summing up array element

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

[Jan]

Is this a homework question?

Answer 1

[Cedric]

Hi, use CUMSUM:

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

[Bathrinath]

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

[Cedric]

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

Answer 2

[Evan]

help cumsum
>> s = cumsum(p)

s =
    3     9    11    16

[Bathrinath]

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

Answer 3

[Andrew Reibold]

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

[Bathrinath]

Thanks it works

Answer 4

[Iain]

 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

[Jan]

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.