Build a matrix of zeros with the size of the initial matrix according to indices

Hi,

I have an initial matrix M(466,504) where I extracted the 30 highest values in each row with the command:

[val,idx] = maxk(M,30,2)

I would like to get now a matrix of zeros of same size but with those 30 values at their initial position.

How could I build such a matrix ?

[Stephen23]

You would need:

1. the 30 highest values,
2. their indices,
3. the size of the original matrix.

Do you have that information?

[JohnB]

I think your answer was also right !

I kept it in my mail session ;)

Thanks for your help Too !

Answer 1

[Guillaume]

newM = zeros(size(M));
newM(sub2ind(size(M), repmat((1:size(idx, 1))', 1, size(idx, 2)), idx)) = val

[JohnB]

It doesn't give the right dimensions :S ...

[Guillaume]

"It doesn't give the right dimensions"

Could you be a bit clearer about the problem you're having? "the right dimensions" of what? The code I've provided does exactly what you asked: e.g. with the 3 highest values of each row:

>> M = magic(10)
M =
    92    99     1     8    15    67    74    51    58    40
    98    80     7    14    16    73    55    57    64    41
     4    81    88    20    22    54    56    63    70    47
    85    87    19    21     3    60    62    69    71    28
    86    93    25     2     9    61    68    75    52    34
    17    24    76    83    90    42    49    26    33    65
    23     5    82    89    91    48    30    32    39    66
    79     6    13    95    97    29    31    38    45    72
    10    12    94    96    78    35    37    44    46    53
    11    18   100    77    84    36    43    50    27    59
    
>> [val, idx] = maxk(M, 3, 2);

>> newM = zeros(size(M));
newM(sub2ind(size(M), repmat((1:size(idx, 1))', 1, size(idx, 2)), idx)) = val
newM =
    92    99     0     0     0     0    74     0     0     0
    98    80     0     0     0    73     0     0     0     0
     0    81    88     0     0     0     0     0    70     0
    85    87     0     0     0     0     0     0    71     0
    86    93     0     0     0     0     0    75     0     0
     0     0    76    83    90     0     0     0     0     0
     0     0    82    89    91     0     0     0     0     0
    79     0     0    95    97     0     0     0     0     0
     0     0    94    96    78     0     0     0     0     0
     0     0   100    77    84     0     0     0     0     0

As you can see you get the highest 3 values of each row in exactly the same position as they were originally, in a matrix the same size as the original one.

[JohnB]

Today it is working x) Yesterday It doesn't ... Maybe I missed some characters by myself :S

Anyway Huge Regards !