I have already looked here and here, but I'm not sure I found what I need.
I have an irregular file (that represents neighbors of particles 1 to 5) that looks like that
2 3 5
1 3
1 2
1
I am want to figure out a way to load it (as 'something' called A) and do the following things:
Count the number of elements on one line (for instance size(A(1,:)) shall give me 3)
Given an array B (of size 5) select the elements of B corresponding to the indices given by a line (something like B(A(1,:)) shall give me [B(2) B(3) B(5)])
Since you want arrays with size depending on their first index, you're probably left with cells. Your cell A could be such that A{1} equals to [2 3 5] and A{2} to [1 3] in your example etc. To do this you can read your file infile by
fid=fopen(infile,'rt');
A=[];
while 1
nextline=fgets(fid);
if nextline==-1 %EOF reached
break;
end
A{end+1}=sscanf(nextline,'%d');
end
fclose(fid);
%demonstrate use for indexing
B=randi(10,5,1);
B(A{3}) %2-element vector
B(A{4}) %empty vector
Then A{i} is a vector corresponding to the ith line in your file. If the line is empty, then it's the empty vector. You can use it to index B as you want to, see the example above. Note that you should not add a newline at the very end of your infile, otherwise you'll have a spurious empty element for A. If you know in advance how many lines you need to read, this is not an issue.
And the number of entries in line i are given by length(A{i}), with i=1:length(A).
Related
For example, I can create a zeros(100). But I want the entry of row 58 and column 59 to be 1. But I need temporary variable and multiple lines to do this.
a. Let this matrix be M. How can I do this in one line? M = ....?
P.S.
b. Better still, sometimes I want two or more entries of the zero matrix be 1.
Again, how can I do this?
If I can do a. in one-line, of course I can add them up. But is there any special function to do fill zero matrix entries with 1?
First, remember that a one line expression isn't always the most effective. It could also be harder to read/understand.
One way to do this is by using a sparse matrix
The following example creates a 10x10 zero-matrix with ones at [5,2] (row 5, col 2) and [7 5]
full(sparse([5 7],[2 5],1,10,10))
Use full to convert it from a sparse matrix to a "full" one
Another (faster but maybe not as intuitive) alternative is to use accumarray
accumarray([5 2;7 5],1,[10,10])
Remember that the index values above is used directly in the expression to get on one line, the better option would be to create them separately
points = [5 2; 7 5]
or perhaps,
rowIdx = [5 7];
colIdx = [2 5];
Lets say i have a matrix A of 300x65. the last column(65th) contains ordered values (1,2,3). the first 102 elements are '1', the second 50 elements are '2' and the remainder will be '3'.
I have another matrix B, which is 3x65 and i want to copy the first row of B by the number of '1's in matrix A. The second row of B should be copied by the number of '2's in in matrix A and the 3th row should be copied by the remaining value of matrix A. By doing this, matrix B should result in a 300x65 matrix.
I've tried to use the repmat function of matlab with no succes, does anyone know how to do this?
There are many inconsistencies in your problem
first if you copy 1 row of B for every element of A(which will end up happening by your description) that will result in a matrix 19500x65
secondly copy its self is a vague term, do you mean duplicate? do you want to store the copied value into a new var?
what I gathered from your problem is you want to preform some operation between A and B to create a matrix and store it in B which in itself will cause the process to warp as it goes if you do not have another variable to store the result in
so i suggest using a third variable c to store the result in and then if you need it to be in b set b = C
also for whatever process you badly described I recommend learning to use a 'for' loop effectively because it seems like that is what you would need to use
syntax for 'for' loop
for i = [start:increment:end]
//loops for the length of [start:increment:end]
//sets i to the nth element of [start:increment:end] where n is the number of times the loop has run
end
If I understand your question, this should do it
index = A(:,end); % will be a column of numbers with values of 1, 2, or 3
newB = B(index,:); % B has 3 rows, which are copied as required by "index"
This should result in newB having the same number of rows as A and the same number of columns as the original B
X =
4 3
8 3
I want to extract the element in each row of X and do some operation on each of them separably (4,3) and (8,3). however the size of may be different based on some parameters in my code, so I want general formula to do such thing,
How I can use the for loop for solving this issue ?
This link shows how to extract specific lines (or columns) from a matrix http://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html
All you have to do is write a loop on an index ii to go through every line.
for ii=1:size(X,1)
a=myfun(X(ii,:));
end
Could somebody explain the following code snippet? I have no background in computer science or programming and just recently became aware of Matlab. I understand the preallocation part from data=ceil(rand(7,5)*10)... to ...N*(N-1)/2).
I need to understand every aspect of how matlab processes the code from kk=0 to the end. Also, the reasons why the code is codified in that manner. There's no need to explain the function of: bsxfun(#minus), just how it operates in the scheme of the code.
data=ceil(rand(7,5)*10);
N = size(data,2);
b=cell(N-1,1);
c=NaN(size(data,1),N*(N-1)/2);
kk=0;
for ii=1:N-1
b{ii} = bsxfun(#minus,data(:,ii),data(:,ii+1:end));
c(:,kk+(1:N-ii)) = bsxfun(#minus,data(:,ii),data(:,ii+1:end));
kk=kk+N-ii;
end
Start at zero
kk=0;
Loop with ii going from 1 up to N-1 incrementing by 1 every iteration. Type 1:10 in the command line of matlab and you'll see that it outputs 1 2 3 4 5 6 7 8 9 10. Thuis colon operator is a very important operator to understand in matlab.
for ii=1:N-1
b{ii} = ... this just stores a matrix in the next element of the cell vector b. Cell arrays can hold anything in each of their elements, this is necessary as in this case each iteration is creating a matrix with one fewer column than the previous iteration.
data(:,ii) --> just get the iith column of the matrix data (: means get all the rows)
data(:, ii + 1:end) means get a subset of the matrix data consisting of all the rows but only of columns that appear after column ii
bsxfun(#minus, data(:,ii), data(:,ii+1:end)) --> for each column in the matrix data(:, ii+1:end), subtract the single column data(:,ii)
b{ii} = bsxfun(#minus,data(:,ii),data(:,ii+1:end));
%This does the same thing as the line above but instead of storing the resulting matrix of the loop in a separate cell of a cell array, this is appending the original array with the new matrix. Note that the new matrix will have the same number of rows each time but one fewer column, so this appends as new columns.
%c(:,kk + (1:N-ii)) = .... --> So 1:(N-ii) produces the numbers 1 up to the number of columns in the result of this iteration. In matlab, you can index an array using another array. So for example try this in the command line of matlab: a = [0 0 0 0 0]; a([1 3 5]) = 1. The result you should see is a = 1 0 1 0 1. but you can also extend a matrix like this so for example now type a(6) = 2. The result: a = 1 0 1 0 1 2. So by using c(:, 1:N-ii) we are indexing all the rows of c and also the right number of columns (in order). Adding the kk is just offsetting it so that we do not overwrite our previous results.
c(:,kk+(1:N-ii)) = bsxfun(#minus,data(:,ii),data(:,ii+1:end));
Now we just increment kk by the number of new columns we added so that in the next iteration, c is appended at the end.
kk=kk+N-ii;
end;
I suggest that you put a breakpoint in this code and step through it line by line and look at how the variables change in matlab. To do this click on the little dashed line next to k=0; in the mfile, you will see a red dot appear there, and then run the code. The code will only execute as far as the dot, you are now in debug mode. If you hover over a variable in debug mode matlab will show its contents in a tool tip. For a really big variable check it out in the workspace. Now step through the code line by line and use my explanations above to make sure you understand how each line is changing each variable. For more complex lines like b{ii} = bsxfun(#minus,data(:,ii),data(:,ii+1:end)); you should highlight code snippets and ruin these in the command line to see what each part is doing so for example run data(:,ii) to see what that does and then try data(:,ii+1:end)) or even just ii+1:end (well in that case it wont work, replace end with size(data, 2)). Debugging is the best way to understand code that confuses you.
bsxfun(#minus,A,B)
is almost the same as
A-B
The difference is that the bsxfun version will handle inputs of different size: In each dimension (“direction,” if you find it easier to think about that way), if one of the inputs is scalar and the other one a vector, the scalar one will simply be repeated sufficiently often.
http://www.mathworks.com/help/techdoc/ref/bsxfun.html
I am trying to copy a few elements from a matrix, but not a whole row, and not a single element.
For example, in the following matrix:
a = 1 2
3 4
5 6
7 8
9 0
How would I copy out just the following data?
b = 1
3
5
i.e. rows 1:3 in column 1 only... I know that you can remove an entire column like this:
b = a(:,1)
... and I appreciate that could just do this and then dump the last two rows, but I'd like to use more streamlined code as I am running a very resource-intensive solution.
Elements in a matrix in MATLAB are stored in column-major order. Which means, you could even use a single index and say:
b = a(1:3);
Since the first 3 elements ARE 1,3,5. Similarly, a(6) is 2, a(7) is 4 etc. Look at the sub2ind method to understand more:
http://www.mathworks.com/help/techdoc/ref/sub2ind.html
You are not "removing" the second column, you are referencing the other column.
You should read some of the Matlab docs, they provide some help about the syntax for accessing portions of matrices:
http://www.mathworks.com/help/techdoc/learn_matlab/f2-12841.html#f2-428