function letter=read_char(imgn, num_letters)
global templates
comp=[ ];
for n=1:num_letters
sem=corr2(imgn, templates{1,n});
comp=[comp sem];
end
vd=find(comp==max(comp));
Can someon please explain what the 'FOR' loop does and how 'vd' is calculated?
Also, what is the variable 'comp' and what will it looks like as the array also contains itself and another variable 'sem' which is calculated from the corr2 function.
thanks
The for loop creates a loop variable n that starts at 1 for the first loop and is incremented by 1 for each successive loop until it reaches num_letters. This means the loop will execute num_letters times.
The variable comp is first initialized to the empty matrix []. Within the loop, a 2-D correlation coefficient is computed between the matrix imgn and another matrix templates{1,n} (indexed from a cell array) using the function CORR2. The correlation coefficient sem is appended to the array comp using horizontal concatenation. This will ultimately result in comp being a 1-by-num_letters array of correlation coefficients.
The variable vd stores the array indices where values in comp are equal to the maximum value found in comp. This is done using the functions MAX and FIND and the comparison operator ==.
In the future, I would urge you to first use the online documentation to try and help you better understand how MATLAB works. It is very good documentation. I've learned most of what I know from it. ;)
Related
I'm trying to vectorize one function in Matlab, but I have a problem with assigning values.
function [val] = clenshaw(coeffs,x)
b=zeros(1,length(coeffs)+2);
for k=length(coeffs):-1:2
b(k)=coeffs(k)-b(k+2)+2*b(k+1).*x;
end
val=coeffs(1)-b(3)+b(2).*x;
The purpose of this function is to use Clenshaw's algorithm to compute a value of one polynomial with coefficients "coeffs" at point x.
It work fine when x is a single value, but I'd like it to work with vector of arguments too.
When I try to pass a vector I get an error:
Unable to perform assignment because the left
and right sides have a different number of
elements.
Error in clenshaw (line 7)
b(k)=coeffs(k)-b(k+2)+2*b(k+1).*x;
I understand that there is a problem, because I'm trying to assign vector to a scalar variable b(k).
I tried making b a matrix instead of a vector, however I still cannot get the return output I'd like to have which would be a vector of values of this function at points from vector x.
Thank you for helping and sorry if something isn't entirely clear, because English is not my native language.
The vectorized version of your function looks like this:
function [val] = clenshaw(coeffs,x)
b=zeros(length(x),length(coeffs)+2);
for k=length(coeffs):-1:2
b(:,k)=coeffs(k)-b(:,k+2)+2*b(:,k+1).*transpose(x);
end
val=coeffs(1)-b(:,3)+b(:,2).*transpose(x);
end
b needs to be a matrix. In your loop, you have to perform every operation per row of b. So you need to write b(:,k) instead of b(k). Since b(:,k) is a vector and not a scalar, you also have to be careful with the dimensions when using the .* operator. To get the correct results, you need to transpose x. The same goes for the calculation of val. If you don't like the transposition, just swap the rows and cols of b and you get this:
function [val] = clenshaw(coeffs,x)
b=zeros(length(coeffs)+2, length(x));
for k=length(coeffs):-1:2
b(k,:)=coeffs(k)-b(k+2,:)+2*b(k+1,:).*x;
end
val=coeffs(1)-b(3,:)+b(2,:).*x;
end
However, the first version returns a column vector and the second a row vector. So you might need to transpose the result if the vector type is important.
I am a beginner in Matlab and have not been able to find an answer to my question so far. Your help will definitely be very much appreciated.
I have 70 matrices (100x100), named SUBJ_1, SUBJ_2 etc. I would like to create a loop so that I would calculate some metrics (i.e. max and min values) for each matrix, and save the output in a 70x2 result matrix (where each row would correspond to the consecutively named SUBJ_ matrix).
I am struggling with both stages - how to use the names of individual variables in a 'for' loop and how to properly save individual outputs in a combined array.
Many thanks and all the best!
Don't use such variable names, create a big cell array named SUBJ and put each Matrix in it.
r=zeros(numel(SUBJ),2)
for idx=1:numel(SUBJ)
r(idx,1)=min(min(SUBJ{idx}))
r(idx,2)=max(max(SUBJ{idx}))
end
min and max are called twice because first call creates maximum among rows, second call among columns.
Even though this is in principle possible in Matlab, I would not recommend it: too slow and cumbersome to implement.
You could instead use a 3-D matrix (100x100x70) SUBJ which would contain all the SUBJ_1 etc. in one matrix. This would allow you to calculate min/max etc. with just one line of code. Matlab will take care of the loops internally:
OUTPUT(:,1) = min(min(SUBJ,[],1)[],2);
OUTPUT(:,2) = max(max(SUBJ,[],1)[],2);
Like this, OUTPUT(1,1) contains min(min(SUBJ(:,:,1))) and so on...
As to how to use the names of individual variables in a 'for' loop, here gives an example:
SUBJ = [];
for idx = 1:70
term = eval(['SUBJ_',num2str(idx)]);
SUBJ = [SUBJ; max(max(term)),min(min(term))];
end
I don't understand this piece of code of a for loop in matlab, I know that loops in matlab usually look like: for ii=1:2:100 so that it starts in 1 until 100 and in each iteration you add 2.
But here I've got this condition in the loop and I don't get what it does:
for ii=[1:w:rd(1)-w-border, rd(1)-w-border+1],
...
end;
w and border are integers passed as arguments and rd is size of the image/matrix (rd = size(image);)
Can someone explain me how for loops work in matlab with this kind of condition?
Thanks in advance.
For loop in matlab can execute statements for a defined set of index values:
For example, the following code will display all the element in the set [1,5,8,17]:
for s = [1,5,8,17]
disp(s)
end
Your code for ii=[1:w:rd(1)-w-border, rd(1)-w-border+1] is similar.
Its just like a set 1:w:rd(1)-w-border with an additional element rd(1)-w-border+1.
Its like writing this set [1,2,3,4,5,8] as [1:1:5, 8]
I hope its clear now.
the for argument is a vector. the loop iterator ii takes one value for the vector for each iteration of the loop. As you mentioned, the vector can be equally spaced one like 1:2:100. But it can also be arbitrary, for example for ii = [4,6,1,8] ....
In you case the for argument vector is partly "equally spaced" vector: 1:w:rd(1)-w-border plus another element rd(1)-border+1.
I am trying to use MATLAB in order to generate a variable whose elements are either 0 or 1. I want to define this variable using some kind of concatenation (equivalent of Java string append) so that I can add as many 0's and 1's according to some upper limit.
I can only think of using a for loop to append values to an existing variable. Something like
variable=1;
for i=1:N
if ( i%2==0)
variable = variable.append('0')
else
variable = variable.append('1')
i=i+1;
end
Is there a better way to do this?
In MATLAB, you can almost always avoid a loop by treating arrays in a vectorized way.
The result of pseudo-code you provided can be obtained in a single line as:
variable = mod((1:N),2);
The above line generates a row vector [1,2,...,N] (with the code (1:N), use (1:N)' if you need a column vector) and the mod function (as most MATLAB functions) is applied to each element when it receives an array.
That's not valid Matlab code:
The % indicates the start of a comment, hence introducing a syntax error.
There is no append method (at least not for arrays).
Theres no need to increment the index in a for loop.
Aside of that it's a bad idea to have Matlab "grow" variables, as memory needs to be reallocated at each time, slowing it down considerably. The correct approach is:
variable=zeros(N,1);
for i=1:N
variable(i)=mod(i,2);
end
If you really do want to grow variables (some times it is inevitable) you can use this:
variable=[variable;1];
Use ; for appending rows, use , for appending columns (does the same as vertcat and horzcat). Use cat if you have more than 2 dimensions in your array.
I want to apply a function to all columns in a matrix with MATLAB. For example, I'd like to be able to call smooth on every column of a matrix, instead of having smooth treat the matrix as a vector (which is the default behaviour if you call smooth(matrix)).
I'm sure there must be a more idiomatic way to do this, but I can't find it, so I've defined a map_column function:
function result = map_column(m, func)
result = m;
for col = 1:size(m,2)
result(:,col) = func(m(:,col));
end
end
which I can call with:
smoothed = map_column(input, #(c) (smooth(c, 9)));
Is there anything wrong with this code? How could I improve it?
The MATLAB "for" statement actually loops over the columns of whatever's supplied - normally, this just results in a sequence of scalars since the vector passed into for (as in your example above) is a row vector. This means that you can rewrite the above code like this:
function result = map_column(m, func)
result = [];
for m_col = m
result = horzcat(result, func(m_col));
end
If func does not return a column vector, then you can add something like
f = func(m_col);
result = horzcat(result, f(:));
to force it into a column.
Your solution is fine.
Note that horizcat exacts a substantial performance penalty for large matrices. It makes the code be O(N^2) instead of O(N). For a 100x10,000 matrix, your implementation takes 2.6s on my machine, the horizcat one takes 64.5s. For a 100x5000 matrix, the horizcat implementation takes 15.7s.
If you wanted, you could generalize your function a little and make it be able to iterate over the final dimension or even over arbitrary dimensions (not just columns).
Maybe you could always transform the matrix with the ' operator and then transform the result back.
smoothed = smooth(input', 9)';
That at least works with the fft function.
A way to cause an implicit loop across the columns of a matrix is to use cellfun. That is, you must first convert the matrix to a cell array, each cell will hold one column. Then call cellfun. For example:
A = randn(10,5);
See that here I've computed the standard deviation for each column.
cellfun(#std,mat2cell(A,size(A,1),ones(1,size(A,2))))
ans =
0.78681 1.1473 0.89789 0.66635 1.3482
Of course, many functions in MATLAB are already set up to work on rows or columns of an array as the user indicates. This is true of std of course, but this is a convenient way to test that cellfun worked successfully.
std(A,[],1)
ans =
0.78681 1.1473 0.89789 0.66635 1.3482
Don't forget to preallocate the result matrix if you are dealing with large matrices. Otherwise your CPU will spend lots of cycles repeatedly re-allocating the matrix every time it adds a new row/column.
If this is a common use-case for your function, it would perhaps be a good idea to make the function iterate through the columns automatically if the input is not a vector.
This doesn't exactly solve your problem but it would simplify the functions' usage. In that case, the output should be a matrix, too.
You can also transform the matrix to one long column by using m(:,:) = m(:). However, it depends on your function if this would make sense.