(length(x), 2); %create the output matrix Y
Y(1:2, 1) = x(1:2);      %popualte the first 2 row of column 1 with x(1:2) // thanks to @Dan

for i=3:length(x);
    Y(i, 1) = x(i);             %populate the first column
    Y(i, 2) = abs(x(i)-x(i-1)); %populate the second column
end

gives

Y =

0.7513         0
0.2551         0
0.5060    0.2509
0.6991    0.1931
0.8909    0.1918
0.9593    0.0684
0.5472    0.4121
0.1386    0.4086
0.1493    0.0107
0.2575    0.1082

edited Sep 20, 2013 at 8:11

answered Sep 20, 2013 at 7:53

marsei

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2 Comments

PooperScooper Over a year ago

I'm definitely gonna need the for loop in there because im just using the difference as a simpler function. I will need to calculate error estimates in a sequence. Do you know how i could get the results of the for loop into a matrix?

Dan Over a year ago

I suggest you add Y(1:2, 1) = x(1:2); before the loop for completeness.

StrongBad · Accepted Answer · 2013-09-20 09:10:45Z

1

In MATLAB there is a "big" difference between 0 and 0.0000. The former is really zero while the latter can be almost as big as 0.00005. You can "fix" it by changing the output format with format long. You also need to use i in the assignment ...

format long;
n=length(x);
z = zeros(n, 2);
z(:,1) = x;
for i=3:n;
  z(i,2)=abs(x(i)-x(i-1));
end

edited Sep 20, 2013 at 9:10

answered Sep 20, 2013 at 8:53

StrongBad

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1 Comment

PooperScooper Over a year ago

I did format long and this was my output. It used only the final iteration as the value for 2nd column

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