Matlab for circular pit

matlab use for Nested loop traversal array , There may be bug.matlab/octave Environmental Science ：

• linux：Matlab R2018a [1]
• windows：GNU Octave, version 5.2.0

With for x = vector Form traversal , There are several cases ：

• Traverse row vector elements ,OK;
• Traverse column vector elements ,OK;
• nesting ： Layman column , Something happened ： The outer traversal line is normal , The inner traversal column is assigned to the whole column every time ;
• nesting ： Outside column inside line , Something happened ： The outer traversal column does not involve nesting at first for It's normal , Getting columns in the inner loop is the whole column ; The inner traversal line is normal ;
• nesting ： Layman and expert ,OK;
• nesting ： Outer column inner column , Something happened ： The outer traversal column does not involve nesting at first for It's normal , Getting columns in the inner loop is the whole column ; And the inner traversal column is also the whole column ;

See the code for details. 、 Comments and output ：

clear all; clc;

a = [1 2 3];  %  Row vector 
fprintf("a size: [%s]\n", num2str(size(a)));
fprintf("single loop a\n");
for i = a
    fprintf("i: %d\n", i);  % OK
end

b = [4; 5; 6];  %  Column vector 
fprintf("b size: [%s]\n", num2str(size(b)));
fprintf("single loop b\n");
for i = b
    fprintf("i: %d\n", i);  % OK
end

fprintf("nested loop: row->col\n");
for ia = a
    fprintf("--- a: %d\n", ia);  % OK
    for ib = b
        fprintf("a: %s, b: [%s], %s\n", num2str(ia), num2str(size(ib)), num2str(ib));  % ia OK,ib  Entire column 
    end
end

fprintf("nested loop: col->row\n");
for ib = b
    fprintf("--- b: %d\n", ib);  % OK
    for ia = a
        fprintf("b: %s, a: [%s], %s\n", num2str(ib), num2str(size(ia)), num2str(ia));  % ib  Entire column ,ia OK
    end
end

fprintf("nested loop: row->row\n");
for ia = a
    fprintf("--- a: %d\n", ia);  % OK
    for ia2 = a
        fprintf("a: %s, a2: [%s], %s\n", num2str(ia), num2str(size(ia2)), num2str(ia2));  % OK
    end
end

fprintf("nested loop: col->col\n");
for ib = b
    fprintf("--- b: %d\n", ib);  % OK
    for ib2 = b
        fprintf("b: %s, b2: [%s], %s\n", num2str(ib), num2str(size(ib2)), num2str(ib2));  % ib、ib2  It's the whole column 
    end
end

fprintf("nested loop: row->row->row\n");
c = [7, 8, 9];
for ia = a
    fprintf("a: [%s], %d\n", num2str(size(ia)), ia);  % OK
    for ib = b'  %  Change to line 
        fprintf("|- b: [%s], %d\n", num2str(size(ib)), ib);  % OK
        for ic = c
            fprintf("|- |- c: [%s], %d\n", num2str(size(ic)), ic);  % OK
        end
    end
end

• Output

a size: [1  3]
single loop a
i: 1
i: 2
i: 3

b size: [3  1]
single loop b
i: 4
i: 5
i: 6

nested loop: row->col
--- a: 1
a: 1, b: [3  1], 456
--- a: 2
a: 2, b: [3  1], 456
--- a: 3
a: 3, b: [3  1], 456

nested loop: col->row
--- b: 4
--- b: 5
--- b: 6
b: 456, a: [1  1], 1
b: 456, a: [1  1], 2
b: 456, a: [1  1], 3

nested loop: row->row
--- a: 1
a: 1, a2: [1  1], 1
a: 1, a2: [1  1], 2
a: 1, a2: [1  1], 3
--- a: 2
a: 2, a2: [1  1], 1
a: 2, a2: [1  1], 2
a: 2, a2: [1  1], 3
--- a: 3
a: 3, a2: [1  1], 1
a: 3, a2: [1  1], 2
a: 3, a2: [1  1], 3

nested loop: col->col
--- b: 4
--- b: 5
--- b: 6
b: 456, b2: [3  1], 456

nested loop: row->row->row
a: [1  1], 1
|- a: 1, b: [1  1], 4
|- |- a: 1, b: 4, c: [1  1], 7
|- |- a: 1, b: 4, c: [1  1], 8
|- |- a: 1, b: 4, c: [1  1], 9
|- a: 1, b: [1  1], 5
|- |- a: 1, b: 5, c: [1  1], 7
|- |- a: 1, b: 5, c: [1  1], 8
|- |- a: 1, b: 5, c: [1  1], 9
|- a: 1, b: [1  1], 6
|- |- a: 1, b: 6, c: [1  1], 7
|- |- a: 1, b: 6, c: [1  1], 8
|- |- a: 1, b: 6, c: [1  1], 9
a: [1  1], 2
|- a: 2, b: [1  1], 4
|- |- a: 2, b: 4, c: [1  1], 7
|- |- a: 2, b: 4, c: [1  1], 8
|- |- a: 2, b: 4, c: [1  1], 9
|- a: 2, b: [1  1], 5
|- |- a: 2, b: 5, c: [1  1], 7
|- |- a: 2, b: 5, c: [1  1], 8
|- |- a: 2, b: 5, c: [1  1], 9
|- a: 2, b: [1  1], 6
|- |- a: 2, b: 6, c: [1  1], 7
|- |- a: 2, b: 6, c: [1  1], 8
|- |- a: 2, b: 6, c: [1  1], 9
a: [1  1], 3
|- a: 3, b: [1  1], 4
|- |- a: 3, b: 4, c: [1  1], 7
|- |- a: 3, b: 4, c: [1  1], 8
|- |- a: 3, b: 4, c: [1  1], 9
|- a: 3, b: [1  1], 5
|- |- a: 3, b: 5, c: [1  1], 7
|- |- a: 3, b: 5, c: [1  1], 8
|- |- a: 3, b: 5, c: [1  1], 9
|- a: 3, b: [1  1], 6
|- |- a: 3, b: 6, c: [1  1], 7
|- |- a: 3, b: 6, c: [1  1], 8
|- |- a: 3, b: 6, c: [1  1], 9

It is more stable to traverse by subscript , But when traversing a matrix 、 When higher-order tensors are used, attention should be paid to adding : Means take all （ see [2]）：

clear all; clc;

a = [1 2 3];  %  That's ok 
b = [4; 5; 6];  %  Column 

for ia = 1 : length(a)  %  Subscript 
    xa = a(ia);
    for ib = 1 : length(b)  %  Subscript 
        xb = b(ib);
        fprintf("a: %d, b: %d\n", xa, xb);  % OK
    end
end

c = reshape(1 : 12, 3, 4);  %  matrix 
for ic = 1 : size(c, 1)  %  Subscript 
    d = c(ic);  %  The second dimension is not added  `:`, Something happened ： You can only get the first column 
    e = c(ic, :);  %  The second Vega  `:`,OK
    fprintf("d: %d\n", d);
    fprintf("e:"), disp(e);
end

• Output

a: 1, b: 4
a: 1, b: 5
a: 1, b: 6
a: 2, b: 4
a: 2, b: 5
a: 2, b: 6
a: 3, b: 4
a: 3, b: 5
a: 3, b: 6

d: 1
e:     1     4     7    10

d: 2
e:     2     5     8    11

d: 3
e:     3     6     9    12

References

1. docker Command line installation matlab R2018a、matconvnet、gpu Environmental Science
2. matlab、numpy Of indexing differences