using fread() to read part of a file

Hi everyone I came with another frustrating question at least for me which is: I have a matrix of [10000,26] stored in a file using fwrite() function. This means I have to use fread() to read from that file. Well the problem is that when I tried to load part of the file, lets say [100,26], the fread() keeps reading the first column. So I built a side program to see how the fread() behaves

count=1;
zz=zeros(10000,26);
for i=1:10000
    for j=1:26
        zz(i,j)=count;
        count=count+1;
    end

end
vidbin=fopen('test.bin','wb');
fwrite(vidbin,zz,'float');
fclose(vidbin);

I found out when I tried to load a portion of the file as

vidbin=fopen('test.bin','rb');
myarr=fread(vidbin,[100,26],'float');
fclose(vidbin);

The array myarr is not as suppose to be. it looks like 1 2601 . . . 65001 27 2627 . . . 65027 . . . myarr= . . . . . . 2575 5175 67575 It appears as if the fread() reads the first column in zz[] and divides it along the columns of myarr[] as the first row should be [1 2 3 ... 26].

How can I make fread() reads the required part of the file that I want the way I want? Waiting for ideas and solutions, please.

[Stephen23]

Note that it is more robust to avoid using i and j for variable names, as these are both names of the inbuilt imaginary unit.

Answer 1

[Stephen23]

The documentation for fread states that "fread populates A in column order", so when you specify the size of A it keep reading along the "columns" of the file, whereas you are wanting it to stop reading after a certain number of rows.

To resolve the issue, try transposing the matrix (untested):

fwrite(vidbin, zz.', 'float');

and then specifying how many columns, not rows:

myarr = fread(vidbin, [26,100], 'float').';

[Ahmed]

Thank you very much Stephen Cobeldick for your swift answer. I did what you've suggested and it was part of the solution. After transposing the original matrix, fread() kept reading in columns but this time the columns are in order [1, 2, 3,...] not as before. So I reshaped it as [26,100] and then transposed it again to get the final array as supposed to. It is kind of tricky, but we have to to what it takes.

Thank you again for helping me finding the solution

[Stephen23]

I am glad that it works!