Add first element to a cell array

Dear,

I would like to add a new element in the first position in an cell array For example if ArrCell={[1],[2],[3],[4],[5]} is the existing cell and [new] is a matrix. I would like to get ArrCell={[new],[1],[2],[3],[4],[5]}

Of course, I can get this result using a temporal cell array and looping over all the elements of the existing one, but the quesion is if there an efficient way to get this result

Thanks in advance,

Answer 1

[Adam]

ArrCell = [ { new }, ArrCell ];

should work. I'm not at my machine with Matlab at the moment so can't double-check, but fairly sure that works.

[Luis Isaac]

Great, It works!!! Thanks a lot,

[Guillaume]

Note that there is no data structure matlab that allows for efficient insertion at the head of a container (as you're doing here). So, while the above way is efficient in that you don't have to write much code, it's still inefficient in that matlab has to reallocate a whole new array and copy all the data in the background. So your insertion is O(n).

Of course, it does not matter if your cell array is small enough.

[Daniel van de Velden]

Correct answer would be:

ArrCell = [ { new }; ArrCell ];

[Adam]

ArrCell was a row array. You can't concatenate vertically a scalar and a 5-element row array.

[Image Analyst]

new = magic(3) % A matrix
ArrCell={[1],[2],[3],[4],[5]} % A cell array
ArrCell = [ { new }, ArrCell ] % Adam uses comma
ArrCell2 = [ { new }; ArrCell ]; % Daniel uses semicolon

new =

8 1 6

3 5 7

4 9 2

ArrCell =

1×5 cell array

{[1]} {[2]} {[3]} {[4]} {[5]}

ArrCell =

1×6 cell array

{3×3 double} {[1]} {[2]} {[3]} {[4]} {[5]}

Error using vertcat

Dimensions of arrays being concatenated are not consistent.

Error in test1 (line 12)

ArrCell2 = [ { new }; ArrCell ];

Answer 2

[Image Analyst]

But the big question is why are you even using a cell array for that instead of a simple numerical array. Cell arrays are complicated and use a huge amount of overhead. From what you've shown there is no reason to use one. I recommend this instead

Arr = [1,2,3,4,5];
Arr = [new, 1,2,3,4,5]

[Luis Isaac]

I using a cell array because ArrCell is composed of vectors and matrices of different sizes, i.e. size of [new] is different, or could be different, than [1]

[Josh]

appreciate this nudge in a more efficient direction, i fallback on cells arrays out of habit and ease but switching over to numerical arrays where possible...i make animations and cells help when image sizes are changing at least but seem uneccessary when image size is fixed

Answer 3

[Fletcher Garrison]

You can also use:

Arrcell = {1,2,3,4,5};
new = magic(5);
Arrcell = horzcat(new,Arrcell);

Answer 4

[Jonathan]

For future people looking this up:

testCell = {"one", 2, 10, "llama"}

testCell = 1x4 cell array
    {["one"]}    {[2]}    {[10]}    {["llama"]}

testCell = {"newInput", testCell{:}}

testCell = 1x5 cell array
    {["newInput"]}    {["one"]}    {[2]}    {[10]}    {["llama"]}

[Jonathan]

I stand corrected by MATLAB autofix. Apparently this is faster.

testCell ={"one", 2, 10, "llama"}

testCell = 1x4 cell array
    {["one"]}    {[2]}    {[10]}    {["llama"]}

testCell = [{"newInput"}, testCell(:).']

testCell = 1x5 cell array
    {["newInput"]}    {["one"]}    {[2]}    {[10]}    {["llama"]}