Function writing function

Richard A. DeVenezia · Apr 3, 2004

Can a function write another function that has a specific number of
nested loops and then run it?

i.e.
function maker (N) {
// does stuff that creates function doer()
// invoke doer
doer()
}

For a specific value of N, doer might have this construction:

function doer() {
for (i0=0;i0<x;i0++ {
for (i1=0;i1<x;i1++ {
...
for (i#N#=0;i#N#<x;i#N#++ {

// stuff

}...}} // N closing braces
}

i#N# simply means replace #N# with what ever value of N is.

So for N=4, I would want 4 nested loops, for N=5 I would want 5 nested
loops, etc...

Thanks,
Richard

Richard Cornford · Apr 4, 2004

Richard said:
Can a function write another function that has a specific
number of nested loops and then run it?

The - Function - constructor can be invoked with a string holding the
text of source code as its final argument and will create a function
object using that code as the function body (except possibly on systems
implementing the ECMAScript compact profile (ECMA 327) and taking
advantage of the option not to facilitate the use of the Function
constructor).

In any real problem situation the use of the - Function - constructor is
unlikely to be necessary or optimal. Javascript is so dynamic in its
nature that creating functions from strings would usually be avoidable
and the overhead in string manipulation would be difficult to justify
unless the resulting function was destined for repeated use.

i.e.
function maker (N) {
// does stuff that creates function doer()
// invoke doer
doer()
}

For a specific value of N, doer might have this construction:

function doer() {
for (i0=0;i0<x;i0++ {
for (i1=0;i1<x;i1++ {
...
for (i#N#=0;i#N#<x;i#N#++ {

// stuff

}...}} // N closing braces
}

i#N# simply means replace #N# with what ever value of N is.

So for N=4, I would want 4 nested loops, for N=5 I would want
5 nested loops, etc...

Identifying a better alternative hangs of the unspecified "stuff" that
if to happen within the innermost loop. How is it going to take
advantage of loop counter i#N#, and if it doesn't why are the nested
loops needed?

Richard.

Richard A. DeVenezia · Apr 4, 2004

Richard said:
Identifying a better alternative hangs of the unspecified "stuff" that
if to happen within the innermost loop. How is it going to take
advantage of loop counter i#N#, and if it doesn't why are the nested
loops needed?

This function writes a function that enumerates permutations of n things by
iterations.
Yes, the dense interaction of abstract and specific is a difficult brew to
understand and maintain.
Yes, there are recursive techniques that can do the same thing (I haven't
done any benchmarking to see if lots of loops with lots of ifs is faster
than lots of loops of recursive function calls)

<html>
<head>
<title>Perms</title>
<script type="text/javascript">
function Perms (n) {

var f = '// Permutations of ' + n + ' things';
f += '\nvar n=0'

for (var i=0;i<n;i++) {
f += '\nfor (i'+i+'=0;i'+i+'<'+n+';i'+i+'++) {'

for (var j=0;j<i;j++) {
f += '\nif (i'+i+'==i'+j+') continue'
}
}

f += '\nn++'
f += '\n/**/ document.write ("<BR>"+n+": "'

for (var i=0;i<n;i++) {
if (i) f += '+","'
f += '+i'+i
}

f += ') /**/'

f += '\n'

for (var i=0;i<n;i++) {
f += '}'
}

f += '\ndocument.write("<BR>"+n+" permutations")'

perms = new Function (f)

perms()

document.write ('<PRE>'+perms.toString().replace(/</g,"<")+'</PRE>')
}
</script>
</head>
<body>
<script type="text/javascript">
Perms(4)
</script>
</body>
</html>

Lasse Reichstein Nielsen · Apr 5, 2004

Richard A. DeVenezia said:
This function writes a function that enumerates permutations of n things by
iterations.
Yes, the dense interaction of abstract and specific is a difficult brew to
understand and maintain.

Yes, there are recursive techniques that can do the same thing (I haven't
done any benchmarking to see if lots of loops with lots of ifs is faster
than lots of loops of recursive function calls)

My guess is that the recursive version will be simpler and therefore
faster.

The way you generate ifs, the inner loop (which will run n times) will
start with n-1 if statements that will continue the loop in all but
one case. There will be approx. (1/2)*n^2 if statements, which will all
have been tested for each permutation you generate, as well as all the
tests that fail and continue a loop. I believe the time complexity
is quite bad for this approach.

Here is a recursive function for comparison

---
function perm(arr,acc) { // arr(ay): elements yet to permute
// acc(umulator): elements placed so far
if (!acc) {acc=[];}
if (arr.length == 0) {
return [acc.slice(0)]; // return copy of accumulator
}
var result = [];
var tmp = arr.pop(); // pick element to fill hole with
for(var i=0;i<arr.length;i++) {
acc.push(arr); // push each other element on acc
arr = tmp; // fill hole with elem from above
result = result.concat(perm(arr,acc)); // recurse
arr = acc.pop(); // restore element
}
acc.push(tmp); // push picked element
result = result.concat(perm(arr,acc)); // recurse
arr.push(acc.pop()); // restore arr and acc

return result;
}

alert(perm([0,1,2,3]).join("\n"));

Initialize multi-dimensional array	10	Jan 24, 2012
Function is not worked in C	2	Jun 27, 2023
Why struct not globally changed in function?	1	Aug 22, 2023
Function Help	1	Feb 26, 2022
Problem with pointer, array and function parameter	2	Jun 12, 2012
Issues with writing pytest	0	Sep 9, 2022
SENTINEL CONTROL LOOP WHEN DEALING WITH TWO ARRAYS	1	Oct 26, 2023
I Need Help with making a function that draws in a canvas using location data.	1	Dec 17, 2021

Function writing function

Richard A. DeVenezia

Richard Cornford

Richard A. DeVenezia

Lasse Reichstein Nielsen

Ask a Question

Similar Threads

Members online

Forum statistics

Latest Threads