Julia - write to the beginning of file

Question

I've been trying to develop an algorithm which would at some point need to write results either to the beginning of a file, or to the end of it.

I'm trying to create a sorting algorithm which wouldn't use as much RAM as my files to be sorted are too big for my current specs. So for the cost of additional time, I would like to do this directly to file instead to RAM.

I know one can write files in Julia in this manner>

write(outfile,"A, B, C, D\n")

But I cannot seem to find how to write to the beginning of it.

Answer 1

s=open("test.txt", "a+");
write(s,"B");
write(s,"C");
position(s) # => 2
seekstart(s);
position(s) # => 0
write(s,"A"); # be careful you are overwriting B!
position(s) # => 1
close(s);
s=open("test.txt", "r");
read(s,Char) # => 'A'
read(s,Char) # => 'C' # we lost 'B'!

So if you like to prepend! something to a file stream, the above solution do not work.

cdata=readall(s);
seekstart(s);
write(s,prependdata);
write(s,cdata);

Answer 2

You can use two files instead of one, as when you implement a deque with two stacks:

• To append data, append to the first file: it will store the tail of the data.

• To prepend data, append to the second file: it will store the head of the data, but in reverse order.

• When you need the data in a single file (probably just once, at the end of your algorithm), reverse the lines in the second file and concatenate the files.