Matlab from text file to sparse matrix.
I have a huge text file in the following format:
1 2
1 3
1 10
1 11
1 20
1 376
1 665255
2 4
2 126
2 134
2 242
2 247
The first column is the coordinate x
and the second column is the coordinate y
. He points out that if I had to build a matrix
M = zeros(N, N); M(1, 2) = 1; M(1, 3) = 1; . . M(2, 247) = 1;
This text file is huge and cannot be transferred directly to main memory. I have to read this line by line. And store it in a sparse matrix .
I need the following function:
function mat = generate( path )
fid = fopen(path);
tline = fgetl(fid);
% initialize an empty sparse matrix. (I know I assigned Mat(1, 1) = 1)
mat = sparse(1);
while ischar(tline)
tline = fgetl(fid);
if ischar(tline)
C = strsplit(tline);
end
mat(C{1}, C{2}) = 1;
end
fclose(fid);
end
But unfortunately, other than the first line, it just puts trash in my sparse rug. Demo video:
1 7
1 9
2 4
2 9
If I print a sparse checkmate, I get:
(1,1) 1 (50,52) 1 (49,57) 1 (50,57) 1
Any suggestions?
Fixing what you have ...
Your problem is that C
- it is a cell array of characters , not numbers. You need to convert the strings you read from the file to integer values. Instead, strsplit
you can use functions like str2num
and str2double
. Since it tline
is a space-delimited character array in this case, it str2num
is the easiest to use to calculate C
:
C = str2num(tline);
Then you just index C
as an array instead of a cell array:
mat(C(1), C(2)) = 1;
Additional tidbit: If you're wondering how your demo code still worked even if it C
contains characters, that's because MATLAB tends to automatically convert variables to the correct type for certain operations. In this case, the characters were converted to their double ASCII code equivalents: '1'
became 49
, '2'
became 50
, etc. He then used them as indices in mat
.
The simplest alternative ...
You don't even need to worry about all this mess above, as you can replace your entire function in a much simpler way using dlmread
and sparse
:
data = dlmread(filePath); mat = sparse(data(:, 1), data(:, 2), 1); clear data; % Save yourself some memory if you don't need it any more