In GNU Octave I have a 1xn matrix:
octave:1> s=[1;2;3;4;5;6;7]
s =
1
2
3
4
5
6
7
I would like to convert this matrix to a space delimited string like this:
"1 2 3 4 5 6 7"
I tried using mat2str and cellstr without much success. Is this possible?
Octave, How to convert matrix to string:
If your matrix is a single column, invert it so it is a row like this:
octave:1> s=[1;2;3;4;5;6;7]
s =
1
2
3
4
5
6
7
mat2str(s')
Which prints:
ans = [1 2 3 4 5 6 7]
This looks like it is not a string because it has left and right square brackets, but it is a string. You can remove the left and right brackets like this:
substr(s,2,length(s)-2)
Which cuts off the first and last characters, which prints:
s = 1 2 3 4 5 6 7
Which is a string.
Related
I have a matrix (m) of scores for 4 students on 3 different exams.
4 3 1
3 2 5
8 4 6
1 5 2
I want to know, for each student, the exams they did best to worse on. Desired output:
1 2 3
2 3 1
1 3 2
3 1 2
Now, I'm new to the language (and coding in general), so I read GeeksforGeeks' page on sorting in Julia and tried
mapslices(sortperm, -m; dims = 2)
However, this gives something subtly different: a matrix of each row being the index of the sorting.
1 2 3
3 1 2
1 3 2
2 3 1
Perhaps it was obvious, but I now realize this is not actually what I want, but I cannot find a built-in function/fast way to complete this operation. Any ideas? Preferably something which doesn't iterate through items in the matrix/row, as in reality my matrix is very, very large. Thanks!
Such functionality is provided by StatsBase.jl. Here is an example:
julia> using StatsBase
julia> m = [4 3 1
3 2 5
8 4 6
1 5 2]
4×3 Array{Int64,2}:
4 3 1
3 2 5
8 4 6
1 5 2
julia> mapslices(x -> ordinalrank(x, rev=true), m, dims = 2)
4×3 Array{Int64,2}:
1 2 3
2 3 1
1 3 2
3 1 2
You might want to use other rank, depending on how you want to split ties, see here for details.
Figured out something which works!
Run m_index_rank = mapslices(sortperm, -m; dims = 2) on the matrix and get a ranking for each row through index. Then, realizing this is, in each row, an inverse permutation away from the desired output, run mapslices(invperm, m_index_rank; dims = 2) for the desired result.
In one line, this is mapslices(r -> invperm(sortperm(r, rev=true)), m; dims=2) over the desired matrix m. dims = 2 is to carry out the operation row-wise.
I'm marking this resolved for now, but please let me know if there are cleaner/faster ways to do this.
Edit: Replaced my syntactically clunky mapslices(invperm, mapslices(sortperm, -m; dims = 2); dims = 2) with a more natural one, thanks to #phipsgabler
I have the following data:
a b c d
5 9 6 0
3 1 3 2
Characters in the first row, numbers in the second row.
How do I get the character corresponding to the highest number in the second row, and how do I increase the corresponding number in the second row? (For example, here, column b has the highest number, 9, so increase that number by 10%.)
I use Dyalog version 17.1.
With:
⎕←data←3 4⍴'a' 'b' 'c' 'd' 5 9 6 0 3 1 3 2
a b c d
5 9 6 0
3 1 3 2
You can extract the second row with:
2⌷data
5 9 6 0
Now grade it descending, that is, find the indices that would sort it from highest to lowest:
⍒2⌷data
2 3 1 4
The first number is the column we're looking for:
⊃⍒2⌷data
2
Now we can use this to extract the character from the first row:
data[⊂1,⊃⍒2⌷data]
b
But we only need the column index, not the actual character. The full index of the number we want to increase is:
2,⊃⍒2⌷data
2 2
Extracting the data to see that we got the right index:
data[⊂2,⊃⍒2⌷data]
9
Now we can either create a new array with the target value increased by 10%:
1.1×#(⊂2,⊃⍒2⌷data)⊢data
a b c d
5 9.9 6 0
3 1 3 2
Or change it in-place:
data[⊂2,⊃⍒2⌷data]×←1.1
data
a b c d
5 9.9 6 0
3 1 3 2
Try it online!
I'm new to julia and I have a problem.
I am working with Julia (Jupyter notebook) and I do not know how can I do column 3 - column 2 and write the result as a new column at the end of the matrix/array2D.
I have tried this:
newCol = array[(1:end),3] - array[(1:end),2]
Any suggestion?
You can subtract the two columns and then concatenate it with the original array using the normal build-an-array syntax:
julia> arr
2x3 Array{Int32,2}:
1 2 3
5 6 7
julia> [arr [arr[:,3] - arr[:,2]]]
2x4 Array{Int32,2}:
1 2 3 1
5 6 7 1
Or use hcat:
julia> hcat(arr,arr[:,3] - arr[:,2])
2x4 Array{Int32,2}:
1 2 3 1
5 6 7 1
(Note that neither of these act in place, so you'd need to assign the result somewhere if you want to use it later.)
I am trying to plot a vector field from data file. I can put the data into file in any way, but for now data file contains information in this form:
x y Fx Fy
where Fx and Fy are x and y components of a vector.
Any ideas how to plot it?
Given a a file vector.txt in an appropriate location containing exactly eight lines
1 1 2 3
1 2 5 3
1 3 4 5
1 4 2 2
2 1 3 3
2 2 5 6
2 3 4 1
2 4 1 4
(without any additional white space anywhere)
and a fresh Mathematica notebook containing
datain = Partition[ToExpression[ "{" <>
StringReplace[Import["vector.txt"],
RegularExpression["\\s"] -> ","] <> "}"], 4];
arrow = Map[Arrow[{Take[#, 2], Take[#, 2] + Drop[#, 2]}] &, datain];
Graphics[arrow]
Evaluate that and a small collection of arrows appears in the notebook.
I have array say "a"
a =
1 4 5
6 7 2
if i use function
b=sort(a)
gives ans
b =
1 4 2
6 7 5
but i want ans like
b =
5 1 4
2 6 7
mean 2nd row should be sorted but elements of ist row should remain unchanged and should be correspondent to row 2nd.
sortrows(a',2)'
Pulling this apart:
a = 1 4 5
6 7 2
a' = 1 6
4 7
5 2
sortrows(a',2) = 5 2
1 6
4 7
sortrows(a',2)' = 5 1 4
2 6 7
The key here is sortrows sorts by a specified row, all the others follow its order.
You can use the SORT function on just the second row, then use the index output to sort the whole array:
[junk,sortIndex] = sort(a(2,:));
b = a(:,sortIndex);
How about
a = [1 4 5; 6 7 2]
a =
1 4 5
6 7 2
>> [s,idx] = sort(a(2,:))
s =
2 6 7
idx =
3 1 2
>> b = a(:,idx)
b =
5 1 4
2 6 7
in other words, you use the second argument of sort to get the sort order you want, and then you apply it to the whole thing.