summing up array element

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Bathrinath 2013-7-25

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编辑： per isakson 2021-9-11

Hello,

I am new to programming, I have an array like p = [3 6 2 5];

I need c = [3 9 11 16]; First element in the array is as it is, when it goes to next element it has to sum the previous element.

Regards, SBN

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Jan 2013-7-25

Is this a homework question?

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Answer 1

Cedric 2013-7-25

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编辑：Cedric 2013-7-25

在 MATLAB Online 中打开

Hi, use CUMSUM:

 >> p_csum = cumsum(p)
 p_csum =
     3     9    11    16

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Bathrinath 2013-7-25

Thanks it works. Can you give the solution in for loop .

Cedric 2013-7-25

编辑：Cedric 2013-7-25

Did you try building a solution based on a FOR loop? What did you implement so far?

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Answer 2

Evan 2013-7-25

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编辑：Evan 2013-7-25

在 MATLAB Online 中打开

help cumsum
>> s = cumsum(p)
s =
    3     9    11    16

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Bathrinath 2013-7-25

Thanks it works. Can you give the solution in for loop .

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Answer 3

Andrew Reibold 2013-7-25

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编辑：Andrew Reibold 2013-7-25

在 MATLAB Online 中打开

Here is your solution in a really easy loop.

p = [3 6 2 5];           %Can have as many numbers as you want
for n = 2:length(p)      %n can be whatever counter you want.
     p(n) = p(n)+p(n-1); %semi-colon suppresses output
end

-Andrew

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Bathrinath 2013-7-25

Thanks it works

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Answer 4

Iain 2013-7-25

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编辑：Iain 2013-7-25

在 MATLAB Online 中打开

 for i = 1:numel(p)
  c(i) = sum(p(1:i));
 end

Or

 so_far = 0; 
 for i = 1:numel(p)
  so_far = so_far + p(i);
  c(i) = so_far;
 end

or

 for i = 1:numel(p)
  s = 0;  
  for j = 1:i
   s = s+ p(j);
  end
  c(i) = s;
 end

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Jan 2013-7-25

These are inefficient solutions, because the complete sum is calculated in each iteration. While this might not matter for 4 elements, the runtime will grow rapidly for longer data.

It is more efficient to re-use the sum for the elements 1 to i, when the sum until the i+1.th element is obtained. See Andrews answer.

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