find max of a vector between two indice

Hi, I want a code to find the maximum number between two indice of a vector.

I have an indice vector and a data vector . For example my indice vector is :

A=[3 7 13]

and my data vector is:

B=[-5 3 -8 1 -7 2 -9 3 6 2 7 9 -4 2 6]

now the max number between indices 3 and 7 of vector B is number "2" .

and the max number between indices 7 and 13 of vector B is number "9".

So the output should be like a vector named C as follows:

C=[2 9]

I've been thinking on this for hours but haven't found any solution yet :(

Tnx in advance.

[Andrei Keino]

    % 
[M,I] = max(B(3:7);
I = I + 3;

Answer 1

[Lucas García]

>> C = arrayfun(@(x) max(B(A(x):A(x+1))),1:length(A)-1)

C =

     2     9

[Andrei Bobrov]

Hi friends!

My part

C = cellfun(@max,mat2cell(B(A(1):A(end)),1,diff(A)+[1 0]))

Answer 2

[Sean de Wolski]

Easiest, and likely fastest way is just to use a for-loop:

A=[3 7 13];
B=[-5 3 -8 1 -7 2 -9 3 6 2 7 9 -4 2 6];

nmax = length(A)-1;
maxes = zeros(1,nmax);
for ii = 1:nmax
  maxes(ii) = max(B(A(ii):A(ii+1)));
end

TIMINGS Mac 2009b

A=[3 7 13];
B=[-5 3 -8 1 -7 2 -9 3 6 2 7 9 -4 2 6];

t1 = 0;
t2 = 0;
for ii = 1:100
  tic
  nmax = length(A)-1;
  maxes = zeros(1,nmax);
  for ii = 1:nmax
    maxes(ii) = max(B(A(ii):A(ii+1)));
  end
  t1 = t1+toc;
  tic
  C = arrayfun(@(x) max(B(A(x):A(x+1))),1:length(A)-1);
  t2 = t2+toc;
end

t2/t1

ans = 25.029

arrayfun takes about 25x longer on my system

[Lucas García]

I totally agree with you, Sean. arrayfun is more time consuming, but takes less syntax.. Depending on my needs for speed I would choone one or the other.

[Sean de Wolski]

I would write a new specific function that accepts A,B and spits out an answer. Then call it from where we need prettiness.

[Lucas García]

Yes!

Answer 3

[Fangjun Jiang]

A=[3 7 13];
B=[-5 3 -8 1 -7 2 -9 3 6 2 7 9 -4 2 6];
C=zeros(1,numel(A)-1)
for k=1:numel(A)-1
C(k)=max(B(A(k):A(k+1)));
end

Answer 4

[Hajik zagori]

Thank you very much Lucas , Fangjun and Sean.I checked them all and all three solutions worked perfectly.

I don't know which answer I'd better accept because all of them were good.

Anyway I guess I choose the first one.

Best Regards .

Hajik.