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I am looking for a better way to calculate the sum off all elements from index X to index Y, if I have a list of indexes.
For example: 

a = {1, 40, 77} % indexes
cell_elements = {1,1,1.....,1} %100 elements, each 1

My current idea looks something like this: 

        counter = 1;
        for k=1:length(cell_elements)
            if(%Some condition) %condition is true for k=1,40 and 77
                sum = sum+cell_elements(k);
                result(counter) = sum;
                sum=0;
                counter = counter+1;      
            else
                sum = sum+cell_elements(k);
            end
        end

I would like to improve the code, since I have the feeling that the problematic is simple, but due to my lack of experience in matlab my code is too long.

Is there any function, where I could just pass a list of indexes and it will do the same job as the code above?

CroatiaHR
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    Possible duplicate of [Matlab: Sum corresponding values if index is within a range](https://stackoverflow.com/questions/29393234/matlab-sum-corresponding-values-if-index-is-within-a-range) – Duck Dodgers May 29 '19 at 09:51
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    What is the expected result with the given example ? Why are you using cell instead or array ? What is `pkt_size` ? BTW you shadow the built-in function `sum` if you use `sum` as a variable name which is not recommanded. – obchardon May 29 '19 at 10:00

1 Answers1

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  • Use array instead of cell
  • Also since your cell_elements are all 1s, you can use the matrix ones(row, column)

Specify the row and the column, here row is 1 and column is 100 this is the code 

cell_elements = ones(1, 100)
  • Define a as an array a = [1, 40, 77]
  • First 40 elements are elements having indices from 1 to 40, in Matlab this is equivalent to 1:40, in order to include the array a, since a(1) = 1 and a(2) = 40; set 1:40 as a(1):a(2) First 40 elements will be 
First_40_elements = cell_elements(a(1):a(2));

To sum them all together use Matlab built in function sum as follow

Sum_First_40_elements = sum(First_40_elements)
  • For the second 37 elements the indices start from 41 to 77, in Matlab written as 41:77, using array a, a(2)+1:a(3)
    Following the logic above 
Second_37_elements = cell_elements(a(2)+1:a(3));

Sum_Second_37_elements = cell_elements(Second_37_elements);

The entire code is as follow

cell_elements = ones(1,100);

a = [1,40,77];

First_40_elements =  cell_elements(a(1):a(2));

Sum_First_40_elements = sum(First_40_elements);
% 40

Second_37_elements =  cell_elements(a(2)+1:a(3));

Sum_Second_37_elements = sum(Second_37_elements);
% 37

The dynamic way is as follow

cell_elements = ones(1,100);
a = [1,25,40, 67, 80, 95];

element_sum = zeros(1, length(a)-1);
for i = 1:length(a)-1
    if i == 1
        element_sum(i) =  sum(cell_elements(a(i):a(i+1)));
    else
        element_sum(i) =  sum(cell_elements(a(i)+1:a(i+1)));
    end
end

Result

>> element_sum

element_sum =

    25    15    27    13    15

>>
Adam
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  • yes, but I am looking for all values between X and Y, and not the corresponding values at index X and Y. So, I am looking for a neat way to sum everything between X,Y, or if I follow the example you have given: a=[1,40,70] result = [1+1+1+1...+1, 1+1+1..+1] %first element a sum of 40, second of 30, in case all elements are ones – CroatiaHR May 29 '19 at 14:42
  • @CroatiaHR, "_from index X to index Y_" ... why do you have 3 indices then ? Do you mean the sum from index 1 to index 40, then the sum from index 40 to index 77 ? – Hoki May 29 '19 at 16:02
  • Well, this is quite close to what I am looking for. But I believe it is not dynamically when it comes to the size of a. In this way, you have a specific size of 3, but in my program, I want to make it flexible in case it has to read in the size. I will still mark it as an answer, since you did answer on the given example, but I will be happy if you could give me an replay if there is a dynamic approach to it – CroatiaHR Jun 01 '19 at 14:34
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    @CroatiaHR check below, I added a dynamic way using a `for loop` – Adam Jun 01 '19 at 15:18