Write to file inside a loop

I have a loop that each time generates a value 'b1'. I want to create a matrix of all the b1 values to make a histogram at the end. The code that I found is:

q = b1;

save('pqfile.txt','q','-ascii');

this code overwrites and only the last b1 is saved on pqfile. wondering if anyone can help me not overwrite

Thanks

[Rik]

@Ali, did either answer solve your issue? If so, please consider accepting the answer that works best for you, or comment with your remaining questions.

Answer 1

[infinity]

Hello,

You can refer a simple solution as follows

saveb1 = [];

for (your loop)
        your code to compute b1;
        
        saveb1 = [saveb1; b1];
end
save('pqfile.txt','saveb1','-ascii');

In this method, you will save b1 of each loop to a varible saveb1. After the loop, you can save "saveb1" to the text file.

Answer 2

[Rik]

Or a much better practice: if you know the number of iterations you should pre-allocate your output array.

saveb1 = zeros(1,k);
for n=1:k
    %your code to compute b1 goes here
    
    saveb1(k)=b1;
end

Alternatively, if you insist of using a file write:

%prepare an empty file
fid=fopen('saveb1.txt','w');
if fid==-1
    error('file could not be opened')
end
for n=1:k
    %your code to compute b1 goes here
    
    fprintf(fid,'%.1f\n',b1);
end
fclose(fid);

And now you can read the file with all the b1 values.

[Ali Rafiei]

Thank you for your quick answer. The functon that generates b1 already has multiple loops. I was wondering if all of the loops should go into the loop you suggested above?

[Rik]

Yes it would. There might be more efficient options, but simply pasting the code that generates a value for b1 for a given value of n into that loop should do the trick. If it is a slow process you could use the file write (or a normal save/load) to check which values have already been calculated and fill those empty slots.