How do I sum the first 3 values after each zero

Question

Chris Matthews 2018-5-16

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https://ww2.mathworks.cn/matlabcentral/answers/401121-how-do-i-sum-the-first-3-values-after-each-zero

评论： Image Analyst 2018-5-17

Hi all, I have matrix X, and want to sum the first 3 values after each zero. So far I've managed to sum each block of values after a zero, but I need to modify this to output only the first 3 values after a zero.

    clear all
  clc
x=[0 0 1 2 4 2 5 1 0 0 3 7 9 6 9 0 0 0 0 3 7 0 0 0 2];
t = [0,x,0];
c = cumsum(t);
f = find(diff(t~=0)~=0);
r = c(f(2:2:end))-c(f(1:2:end));
%output is r = [15,34,10,2];
%but i want r = [7, 19, 10, 2];

Thanks guys!

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Image Analyst 2018-5-17

It looks like you want the sum of the first 3 values not after EACH zero, but after the LAST zero in a run of zeros. Because the first zero is at index 1 and the sum of the first 3 numbers after that (at indexes 2, 3, and 4) is 3, not 7.

Chris Matthews 2018-5-17

Yep, that's true. The code works though, and the output is what I need. Thanks for clarifying.

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Answer 1

per isakson 2018-5-17

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This

x   = [0 0 1 2 4 2 5 1 0 0 3 7 9 6 9 0 0 0 0 3 7 0 0 0 2];
dx  = [ 0, diff( x ) ];
len = length( x );
is0 = [ false, ( x == 0 ) ];
is0(end) = [];
ix_start = find( is0 & not(dx==0) );
out = nan( size( ix_start ) );
for jj = 1 : length( ix_start )
    out(jj) = sum( x( ix_start(jj) : min( ix_start(jj)+2, len ) ) );
end

outputs

>> disp( out )
     7    19    10     2

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Chris Matthews 2018-5-17

Not sure if I should post as a separate question, but I was wondering then, how I could calculate the sum of the differences of the absolute values of the first 3 values from a zero?

So my output should be:

[ (1-0 + 2-1 + 4-2) , (3-0 + 7-3 + 9-7), .....]

Please note that in my actual matrix X, there are positive and negative values so thats why i need the code to take the absolute value.

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Answer 2

Image Analyst 2018-5-17

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Here is another way that works, if you have the Image Processing Toolbox.

x   = [0 0 1 2 4 2 5 1 0 0 3 7 9 6 9 0 0 0 0 3 7 0 0 0 2];
% Get values after the last zero in each run of zeros.
props = regionprops(x~=0, x, 'PixelValues');
% Determine the "r" array:
for k = 1 : length(props)
  theseValues = props(k).PixelValues;      % Extract all values after the zero.
  lastIndex = min(length(theseValues), 3); % Determine how many to sum up.
  r(k) = sum(theseValues(1:lastIndex))     % Do the sum.
end

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Chris Matthews 2018-5-17

Thanks Image Analyst. Not sure if I should post as a separate question, but I was wondering then, how I could calculate the sum of the differences of the absolute values of the first 3 values from the last zero in the string of zeros? So my output should be:

[ (1-0 + 2-1 + 4-2) , (3-0 + 7-3 + 9-7), .....]

Please note that in my actual matrix X, there are positive and negative values so that's why I need the code to take the absolute value.

Image Analyst 2018-5-17

在 MATLAB Online 中打开

Just do it twice:

x   = [0 0 1 2 4 2 5 1 0 0 3 7 9 6 9 0 0 0 0 3 7 0 0 0 2];
differences = [0, diff(x)]
% Get values after the last zero in each run of zeros.
props = regionprops(x~=0, x, 'PixelValues');
propsDiff = regionprops(x~=0, differences, 'PixelValues');
% Determine the "r" array:
for k = 1 : length(props)
  theseValues = props(k).PixelValues;      % Extract all values after the zero.
  lastIndex = min(length(theseValues), 3); % Determine how many to sum up.
  r(k) = sum(theseValues(1:lastIndex));    % Do the sum.
  % Do the same for the differences.
  theseValues = propsDiff(k).PixelValues;      % Extract all values after the zero.
  rDiff(k) = sum(theseValues(1:lastIndex));    % Do the sum.
end
r
rDiff