Creating new vector wich adds previous value with for loop

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Johan Lilliestråle
评论： Johan Lilliestråle 2015-2-19
Hey everybody
Lets say that I have the vector:
a = [1, 1, ,1 ,1];
Then I want to create a new vector which adds all previous values, call it b
b = [1, 2, 3, 4];
the first value is the sum of the a(1,1), the second value is the sum of the previous value (which was 1) plus the new a(1,2) = 1. Then we get 1+1 =2. The third value is th1 sum of the previous value 2 and a(1,3) = 1 which is 1+2=3. And so on and on. If i want to do this in a for loop would that be:
a = [1, 1, ,1 ,1];
b = zeros(1,4)
for i=1:4;
b = 0;
b(i) = b + a(i);
end
I have tried that code but it doesnt work. If anyone a suggestion how to create an foor loop for this problem I would be grateful to see it. Cheers

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回答（2 个）

James Tursa
James Tursa 2015-2-19
编辑：James Tursa 2015-2-19
b = cumsum(a);
To fix your explicit loop, don't set b = 0 at each iteration, and use proper indexing into the vectors. E.g.,
a = [1, 1, 1, 1];
b = a;
for i=2:4;
b(i) = b(i-1) + a(i);
end
Star Strider
Star Strider 2015-2-19
There’s an easier way to do it with the repmat and triu functions:
a_sum = sum(triu(repmat(a, 4, 1)));
This will give you exactly what you want.

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