Say that I have an empty array as follows:
s=[];
Say that for instance we have the following loop:
for j=1:2
for i=1:10
if a(i,j)>0
...
end
end
end
Instead of ..., I want to add elements to s. How do you do that in MatLab?
I would recommend you avoid loops altogether. They are slow in MATLAB.
Let's say you want to set all values in S(i,j) to 1 that correspond to A(i,j) > 0. You could do:
S = zeros(size(A)); % always a good idea to initialize your array
S(A > 0) = 1; % and done.
More succinctly:
S = A > 0;
This specifies that you are changing the values of S to 1 corresponding to those values of A where A > 0.
If you want to set the value of S to the corresponding value of A then you would just use:
S = A(A > 0);
Keep track of another index and just add elements as you go along:
idx = 1
for j=1:2
for i=1:10
if a(i,j)>0
s(idx) = a(i,j)
idx = idx + 1
end
end
end
Though for your particular problem, you could just write
a(a>0)
Related
I made a class that represents a matrix using a multidimensional array. The initialize method goes through every position and sets the value of the elements to the one specified, and the to_s method does the same but it just concatenates every element to a string. I'm writing an insert method that changes the value of the element of a given position to a given value, but it changes a whole value instead of just an element.
class Matrix
attr_accessor :rows, :cols, :arr
# Goes through the matrix setting everything with the "n" value
def initialize(r, c, n = "a")
#rows = r
#cols = c
#arr = Array.new(#rows)
tmpArray = Array.new(#cols)
i = 0
while i < #rows
j = 0
while j < #cols
tmpArray[j] = n
j += 1
end
#arr[i] = tmpArray
i += 1
end
return #arr
end
def to_s
i = 0
str = String.new
while i < #rows
j = 0
str << "("
while j < #cols
str << " "
if #arr[i][j].is_a?String
str << #arr[i][j]
else
str << #arr[i][j].to_s
end
j += 1
end
str << " )\n"
i += 1
end
return str
end
# Calls and prints to_s
def write
print self.to_s
return self.to_s
end
# Rewrites the element (r, c) as the value in "n"
def insert(r, c, n)
#arr[r][c] = n
self.write
return self
end
end
The thing is that, when I print the matrix I notice that I didn't change just an element, but a whole column of the matrix.
a = Matrix.new(2, 2, 0)
a.insert(0, 0, 1)
a.write
# Expected output: ( 1 0 )
# ( 0 0 )
# Real output: ( 1 0 )
# ( 1 0 )
The to_s method isn't failing. I already made a trace of it and tested it. I'm printing the real values that are in the positions of the matrix.
Your program calls Array.new only twice, and it doesn't create arrays using any other methods, and it doesn't use dup or clone to copy any arrays. Therefore, you only have two different array objects in your program. Each element of #arr actually points to the same row array.
One idea for a solution would be to replace this line:
#arr[i] = tmpArray
with this:
#arr[i] = tmpArray.dup
However, your code is pretty long and I have not tried running it, so I do not know if that would be a complete solution.
I'm stuck on an algorithm for a function called adjacentElementsProduct that accepts an array as the argument. It's supposed to return the largest product of adjacent numbers in the array. For example, if the argument is [2,4,1,3,2,6] it would return 12 because of the pair of 2 and 6.
my code is
def adjacentElementsProduct(inputArray)
idx1 = 0
idx2 = 1
while idx2 < inputArray.length
pair = [inputArray[idx1], inputArray[idx1 + 1]]
next_pair = [inputArray[idx2], inputArray[idx2 + 1]]
if next_pair.reduce(:+) > pair.reduce(:+)
pair = next_pair
idx1 += 1
idx2 += 1
else
idx1 += 1
idx2 += 1
end
end
pair.reduce(:+)
end
I just can't figure out where my code is not working. I'm just looking for a push in the right direction because I know just being given the answer won't help me as much. Can anyone help me?
The code makes no sense :).
You are using + instead of *
And in the loop you always assign pair = [inputArray[idx1], inputArray[idx1 + 1]].
So you always return the last pair or the previous. If the maximum product is at the beginning, you still keep advancing the pair variable until the end of the loop.
Besides, the solution is quite complicated.
def adjacentElementsProduct(inputArray)
index = 0
length = inputArray.length
max = 0
while index < length-1 do
result = inputArray[index] * inputArray[index+1]
max = result if result > max
index += 1
end
max
end
I have a question regarding indexing and loops in MATLAB. I have a vector of length n (named data in the code below). I want to examine this vector 4 elements at a time inside of a for loop. How can I do this? My attempt included below does not work because it will exceed the array dimensions at the end of the loop.
for k = 1:length(data)
index = k:k+3;
cur_data = data(index);
pre_q_data1 = cur_data(1);
pre_q_data2 = cur_data(2);
% Interweaving the data
q = [pre_q_data1; pre_q_data2];
qdata = q(:)';
pre_i_data1 = cur_data(3);
pre_i_data2 = cur_data(4);
i = [pre_i_data1; pre_i_data2];
idata = i(:)';
end
You shouldn't have k go all the way to length(data) if you're planning on indexing up to k+3.
I've also taken the liberty of greatly simplifying your code, but feel free to ignore that!
for k = 1:length(data)-3
% maximum k = length(data)-3, so maximum index = length(data)-3+3=length(data)
index = k:k+3;
cur_data = data(k:k+3);
% Interweaving the data
q = cur_data(1:2); % transpose at end of line here if need be
i = cur_data(3:4); % could just use data(k+2:k+3) and not use cur_data
end
This is a simple sorting function that I wrote in Matlab:
function [matrix] = sorting(matrix)
for index = 1:length(matrix)-1
if matrix(index) > matrix(index + 1)
temp = matrix(index + 1);
matrix(index + 1) = matrix(index);
matrix(index) = temp;
end
end
check_sorted(matrix)
end
function [matrix] = check_sorted(matrix)
count = 0;
for index = 1:length(matrix)-1
if matrix(index) < matrix(index + 1)
count = count + 1;
end
end
if count+1 < length(matrix)
sorting(matrix);
end
end
The input for sorting function is a 1D array, e.g. [4 3 2 1], and it successfully returns the sorted array [1 2 3 4] for the first time I call it, but then it starts to return unsorted arrays?
You've got a missing semicolon that is causing the results of each call to check_sorted to be displayed, which is confusing things. If you add the semicolon, the output from sorting with the array [2 4 1 3] suggested in the comments is:
>> sorting([2 4 1 3])
ans =
2 1 3 4
Clearly this isn't sorted. The problem is that MATLAB passes function arguments by value, not by reference. Since you're not returning the re-sorted matrix from check_sorted or updating the return matrix in sorting the original matrix never gets updated. You need to change at least one line in each function (changed lines are commented):
function [matrix] = check_sorted(matrix)
count = 0;
for index = 1:length(matrix)-1
if matrix(index) < matrix(index + 1)
count = count + 1;
end
end
if count+1 < length(matrix)
matrix = sorting(matrix); % change: return re-sorted matrix
end
end
function [matrix] = sorting(matrix)
for index = 1:length(matrix)-1
if matrix(index) > matrix(index + 1)
temp = matrix(index + 1);
matrix(index + 1) = matrix(index);
matrix(index) = temp;
end
end
matrix = check_sorted(matrix); % change: return checked matrix
end
Now the matrix will be updated if it is not sorted on the first (or any subsequent) pass and the fully sorted matrix will be returned by sorting.
This is an odd sort of recursion that really isn't necessary. If you change check_sorted to return a boolean value, true for sorted, false for not sorted, you can change that recursion to a while loop around the for loop in sorting:
function [TF] = check_sorted2(matrix)
count = 0;
for index = 1:length(matrix)-1
if matrix(index) < matrix(index + 1)
count = count + 1;
end
end
TF = count+1 == length(matrix); % TF = true if matrix is sorted
% TF = false otherwise
end
function [matrix] = sorting2(matrix)
while ~check_sorted2(matrix) % keep going until matrix is sorted
for index = 1:length(matrix)-1
if matrix(index) > matrix(index + 1)
temp = matrix(index + 1);
matrix(index + 1) = matrix(index);
matrix(index) = temp;
end
end
end
end
Of course the whole thing can be optimized and vectorized, but this will at least get you going.
i tested your algorithms and it worked. so something else might be wrong. but this algorithm is very very inefficient. you may google sort and pick one that suits you.
if you really want to stick with the algorithm, you could improve it by shortening the two loops. for example, after first call to sorting, for each subsequent call to sorting, you could shorten the loop cycle by 1 because the first call to sorting would put the largest number to the end of the array, the second call would put the second largest to the second from the end, and so on. this is so called bubble sorting. also in check_sorted, you don't need to go through the entire length of the array to check if the array has been sorted. as soon as you see matrix(index) > matrix(index + 1), you can immediately exit the loop (after you set a flag to indicate the array hasn't been sorted).
I'm still confused why am not able to know the results of this small algorithm of my array. the array has almost 1000 number 1-D. am trying to find the peak and the index of each peak. I did found the peaks, but I can't find the index of them. Could you please help me out. I want to plot all my values regardless the indexes.
%clear all
%close all
%clc
%// not generally appreciated
%-----------------------------------
%message1.txt.
%-----------------------------------
% t=linspace(0,tmax,length(x)); %get all numbers
% t1_n=0:0.05:tmax;
x=load('ww.txt');
tmax= length(x) ;
tt= 0:tmax -1;
x4 = x(1:5:end);
t1_n = 1:5:tt;
x1_n_ref=0;
k=0;
for i=1:length(x4)
if x4(i)>170
if x1_n_ref-x4(i)<0
x1_n_ref=x4(i);
alpha=1;
elseif alpha==1 && x1_n_ref-x4(i)>0
k=k+1;
peak(k)=x1_n_ref; // This is my peak value. but I also want to know the index of it. which will represent the time.
%peak_time(k) = t1_n(i); // this is my issue.
alpha=2;
end
else
x1_n_ref=0;
end
end
%----------------------
figure(1)
% plot(t,x,'k','linewidth',2)
hold on
% subplot(2,1,1)
grid
plot( x4,'b'); % ,tt,x,'k'
legend('down-sampling by 5');
Here is you error:
tmax= length(x) ;
tt= 0:tmax -1;
x4 = x(1:5:end);
t1_n = 1:5:tt; % <---
tt is an array containing numbers 0 through tmax-1. Defining t1_n as t1_n = 1:5:tt will not create an array, but an empty matrix. Why? Expression t1_n = 1:5:tt will use only the first value of array tt, hence reduce to t1_n = 1:5:tt = 1:5:0 = <empty matrix>. Naturally, when you later on try to access t1_n as if it were an array (peak_time(k) = t1_n(i)), you'll get an error.
You probably want to exchange t1_n = 1:5:tt with
t1_n = 1:5:tmax;
You need to index the tt array correctly.
you can use
t1_n = tt(1:5:end); % note that this will give a zero based index, rather than a 1 based index, due to t1_n starting at 0. you can use t1_n = 1:tmax if you want 1 based (matlab style)
you can also cut down the code a little, there are some variables that dont seem to be used, or may not be necessary -- including the t1_n variable:
x=load('ww.txt');
tmax= length(x);
x4 = x(1:5:end);
xmin = 170
% now change the code
maxnopeaks = round(tmax/2);
peaks(maxnopeaks)=0; % preallocate the peaks for speed
index(maxnopeaks)=0; % preallocate index for speed
i = 0;
for n = 2 : tmax-1
if x(n) > xmin
if x(n) >= x(n-1) & x(n) >= x(n+1)
i = i+1;
peaks(i) = t(n);
index(i) = n;
end
end
end
% now trim the excess values (if any)
peaks = peaks(1:i);
index = index(1:i);