Stream merge

2-stream merge: Read two sorted streams of items from external source (e.g. disk, or network), and write one stream of sorted items to external sink.; Common algorithm: keep 1 buffered item from each source, select minimal of them, write it, fetch another item from that stream from which the written item was.
N-stream merge: The same as above, but reading from N sources.; Common algorithm: same as above, but keep buffered items and their source descriptors in a heap.

Assuming streams are very big. You must not suck them whole in the memory, but read them as streams.

Python

Built-in function open opens a file for reading and returns a line-by-line iterator (stream) over the file.

There exists a standard library function heapq.merge that takes any number of sorted stream iterators and merges them into one sorted iterator, using a heap.

<lang python>import heapq import sys

sources = sys.argv[1:] for item in heapq.merge(open(source) for source in sources):

   print(item)</lang>

Shell

sort --merge source1 source2 sourceN > sink

REXX

<lang rexx>/**********************************************************************

Merge 1.txt ... n.txt into m.txt
1.txt 2.txt 3.txt 4.txt
1 19 1999 2e3
17 33 2999 3000
8 500 3999
- - - - /

n=4 high='ffff'x p.= Do i=1 To n

 f.i=i'.txt'
 Call get i
 End

Do Forever

 min=high
 Do i=1 To n
   If x.i<<min Then Do    /* avoid numerical comparison */
     imin=i
     min=x.i
     End
   End
 If min<<high Then Do
   Call o x.imin
   Call get imin
   End
 Else Do
   Call lineout oid
   Leave
   End
 End

Exit get: Procedure Expose f. x. high p.

 Parse Arg ii
 If lines(f.ii)=0 Then
   x.ii=high
 Else Do
   x.ii=linein(f.ii)
   If x.ii<<p.ii Then Do
     Say 'Input file' f.ii 'is not sorted ascendingly'
     Say p.ii 'precedes' x.ii
     Exit
     End
   p.ii=x.ii
   End
 Return

o: Say arg(1)

  Return lineout(oid,arg(1))</lang>

Output: