how to compare an element of array to the rest of the elements

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aboltabol
aboltabol 2015-9-26
评论： aboltabol 2015-9-26
I am trying to compare each element of an array with its subsequent elements and get the count of number of times the current element is greater than the subsequent elements.
the following is my code
function [ z ] = test(i,size_data, num,j,k,sum )
% test
num=xlsread('MKdata.xlsx');
size_data=size(num);
j=0
k=0
i=1
for (i=i:size_data)
if num(i) > num([i+1:size_data])
j=j+1
else k=k-1
end
i=i+1
end
sum=j+k
end
the test data set i am using is
1733 3002 875 164 1464 1399 1039 1096 1812 2347 575 1393 411 283 1407 1294 1102 1886 3058 4587
results shows that every increment of i adds 1 to k and finally i get k=-20, j=0 whereas it should be 26
please guide me, please point out the mistake in logic or syntax

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采纳的回答

Image Analyst
Image Analyst 2015-9-26
Wouldn't you just do this (vectorized):
% Define some arbitrary random data
data = randi(9, 1, 20)
% Define the element we're going to consider
index = 3;
% Compare each element of an array with its subsequent elements
% Get logical index that is true when current element is greater.
itIsGreater = data(index) > data(index+1:end)
% Get the count of number of times the current element is greater than the subsequent elements.
theCount = sum(itIsGreater)
You didn't say in words that you want to do that for all possible starting indexes, but if you do, then put it in a loop and make the count depend on starting index.
% Define some arbitrary random data
data = randi(9, 1, 20)
theCount = zeros(1, length(data));
for index = 1 : length(data)
% Compare each element of an array with its subsequent elements
% Get logical index that is true when current element is greater.
itIsGreater = data(index) > data(index+1:end)
if ~isempty(itIsGreater) % if there is at least 1 greater...
% Get the count of number of times the current element is greater than the subsequent elements.
theCount(index) = sum(itIsGreater)
end
end
% Print to command window..
theCount

更多回答（2 个）

Walter Roberson
Walter Roberson 2015-9-26
When you have
if num(i) > num([i+1:size_data])
then you are comparing one value to a number of values, which will give you a vector of logical results, 0 for any places it does not hold and 1 for any places it holds. "if" is only considered true if all of the elements it is asked to process are true, so your "if" would be considered true only if num(i) was greater than all of the remaining elements in num.
I cannot tell you how to correct your code because you neglected to indicate the circumstances under which you want j or k to be incremented.
Meanwhile I recommend you look at the documentation for all() and for any()
  1 个评论
aboltabol
aboltabol 2015-9-26
Sir, Thanks for your reply I want j to be incremented each time an element, to which num(i) is being compared, is greater than num(i). if the element is less than num(i) then i want k to be incremented. this has to be done for each element, for all its subsequent elements. so for a series having 20 entries the first entry will be compared to the following 19 entries and the 19th entry will be compared to 20th entry only

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Jos
Jos 2015-9-26
As Walter said num(i)>num(i+1:end) compares num(i) to all subsequent elements with result 1 for each time true. You can use this by summing the result, which will give what you want. The code below gives your 26 as answer
size_data=size(num,2);
j=0;
k=0;
for i=1:size_data-1
j = j + sum(num(i)>num(i+1:end));
k = k + sum(num(i)<num(i+1:end));
end
sum=k-j;

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