How would I be able to do the equivalent of this with strings:
a = [1 2 3; 4 5 6];
c = [];
for i=1:5
b = a(1,:)+i;
c = [c;b];
end
c =
2 3 4
3 4 5
4 5 6
5 6 7
6 7 8
Basically looking to combine several strings into a Matrix.
You're growing a variable in a loop, which is a kind of sin in Matlab :) So I'm going to show you some better ways of doing array concatenation.
There's cell strings:
>> C = {
'In a cell string, it'
'doesn''t matter'
'if the strings'
'are not of equal lenght'};
>> C{2}
ans =
doesn't matter
Which you could use in a loop like so:
% NOTE: always pre-allocate everything before a loop
C = cell(5,1);
for ii = 1:5
% assign some random characters
C{ii} = char( '0'+round(rand(1+round(rand*10),1)*('z'-'0')) );
end
There's ordinary arrays, which have as a drawback that you have to know the size of all your strings beforehand:
a = [...
'testy' % works
'droop'
];
b = [...
'testing' % ERROR: CAT arguments dimensions
'if this works too' % are not consistent.
];
for these cases, use char:
>> b = char(...
'testing',...
'if this works too'...
);
b =
'testing '
'if this works too'
Note how char pads the first string with spaces to fit the length of the second string. Now again: don't use this in a loop, unless you've pre-allocated the array, or if there really is no other way to go.
Type help strfun on the Matlab command prompt to get an overview of all string-related functions available in Matlab.
You mean storing a string on each matrix position? You can't do that, since matrices are defined over basic types. You can have a CHAR on each position:
>> a = 'bla';
>> b = [a; a]
b <2x3 char> =
bla
bla
>> b(2,3) = 'e'
b =
bla
ble
If you want to store matrices, use a cell array (MATLAB reference, Blog of Loren Shure), which are kind of similar but using "{}" instead of "()":
>> c = {a; a}
c =
'bla'
'bla'
>> c{2}
ans =
bla
Related
How to access the whole elements hello and ahoy in Octave? Only the first character in each string is being printed.
octave:1> s = ["hello";"ahoy"]
s =
hello
ahoy
octave:2> s(1)
ans = h
octave:3> s(2)
ans = a
Use cell arrays instead.
octave:1> s = { 'hello'; 'ahoy' };
octave:2> s{1}
ans = hello
octave:3> s{2}
ans = ahoy
See https://octave.org/doc/v5.2.0/Cell-Arrays.html#Cell-Arrays
Check the size and type of s to understand what's going on:
octave:5> size(s)
ans =
2 5
octave:6> class(s)
ans = char
It's a 2x5 matrix of characters. To index, use matrix indexing. For example getting the first row:
octave:7> s(1,:)
ans = hello
A = {'a','b','c','b','a',...}
A is a <1X400> cell array and I want to create a matrix from A such that if the cell is a, the matrix shows 1, if it is b, it shows as 2 in the matrix and 3 for c.
Thank you.
Specific Case
For a simple specific case as listed in the question, you can use char to convert all the cell elements to characters and then subtract 96 from it, which is ascii equivalent of 'a'-1 -
A_numeric = char(A)-96
Sample run -
>> A
A =
'a' 'b' 'c' 'b' 'a'
>> A_numeric = char(A)-96
A_numeric =
1
2
3
2
1
Generic Case
For a generic substitution case, you need to do a bit more of work like so -
%// Inputs
A = {'correct','boss','cat','boss','correct','cat'}
newcellval = {'correct','cat','boss'}
newnumval = [8,2,5]
[unqcell,~,idx] = unique(A,'stable')
[~,newcell_idx,unqcell_idx] = intersect(newcellval,unqcell,'stable')
A_numeric = newnumval(changem(idx,newcell_idx,unqcell_idx))
Sample input-output -
>> A,newcellval,newnumval
A =
'correct' 'boss' 'cat' 'boss' 'correct' 'cat'
newcellval =
'correct' 'cat' 'boss'
newnumval =
8 2 5
>> A_numeric
A_numeric =
8 5 2 5 8 2
That's easy:
result = cell2mat(A)-'a'+1
For a generic association of letters to numbers 1,2,3...:
letters2numbers = 'abc'; %// 'a'->1, 'b'->2 etc.
[~, result] = ismember(cell2mat(A), letters2numbers)
For a generic association of strings to numbers 1,2,3...:
strings2numbers = {'hi', 'hello', 'hey', 'good morning', 'howdy'};
A = {'hello', 'hi', 'hello', 'howdy', 'bye'};
[~, result] = ismember(A, strings2numbers)
In this example,
result =
2 1 2 5 0
use a For Loop which iterate over A and convert character to number
for loop = 1:length(A)
outMat(loop) = char(A(loop)) - 96
end
I hope it works.
I have 2 cell of strings and I would like to order them according to the first one.
A = {'a';'b';'c'}
B = {'b';'a';'c'}
idx = [2,1,3] % TO FIND
B=B(idx);
I would like to find a way to find idx...
Use the second output of ismember. ismember tells you whether or not values in the first set are anywhere in the second set. The second output tells you where these values are located if we find anything. As such:
A = {'a';'b';'c'}
B = {'b';'a';'c'}
[~,idx] = ismember(A, B);
Note that there is a minor typo when you declared your cell arrays. You have a colon in between b and c for A and a and c for B. I placed a semi-colon there for both for correctness.
Therefore, we get:
idx =
2
1
3
Benchmarking
We have three very good algorithms here. As such, let's see how this performs by doing a benchmarking test. What I'm going to do is generate a 10000 x 1 random character array of lower case letters. This will then be encapsulated into a 10000 x 1 cell array, where each cell is a single character array. I construct A this way, and B is a random permutation of the elements in A. This is the code that I wrote to do this for us:
letters = char(97 + (0:25));
rng(123); %// Set seed for reproducibility
ind = randi(26, [10000, 1]);
lettersMat = letters(ind);
A = mat2cell(lettersMat, ones(10000,1), 1);
B = A(randperm(10000));
Now... here comes the testing code:
clear all;
close all;
letters = char(97 + (0:25));
rng(123); %// Set seed for reproducibility
ind = randi(26, [10000, 1]);
lettersMat = letters(ind);
A = mat2cell(lettersMat, 1, ones(10000,1));
B = A(randperm(10000));
tic;
[~,idx] = ismember(A,B);
t = toc;
fprintf('ismember: %f\n', t);
clear idx; %// Make sure test is unbiased
tic;
[~,idx] = max(bsxfun(#eq,char(A),char(B)'));
t = toc;
fprintf('bsxfun: %f\n', t);
clear idx; %// Make sure test is unbiased
tic;
[~, indA] = sort(A);
[~, indB] = sort(B);
idx = indB(indA);
t = toc;
fprintf('sort: %f\n', t);
This is what I get for timing:
ismember: 0.058947
bsxfun: 0.110809
sort: 0.006054
Luis Mendo's approach is the fastest, followed by ismember, and then finally bsxfun. For code compactness, ismember is preferred but for performance, sort is better. Personally, I think bsxfun should win because it's such a nice function to use ;).
This seems to be significantly faster than using ismember (although admittedly less clear than #rayryeng's answer). With thanks to #Divakar for his correction on this answer.
[~, indA] = sort(A);
[~, indB] = sort(B);
idx = indA(indB);
I had to jump in as it seems runtime performance could be a criteria here :)
Assuming that you are dealing with scalar strings(one character in each cell), here's my take that works even when you have not-commmon elements between A and B and uses the very powerful bsxfun and as such I am really hoping this would be runtime-efficient -
[v,idx] = max(bsxfun(#eq,char(A),char(B)'));
idx = v.*idx
Example -
A =
'a' 'b' 'c' 'd'
B =
'b' 'a' 'c' 'e'
idx =
2 1 3 0
For a specific case when you have no not-common elements between A and B, it becomes a one-liner -
[~,idx] = max(bsxfun(#eq,char(A),char(B)'))
Example -
A =
'a' 'b' 'c'
B =
'b' 'a' 'c'
idx =
2 1 3
I have a string (eg. 'STA') and I want to make a cell array that will be a concatenation of my sting with a numbers from 1 to X.
I want the code to do something like the fore loop here below:
for i = 1:Num
a = [{a} {strcat('STA',num2str(i))}]
end
I want the end results to be in the form of {<1xNum cell>}
a = 'STA1' 'STA2' 'STA3' ...
(I want to set this to a uitable in the ColumnFormat array)
ColumnFormat = {{a},... % 1
'numeric',... % 2
'numeric'}; % 3
I'm not sure about starting with STA1, but this should get you a list that starts with STA (from which I guess you could remove the first entry).
N = 5;
[X{1:N+1}] = deal('STA');
a = genvarname(X);
a = a(2:end);
You can do it with combination of NUM2STR (converts numbers to strings), CELLSTR (converts strings to cell array), STRTRIM (removes extra spaces)and STRCAT (combines with another string) functions.
You need (:) to make sure the numeric vector is column.
x = 1:Num;
a = strcat( 'STA', strtrim( cellstr( num2str(x(:)) ) ) );
As an alternative for matrix with more dimensions I have this helper function:
function c = num2cellstr(xx, varargin)
%Converts matrix of numeric data to cell array of strings
c = cellfun(#(x) num2str(x,varargin{:}), num2cell(xx), 'UniformOutput', false);
Try this:
N = 10;
a = cell(1,N);
for i = 1:N
a(i) = {['STA',num2str(i)]};
end
I've a string like this "FBECGHD" and i need to use MATLAB and generate all the required possible permutations? In there a specific MATLAB function that does this task or should I define a custom MATLAB function that perform this task?
Use the perms function. A string in matlab is a list of characters, so it will permute them:
A = 'FBECGHD';
perms(A)
You can also store the output (e.g. P = perms(A)), and, if A is an N-character string, P is a N!-by-N array, where each row corresponds to a permutation.
If you are interested in unique permutations, you can use:
unique(perms(A), 'rows')
to remove duplicates (otherwise something like 'ABB' would give 6 results, instead of the 3 that you might expect).
As Richante answered, P = perms(A) is very handy for this. You may also notice that P is of type char and it's not convenient to subset/select individual permutation. Below worked for me:
str = 'FBECGHD';
A = perms(str);
B = cellstr(reshape(A,7,[])');
C = unique(B);
It also appears that unique(A, 'rows') is not removing duplicate values:
>> A=[11, 11];
>> unique(A, 'rows')
ans =
11 11
However, unique(A) would:
>> unique(A)
ans =
11
I am not a matlab pro by any means and I didn't investigate this exhaustively but at least in some cases it appears that reshape is not what you want. Notice that below gives 999 and 191 as permutations of 199 which isn't true. The reshape function as written appears to operate "column-wise" on A:
>> str = '199';
A = perms(str);
B = cellstr(reshape(A,3,[])');
C = unique(B);
>> C
C =
'191'
'199'
'911'
'919'
'999'
Below does not produce 999 or 191:
B = {};
index = 1;
while true
try
substring = A(index,:);
B{index}=substring;
index = index + 1;
catch
break
end
end
C = unique(B)
C =
'199' '919' '991'