Create table in loop
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Hello,
How can I create a table so that it grows with each iteration of a loop? I am trying to display all of the results from the loop but only manage to output the last.
Thank you for your time and help in advance!
idx = nchoosek(1:size(M), 3); %matrix of index trios
for k = 1:size(idx,1)
ix = idx(k ,:); % current idx trio
Lx = M(ix,1);
Ly = M(ix,2);
gap = M(ix,3);
L = sqrt((Lx.^2 + Ly.^2)/2);
lsq = lsqnonlin(@(phi) errFun(phi, L, gap), [1.5; 18; 1]);
a = lsq(1);
end
table(ix(:)', Lx', Ly', a)
11 个评论
Adam Danz
2020-8-27
The first line produces an error. One of the following might be what you're looking for
- idx = nchoosek(1:numel(M), 3); where numel(M) >=3
- idx = nchoosek(1:size(M,1), 3); where size(M,1) >=3
- idx = nchoosek(1:size(M,2), 3); where size(M,2) >=3
It also appears that ix, Lx, and Ly are vectors with more than 1 element but a only has 1 element. Is that correct?
Valeria Villegas-Medina
2020-8-27
Valeria Villegas-Medina
2020-8-31
Adam Danz
2020-8-31
If the errors pertain to your implementation of any of the solutions, I can probably help out.
Valeria Villegas-Medina
2020-8-31
Adam Danz
2020-8-31
Are you getting an error message?
What is M and how I can recreate the problem?
Valeria Villegas-Medina
2020-8-31
Adam Danz
2020-8-31
Have you looked at the variable values and do they make sense?
I suggest putting a break point on the line that contains lsqnonlin and run your code in debug mode so you can see what the input variables look like on each iteration [instructions].
For example, on the first iteration, the loop variables Lx, Ly, gap, and L, are all NaNs except for the first row. Why would that be a good input to lsqnonlin?
Did you mean to call lsqnonline after the loop instead of within the loop?
Valeria Villegas-Medina
2020-8-31
Adam Danz
2020-8-31
At a conceptual level, what's the reason behind fitting a nonlinear curve to matrices full of 1s (or 0s)? Anyway, the error is unrelated to the solutions in my answer so if you continue to have questions with lsqnonlin, I think the first step it to think through it at a conceptual level to figure out what you're supposed to be fitting.
回答（1 个）
1. Store the values within the loop and build the table later
Unless you're already working with table variables, it probably easier to store the values in each iteration and then build the table later. More importantly, this method is much faster and efficient than the other methods below (see last section on speed comparison).
In these demos, A and B are row vectors and C is a scalar.
rng('default') % for reproducibility
% Pre-allocate loop vars
A = nan(5,4);
B = nan(5,3);
C = nan(5,1);
% Create vars within loop
for i = 1:5
A(i,:) = rand(1,4);
B(i,:) = rand(1,3);
C(i) = rand(1);
end
% Build table
T = table(A,B,C,'VariableNames',{'BG','RO','GR'});
Result:
T =
5×3 table
BG RO GR
___________________________________________ _______________________________ ________
0.81472 0.90579 0.12699 0.91338 0.63236 0.09754 0.2785 0.54688
0.95751 0.96489 0.15761 0.97059 0.95717 0.48538 0.80028 0.14189
0.42176 0.91574 0.79221 0.95949 0.65574 0.035712 0.84913 0.93399
0.67874 0.75774 0.74313 0.39223 0.65548 0.17119 0.70605 0.031833
0.27692 0.046171 0.097132 0.82346 0.69483 0.3171 0.95022 0.034446
2. Build table within a loop - with preallocation
If you'd rather create the table within the loop and you know the size of the table and variable ahead of time, preallocate the table before the loop and then fill in each row and column using table indexing. Incidentally, preallocation does not save time (comparing this demo and the next one).
rng('default') % for reproducibility
T = table(zeros(5,4),zeros(5,3),zeros(5,1),'VariableNames', {'BG','RO','GR'});
for i = 1:5
T.BG(i,:) = rand(1,4);
T.RO(i,:) = rand(1,3);
T.GR(i) = rand(1);
end
Result:
T =
5×3 table
BG RO GR
___________________________________________ _______________________________ ________
0.81472 0.90579 0.12699 0.91338 0.63236 0.09754 0.2785 0.54688
0.95751 0.96489 0.15761 0.97059 0.95717 0.48538 0.80028 0.14189
0.42176 0.91574 0.79221 0.95949 0.65574 0.035712 0.84913 0.93399
0.67874 0.75774 0.74313 0.39223 0.65548 0.17119 0.70605 0.031833
0.27692 0.046171 0.097132 0.82346 0.69483 0.3171 0.95022 0.034446
3. Build table within a loop - without preallocation
If you don't know the size of the table and variables ahead of time, you can build the table within the loop by vertical concatenation. The sizes within each variable must still be consistent, though.
rng('default') % for reproducibility
T = table();
tempTable = table();
for i = 1:5
tempTable.BG = rand(1,4);
tempTable.RO = rand(1,3);
tempTable.GR = rand(1);
T = [T;tempTable];
end
Result:
T =
5×3 table
BG RO GR
___________________________________________ _______________________________ ________
0.81472 0.90579 0.12699 0.91338 0.63236 0.09754 0.2785 0.54688
0.95751 0.96489 0.15761 0.97059 0.95717 0.48538 0.80028 0.14189
0.42176 0.91574 0.79221 0.95949 0.65574 0.035712 0.84913 0.93399
0.67874 0.75774 0.74313 0.39223 0.65548 0.17119 0.70605 0.031833
0.27692 0.046171 0.097132 0.82346 0.69483 0.3171 0.95022 0.034446
Efficiency and speed
The fastest and most efficient method in this answer is the first one, by far!
Each method was timed 500 times using tic/toc and the median durations were compared.
- 1 was 174 times faster than 2
- 1 was 142 times faster than 3
- 3 was 1.15 times faster than 2 (I checked this twice, #3 was faster than #2 by factors of 1.1 and 1.2)
I haven't looked into my table preallocation is slower than without it but others have reported this as well (example).
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2020-8-27
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