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
....

no. if the remaining is less than 3, e.g. 1 or 2 samples, it will just leave it. — Jessy
– Jessy, Commented Jun 2, 2010 at 13:32

Amro · Accepted Answer · 2010-06-02 15:33:42Z

7

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));

edited Jun 2, 2010 at 15:33

answered Jun 2, 2010 at 15:28

Amro

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1 Comment

High Performance Mark Over a year ago

+1: I've never come across accumarray before, much neater solution than mine. Thanks @Amro.

Jacob · Accepted Answer · 2010-06-03 17:48:25Z

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

edited Jun 3, 2010 at 17:48

answered Jun 2, 2010 at 14:11

Jacob

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13 Comments

Jessy Over a year ago

I wonder what is the different between importdata and textscan?

Jacob Over a year ago

In textscan you need to specify the formatting, importdata figures it out (most of the time).

Jessy Over a year ago

I wonder if the "data" in the file.data ..refer to the name of the file? data.txt?

Jessy Over a year ago

I got this error when implement the code ..Attempt to reference of non-structure array..:(

Jacob Over a year ago

No, the data is the data component after importdata. Can you just type file = importdata('data.txt');disp(file) and tell me what the output it?

|

Olivier Verdier · Accepted Answer · 2010-06-02 13:37:08Z

0

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

answered Jun 2, 2010 at 13:37

Olivier Verdier

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Comments

High Performance Mark · Accepted Answer · 2010-06-02 14:12:42Z

0

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.

answered Jun 2, 2010 at 14:12

High Performance Mark

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3 Comments

Jacob Over a year ago

@Mark: Loops aren't bad always. This has been discussed before in SO. Plus, this solution fixes the interval at 3, ( data(1:3:end,3) + data(2:3:end) + ... ).

High Performance Mark Over a year ago

@Jessy -- how big is your dataset ?

Jessy Over a year ago

thousand of lines --- 318687 lines

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