extract a number N of equally spaced rows within a matrix

Question

Alberto Acri 2023-9-22

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https://ww2.mathworks.cn/matlabcentral/answers/2024512-extract-a-number-n-of-equally-spaced-rows-within-a-matrix

评论： Alberto Acri 2023-9-24

采纳的回答： Fangjun Jiang

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HI! I have a number N and a matrix with 100 rows and 2 columns.

How can I recover N rows of the matrix so that they are as equidistant from each other as possible?

For example:

with N = 10 I will have 10 rows (row 1, 11, 21, 31,...)
with N = 7 I will have 7 rows (row 1, 15, 29,...) In this case the steps are like 100/7 = 14.28 = 14.

a=1:1:100;
a=a';
b=0.1:0.1:10;
b=b';
M=[a,b]; % example matrix
N=10; % number of rows
step = height(M)/N;
step = 14; % round down
v = 1:step:height(M);
M_new = M(v,:);

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Answer 1

Fangjun Jiang 2023-9-22

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编辑：Fangjun Jiang 2023-9-22

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M=repmat((1:100)',1,2);
N=7;
d=M(round(linspace(1,height(M),N)),:)
d = 7×2
     1     1
    18    18
    34    34
    51    51
    67    67
    84    84
   100   100

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Answer 2

Dyuman Joshi 2023-9-22

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https://ww2.mathworks.cn/matlabcentral/answers/2024512-extract-a-number-n-of-equally-spaced-rows-within-a-matrix#answer_1316297

编辑：Dyuman Joshi 2023-9-22

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You are on the right course -

a=(1:1:100)';
b=(0.1:0.1:10)';
M=[a,b]; % example matrix
N=7; % number of rows
step = height(M)/N
step = 14.2857
%Use floor to round down to nearest integer less than or equal to step
step = floor(step) % round down
step = 14
v = 1:step:height(M)
v = 1×8
     1    15    29    43    57    71    85    99
M_new = M(v,:)
M_new = 8×2
    1.0000    0.1000
   15.0000    1.5000
   29.0000    2.9000
   43.0000    4.3000
   57.0000    5.7000
   71.0000    7.1000
   85.0000    8.5000
   99.0000    9.9000

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Fangjun Jiang 2023-9-24

编辑：Fangjun Jiang 2023-9-24

在 MATLAB Online 中打开

when height(M) is 100 and N is 10, it is hard to say which one is the right answer.

d=1:10:100
d = 1×10
     1    11    21    31    41    51    61    71    81    91
d1=1:11:100
d1 = 1×10
     1    12    23    34    45    56    67    78    89   100

But when height(M) is 100 and N is 6, it is clear d is not the right answer while d1 still is right.

d=1:floor(100/6):6*floor(100/6)
d = 1×6
     1    17    33    49    65    81
d1=round(linspace(1,100,6))
d1 = 1×6
     1    21    41    60    80   100

In the resulting d, the even distance is 16. But the last value of d is 81. there is a bigger gap (19) to the last value of M, which is 100. So d is not evenly sampled from the rows of M. This becomes worse in previous example. The last value of d is 901 where the last value of M is 1000, and the "even distance" is 9.

Using round(linspace()) always gives the almost evenly distributed rows, just not the "exact" evenly distributed rows.

OP's numerical example has the same issue as a way to reflect "as equidistant from each other as possible".

Dyuman Joshi 2023-9-24

编辑：Dyuman Joshi 2023-9-24

"when height(M) is 100 and N is 10, it is hard to say which one is the right answer."

OP has specified what the expected outcome is and the logic behind it -

HI! I have a number N and a matrix with 100 rows and 2 columns.

How can I recover N rows of the matrix so that they are as equidistant from each other as possible?

For example:

with N = 10 I will have 10 rows (row 1, 11, 21, 31,...)
with N = 7 I will have 7 rows (row 1, 15, 29,...) In this case the steps are like 100/7 = 14.28 = 14.

Alberto Acri 2023-9-24

Thanks for the replies!

@Fangjun Jiang Yes, it is actually impossible to make an "equally spaced" distribution for any value of N but your case is more useful for my purpose.

@Dyuman Joshi Your solution is certainly fine too. I gave an example to make it clear what I wanted to achieve as a final result. The distribution as you wrote it follows exactly what I wanted to obtain as a result in the question. But it was an example. You'll accept both as an answer but that's not possible.

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