I am working in MATLAB for my image processing project.
I am using a for loop to generate some kind of image data (size of image varies) with each loop iteration. My problem is how do stop it from overwriting the image in next iteration.
Img(i,j)=data
Ideally I would like it to have
Img_1 = data (for 1st iteration)
Img_2 = data (for 2nd iteration)
Img_3 = data (for 3rd iteration)
and so on...
Is there any way, it can be acheived?
Yes, you can use dynamic field names with structures. I wouldn't recommend using separate variable names because your workspace will become unwieldy. Do something like this:
img_struct = struct(); %// Create empty structure
for ii = 1 : num_iterations
%// Do your processing on data
%...
%...
img_struct.(['Img_' num2str(ii)]) = data; %// After iteration
end
This will create a structure called img_struct where it will have fields that are named Img_1, Img_2, etc. To access a particular data from an iteration... say... iteration 1, do:
data = img_struct.Img_1;
Change the _1 to whatever iteration you choose.
Alternatively, you can use cell arrays... same line of thinking:
%// Create empty cell array
img_cell = cell(num_iterations, 1);
for ii = 1 : num_iterations
%// Do your processing on data
%...
%...
img_cell{ii} = data; %// After iteration
end
Cell arrays are arrays that take on any type per element - or they're non-homogeneous arrays. This means that each element can be whatever you want. As such, because your image data varies in size at each iteration, this will do very nicely. To access data at any iteration, simply do:
data = img_cell{ii};
ii is the index of the iteration you want to access.
If you want to literally obtain what you are asking for, you can use the eval() function, which takes a string as input that it will evaluate as if it were a line of code. Example:
for i=1:3
data=ones(i); % assign data, 'ones(i)' used as dummy for test
eval(['Img_' num2str(i) '=data;'])
end
However, I would recommend using cell arrays {}, or alternatively the struct function that rayryeng both suggested.
Related
My code is posted below. It does exactly what I need it to do.
It reads in a file and plots the data that I need. If I want to read in another file and have it go through the same code, without having to write the whole thing a second time with different variables, is that possible? I would like to store the matrices from each loop.
As you can see the file I get is called: Oxygen_1keV_300K.xlsx
I have another file called: Oxygen_1keV_600K.xlsx
and so on.
How can I loop through these files without having to re-code the whole thing? I then want to plot them all on the same graph. It would be nice to store the final matrix Y and Ymean for each file so they are not overwritten.
clear
clc
files = ['Oxygen_1keV_300K','Oxygen_1keV_300K','Oxygen_1keV_600K','Oxygen_1keV_900K'];
celldata = cellstr(file)
k = cell(1,24);
for k=1:24
data{k} = xlsread('C:\Users\Ben\Desktop\Oxygen_1keV_300K.xlsx',['PKA', num2str(k)]);
end
for i=1:24
xfinal{i}=data{1,i}(end,1);
xi{i}=0:0.001:xfinal{i};
xi{i}=transpose(xi{i});
x{i}=data{1,i}(:,1);
y{i}=data{1,i}(:,4);
yi{i} = interp1(x{i},y{i},xi{i});
end
Y = zeros(10001, numel(data));
for ii = 1 : numel(data)
Y(:, ii) = yi{ii}(1 : 10001);
end
Ymean = mean(Y, 2);
figure (1)
x=0:0.001:10;
semilogy(x,Ymean)
Cell arrays make it very easy to store a list of strings that you can access as part of a for loop. In this case, I would suggest putting your file paths in a cell array as a substitute for the string used in your xlsread call
For example,
%The first file is the same as in your example.
%I just made up file names for the next two.
%Use the full file path if the file is not in your current directory
filepath_list = {'C:\Users\Ben\Desktop\Oxygen_1keV_300K.xlsx', 'file2.xlsx', 'file3.xlsx'};
%To store separate results for each file, make Ymean a cell array or matrix too
YMean = zeros(length(filepath_list), 1);
%Now use a for loop to loop over the files
for ii=1:length(filepath_list)
%Here's where your existing code would go
%I only include the sections which change due to the loop
for k=1:24
%The change is that on this line you use the cell array variable to load the next file path
data{k} = xlsread(filepath_list{ii},['PKA', num2str(k)]);
end
% ... do the rest of your processing
%You'll need to index into Ymean to store your result in the corresponding location
YMean(ii) = mean(Y, 2);
end
Cell arrays are a basic matlab variable type. For an introduction, I recommend the documentation for creating and accessing data in cell arrays.
If all your files are in the same directory, you can also use functions like dir or ls to populate the cell array programatically.
I have big data files with currents and voltages. I measure several times my devices but the number of measurements varies. I first ask the user how many rounds of measurements there are, after which I make a loop to extract the currents from the data. My current variables are arrays of 200x3000 doubles. I call them for example Isd_round1, Isd_round2, etc...
for i=1:rounds_number
[filename,pathname]=uigetfile('*.mat', 'Select matlab data');
pathname = cd(pathname);
pathname = strcat(pathname, '\', filename);
Val=load(pathname);
assignin('base', ['Isd1_round' num2str(i)], Val.Isd1)
...etc...
end
After that, I want to plot and compare the currents, but I cannot seem to find a way to call the variables by changing the index. I would like to do something like this:
figure
hold on
for j=1:rounds_number
plot(V, Isd_roundj)
end
And I don't know how to call the variables by changing the index in the loop.
I could also do an array of all the currents but as each current is already an array of (n, m) doubles, how can I create a variable "current" where I would assign "Isd_round1" ?
I would recommend using a cell array to store the data, as was mentioned before. Here is some example code how, to do this:
dataCell = cell(rounds_number,1)
% read data in
for i=1:rounds_number
[filename,pathname]=uigetfile('*.mat', 'Select matlab data');
pathname = cd(pathname);
pathname = strcat(pathname, '\', filename);
Val=load(pathname);
dataCell{i} =Val
end
%plot
for i=1:rounds_number
plot(dataCell{i})
end
%quicker, warning this plots all rounds at once
cellfun(#(x) plot(x),dataCell)
I have a huge text file that needs to be read and processed in MATLAB. This file at some points contain text to indicate that a new data series has started.
I have searched here but cant find any simple solution.
So what I want to do is to read the data in the file, put the data in a table in three different columns and when it finds text it should create a new table. It should repeat this process until the entire document is scanned.
This is how the document looks like:
time V(A,B) I(R1)
Step Information: X=1 (Run: 1/11)
0.000000000000000e+000 -2.680148e-016 0.000000e+00
9.843925313007988e-012 -4.753470e-006 2.216314e-011
1.000052605772457e-011 -4.835427e-006 2.552497e-011
1.031372754715773e-011 -4.999340e-006 -3.042096e-012
1.094013052602406e-011 -5.327165e-006 -1.206968e-011
Step Information: X=1 (Run: 2/11)
0.000000000000000e+000 -2.680148e-016 0.000000e+000
9.843925313007988e-012 -4.753470e-006 2.216314e-011
1.000052605772457e-011 -4.835427e-006 2.552497e-011
1.031372754715773e-011 -4.999340e-006 -3.042096e-012
1.094013052602406e-011 -5.327165e-006 -1.206968e-011
A rather crude approach is to read the file line by line and check if the line consists of three numbers. If it does, then append this to a temporary matrix. When you finally get to a line that doesn't contain three numbers, append this matrix as an element in a cell array, clear the temporary matrix and continue.
Something like this would work, assuming that the file is stored in 'file.txt':
%// Open the file
f = fopen('file.txt', 'r');
%// Initialize empty cell array
data = {};
%// Initialize temporary matrix
temp = [];
%// Loop over the file...
while true
%// Get a line from the file
line = fgetl(f);
%// If we reach the end of the file, get out
if line == -1
%// Last check before we break
%// Check if the temporary matrix isn't empty and add
if ~isempty(temp)
data = [data; temp];
end
break;
end
%// Else, check to see if this line contains three numbers
numbers = textscan(line, '%f %f %f');
%// If this line doesn't consist of three numbers...
if all(cellfun(#isempty, numbers))
%// If the temporary matrix is empty, skip
if isempty(temp)
continue;
end
%// Concatenate to cell array
data = [data; temp];
%// Reset temporary matrix
temp = [];
%// If this does, then create a row vector and concatenate
else
temp = [temp; numbers{:}];
end
end
%// Close the file
fclose(f);
The code is pretty self-explanatory but let's go into it to be sure you know what's going on. First open up the file with fopen to get a "pointer" to the file, then initialize our cell array that will contain our matrices as well as the temporary matrix used when reading in matrices in between header information. After we simply loop over each line of the file and we can grab a line with fgetl using the file pointer we created. We then check to see if we have reached the end of the file and if we have, let's check to see if the temporary matrix has any numerical data in it. If it does, add this into our cell array then finally get out of the loop. We use fclose to close up the file and clean things up.
Now the heart of the operation is what follows after this check. We use textscan and search for three numbers separated by spaces. That's done with the '%f %f %f' format specifier. This should give you a cell array of three elements if you are successful with numbers. If this is correct, then convert this cell array of elements into a row of numbers and concatenate this into the temporary matrix. Doing temp = [temp; numbers{:}]; facilitates this concatenation. Simply put I piece together each number and concatenate them horizontally to create a single row of numbers. I then take this row and concatenate this as another row in the temporary matrix.
Should we finally get to a line where it's all text, this will give you all three elements in the cell array found by textscan to be empty. That's the purpose of the all and cellfun call. We search each element in the cell and see if it's empty. If every element is empty, this is a line that is text. If this situation arises, simply take the temporary matrix and add this as a new entry into your cell array. You'd then reset the temporary matrix and start the logic over again.
However, we also have to take into account that there may be multiple lines that consist of text. That's what the additional if statement is for inside the first if block using all. If we have an additional line of text that precedes a previous line of text, the temporary matrix of values should still be empty and so you should check to see if that is empty before you try and concatenate the temporary matrix. If it's empty, don't bother and just continue.
After running this code, I get the following for my data matrix:
>> format long g
>> celldisp(data)
data{1} =
0 -2.680148e-16 0
9.84392531300799e-12 -4.75347e-06 2.216314e-11
1.00005260577246e-11 -4.835427e-06 2.552497e-11
1.03137275471577e-11 -4.99934e-06 -3.042096e-12
1.09401305260241e-11 -5.327165e-06 -1.206968e-11
data{2} =
0 -2.680148e-16 0
9.84392531300799e-12 -4.75347e-06 2.216314e-11
1.00005260577246e-11 -4.835427e-06 2.552497e-11
1.03137275471577e-11 -4.99934e-06 -3.042096e-12
1.09401305260241e-11 -5.327165e-06 -1.206968e-11
To access a particular "table", do data{ii} where ii is the table you want to access that was read in from top to bottom in your text file.
The most versatile way is to read line by line using textscan. If you want to speed this process up, you can have a dummy read first:
ie. You loop through all the lines without storing the data and decide which lines are the text lines and which are numbers, recording a quick number of lines for each.
You then have enough information about the data to run through quickly the arrays. This will speed up the time it takes to store the data in your new arrays massively.
Your second loop is the one that actually reads the data into the array/s. You should now know which lines to skip. You can also pre-allocate the arrays within the data cell if you wish to.
fid = fopen('file.txt','r');
data = {};
nlines = [];
% now start the loop
k=0; % counter for data sets
while ~feof(fid)
line = fgetl(fid);
% check if is data or text
if all(ismember(line,' 0123456789+.')) % is it data
nlines(k) = nlines(k)+1;
else %is it text
k=k+1;
nlines(k) = 0;
end
end
frewind(fid); % go back to start of file
% You could preallocate the data array here if you wished
% now get the data
for aa = 1 : length(nlines)
if nlines(aa)==0;
continue
end
textscan(fid,'%s\r\n',1); % skip textline
data{aa} = textscan(fid,'%f%f%f\r\n',nlines(k));
end
I have a 2d matrix (A=80,42), I am trying to split it into (80,1) 42 times and save it with a different name. i.e.
M_n1, M_n2, M_n3, … etc (representing the number of column)
I tried
for i= 1:42
M_n(i)=A(:,i)
end
it didn't work
How can I do that without overwrite the result and save each iteration in a file (.txt) ?
You can use eval
for ii = 1:size(A,2)
eval( sprintf( 'M_n%d = A(:,%d);', ii, ii ) );
% now you have M_n? var for you to process
end
However, the use of eval is not recommanded, you might be better off using cell array
M_n = mat2cell( A, [size(A,1)], ones( 1, size(A,2) ) );
Now you have M_n a cell array with 42 cells one for each column of A.
You can access the ii-th column by M_n{ii}
Generally, doing if you consider doing this kind of things: don't.
It does not scale up well, and having them in one array is usually far more convenient.
As long as the results have the same shape, you can use a standard array, if not you can put each result in a cell array eg. :
results = cell(nTests,1)
result{1} = runTest(inputs{1})
or even
results = cellfun(#runTest,inputs,'UniformOutput',false); % where inputs is a cell array
And so on.
If you do want to write the numbers to a file at each iteration, you could do it without the names with csvwrite or the like (since you're only talking about 80 numbers a time).
Another option is using matfile, which lets you write directly to a variable in a .mat file. Consult help matfile for the specifics.
I need to calculate the mean, standard deviation, and other values for a number of variables and I was wondering how to use a loop to my advantage. I have 5 electrodes of data. So to calculate the mean of each I do this:
mean_ch1 = mean(ch1);
mean_ch2 = mean(ch2);
mean_ch3 = mean(ch3);
mean_ch4 = mean(ch4);
mean_ch5 = mean(ch5);
What I want is to be able to condense that code into a line or so. The code I tried does not work:
for i = 1:5
mean_ch(i) = mean(ch(i));
end
I know this code is wrong but it conveys the idea of what I'm trying to accomplish. I want to end up with 5 separate variables that are named by the loop or a cell array with all 5 variables within it allowing for easy recall. I know there must be a way to write this code I'm just not sure how to accomplish it.
You have a few options for how you can do this:
You can put all your channel data into one large matrix first, then compute the mean of the rows or columns using the function MEAN. For example, if each chX variable is an N-by-1 array, you can do the following:
chArray = [ch1 ch2 ch3 ch4 ch5]; %# Make an N-by-5 matrix
meanArray = mean(chArray); %# Take the mean of each column
You can put all your channel data into a cell array first, then compute the mean of each cell using the function CELLFUN:
meanArray = cellfun(#mean,{ch1,ch2,ch3,ch4,ch5});
This would work even if each chX array is a different length from one another.
You can use EVAL to generate the separate variables for each channel mean:
for iChannel = 1:5
varName = ['ch' int2str(iChannel)]; %# Create the name string
eval(['mean_' varName ' = mean(' varName ');']);
end
If it's always exactly 5 channels, you can do
ch = {ch1, ch2, ch3, ch4, ch5}
for j = 1:5
mean_ch(j) = mean(ch{j});
end
A more complicated way would be
for j = 1:nchannels
mean_ch(j) = eval(['mean(ch' num2str(j) ')']);
end
Apart from gnovice's answer. You could use structures and dynamic field names to accomplish your task. First I assume that your channel data variables are all in the format ch* and are the only variables in your MATLAB workspace. The you could do something like the following
%# Move the channel data into a structure with fields ch1, ch2, ....
%# This could be done by saving and reloading the workspace
save('channelData.mat','ch*');
chanData = load('channelData.mat');
%# Next you can then loop through the structure calculating the mean for each channel
flds = fieldnames(chanData); %# get the fieldnames stored in the structure
for i=1:length(flds)
mean_ch(i) = mean(chanData.(flds{i});
end