Whats the difference when using [ ] and : ?

reshape(key,2,[ ])

Can anyone explain how the above function works with an example

Answer 1

[Adam]

key = [1 2 3 4 5 6 7 8 9 10];
res = reshape( key, 2, [] );

would give

res = [ 1 3 5 7 9
        2 4 6 8 10 ];

The [] indicates that the function should just use whatever is leftover as the remaining dimension - i.e. numel( key ) / 2 in this case. So the number of elements in key must be divisable by 2.

Answer 2

[Ameer Hamza]

reshape() work by taking the elements of the input matrix column-wise and arrange them into the shape you want. For example, help explain how it will rearrange the elements

>> key = [
         1  6 11 16;
         2  7 12 17;
         3  8 13 18;
         4  9 14 19;
         5 10 15 20];
>> reshape(key, [], 2)
ans =
     1    11
     2    12
     3    13
     4    14
     5    15
     6    16
     7    17
     8    18
     9    19
    10    20

Answer 3

[Image Analyst]

The syntax is:

reshapedMatrix = reshape(key, desiredRows, desiredColumns);

If either desiredRows or desiredColumns is just empty brackets [] then it takes the one that is there and figures out what the other one should be for you, as a convenience so that you don't have to do the math yourself.

For example, since you input 2 for desiredRows, then if you use [] for desiredColumns, it will compute desiredColumns for you like this:

desiredColumns = numel(key) / 2;

It's a nice convenience, you just have to make sure that you'll have a square matrix (no ragged last row or column just partially filled with numbers. In other you must have numel(key) be a multiple of desiredRows (2 in your example). So

key = 1:6
rk = reshape(key, 2, []) % 2 rows-by-3 columns

works:

key =

1 2 3 4 5 6

rk =

1 3 5

2 4 6

while

key = rand(1, 5);
reshapedMatrix = reshape(key, 2, []); % Throws error

doesn't work because 5 is not a multiple of 2 and you can't have a matrix with the first two columns having 2 rows and the rightmost column having only one row. No "ragged edges" allowed.

key =

1 2 3 4 5 6

rk =

1 3 5

2 4 Can't have nothing in the lower right corner