Problem with loop MATLAB

Question

no  time  scores
1    10    123
2    11    22
3    12    22
4    50    55
5    60    22
6    70    66
.    .     .
.    .     .
n    n     n 

Above a the content of my txt file (thousand of lines).

1st column - number of samples
2nd column - time (from beginning to end ->accumulated)
3rd column - scores

I wanted to create a new file which will be the total of every three sample of the scores divided by the time difference of the same sample.

e.g. (123+22+22)/ (12-10) = 167/2 = 83.5
     (55+22+66)/(70-50) = 143/20 = 7.15

new txt file

83.5
7.15
.
.
.
n

so far I have this code:

fid=fopen('data.txt')
data = textscan(fid,'%*d %d %d')
time = (data{1})
score= (data{2})
for sample=1:length(score)
     ..... // I'm stucked here ..
end
....

Answer 1

For what it's worth, here is how you would go about to do that in Python. It is probably adaptable to Matlab.

import numpy
no, time, scores = numpy.loadtxt('data', skiprows=1).T

 # here I assume that your n is a multiple of 3! otherwise you have to adjust
sums = scores[::3]+scores[1::3]+scores[2::3]
dt = time[2::3]-time[::3]

result = sums/dt

Answer 2

%# Easier to load with importdata
data = importdata('data.txt',' ',1);
%# Get the number of rows
n = size(data,1);
%# Column IDs
time = 2;score = 3;
%# The interval size (3 in your example)
interval = 3;
%# Pre-allocate space
new_data = zeros(numel(interval:interval:n),1);
%# For each new element in the new data
index = 1;
%# This will ignore elements past the closest (floor) multiple of 3 as requested
for i = interval:interval:n
    %# First and last elements in a batch
    a = i-interval+1;
    b = i;
    %# Compute the new data
    new_data(index) = sum( data(a:b,score) )/(data(b,time)-data(a,time));
    %# Increment
    index = index+1;
end

Answer 3

I suggest you use the importdata() function to get your data into your variable called data. Something like this:

data = importdata('data.txt',' ', 1)

replace ' ' by the delimiter your file uses, the 1 specifies that Matlab should ignore 1 header line. Then, to compute your results, try this statement:

(data(1:3:end,3)+data(2:3:end,3)+data(3:3:end,3))./(data(3:3:end,2)-data(1:3:end,2))

This worked on your sample data, should work on the real data you have. If you figure it out yourself you'll learn some useful Matlab.

Then use save() to write the results back to a file.

PS If you find yourself writing loops in Matlab you are probably doing something wrong.

Answer 4

If you are feeling adventurous, here's a vectorized one-line solution using ACCUMARRAY (assuming you already read the file in a matrix variable data like the others have shown):

NUM = 3;
result = accumarray(reshape(repmat(1:size(data,1)/NUM,NUM,1),[],1),data(:,3)) ...
    ./ (data(NUM:NUM:end,2)-data(1:NUM:end,2))

Note that here the number of samples NUM=3 is a parameter and can be substituted by any other value.

Also, reading your comment above, if the number of samples is not a multiple of this number (3), then simply discard the remaining samples by doing this beforehand:

data = data(1:fix(size(data,1)/NUM)*NUM,:);

---

Im sorry, here's a much simpler one :P

result  = sum(reshape(data(:,3), NUM, []))' ./ (data(NUM:NUM:end,2)-data(1:NUM:end,2));