Replacing unassigned values in a vector with nans instead of zeros

Question

jlt199 2016-9-20

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/303763-replacing-unassigned-values-in-a-vector-with-nans-instead-of-zeros

回答： Jonathan Avesar 2019-9-17

Morning everyone,

I'm resizing vectors in a loop. Not good I know and probably why it takes 30 minutes to run, but beside from reading through all the files a first time to determine the correct length of the vectors and then reading through a second time to fill in the vectors I can't see a better way. As there is no easy method of determining the length I need.

However, currently if a file can't be read - some of the files are dodgy and I put a try-catch statement around them - then unassigned values in the vector automatically equal zero. Is there a way to change that behaviour so that an unassigned vector value is a nan?

For example

r(1) = 2;
r(3) = 4;
>> r
r =
     1     0     2

is there a way to make r(2) a nan without explicitly assigning it?

2 个评论
显示无隐藏无

Chad Greene 2016-9-20

在 MATLAB Online 中打开

Is preallocating r an option? I'm thinking something like this:

r = nan(1,3);
r(1) = 2;
r(3) = 4

jlt199 2016-9-20

preallocating to the correct number is not an option as it is unknown

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Steven Lord 2016-9-20

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/303763-replacing-unassigned-values-in-a-vector-with-nans-instead-of-zeros#answer_235388

Preallocate r to be a NaN vector with a number of elements equal to some upper bound on the number of elements that it could possibly contain (for instance, if you have 10 files and each could add 7 elements to the vector preallocate it to be nan(1, 70)) then fill it in and trim off the excess elements after the for loop is complete.

5 个评论
显示 3更早的评论隐藏 3更早的评论

jlt199 2016-9-20

在 MATLAB Online 中打开

Thanks Guillaume, I'm reading them using the following code:

fid = 'W:/Algorithm Dept/Flat Plat Defects/Circular Defects/';
files = dir([fid,'*.txt']);
for i = 1:length(files)
  ML = readtable([fid,file.name],'HeaderLines',8,'ReadVariableNames',false);
...
end

Guillaume 2016-9-21

在 MATLAB Online 中打开

The reading is efficient, but I don't see how that portion of code links to your question. Assuming you're concatenating the tables vertically and you want nans for the missing files, this is how I'd do it:

%...
ML = cell(numel(files));
for filenumber = 1:numel(files)
   try
      ML{filenumber} = readtable(...);
   catch
      ...
   end
end
failedread = cellfun(@isempty, ML);
maxheight = max(cellfun(@height, ML(~failedread)));
width = cellfun(@width, ML(~failedread));
assert(numel(width) > 0 && all(width == width(1)), 'table concatenation will fail')
ML{failedread} = array2table(nan(maxheight, width(1)));
allML = vertcat(ML{:});