Return from a loop on a condition

Question

I have the following function:

(defun chooseBest (TSPs dTSPs)
    (let ((minim (minPos dTSPs 0 0)) (j 0) (best nil)) 
        (loop for i in TSPs
             do (cond ((= j minim) (progn (setf best i) (setq j (+ j 1))))
                      (t (setq j (+ j 1)))))        
        best))

This function receives two lists:

TSPs - a list of paths like this one:

(((1 3600 2300) (2 3100 3300))
 ((3 4700 5750) (22 6650 2300) (23 5400 1600)))

and a list of distances associated with these:

(distancePath1 distancePath2)

The function minPos returns the position of the list with a smaller number that is the path with smaller distance.

Given this, the chooseBest function will return the smaller path.

But I want to improve it, because it will search the whole list for the small number and it is a waste because the minim has that position. For example minim is 2, but it is evaluating a TSPs list with 100 paths, I want to return immediately on 2 and break the loop, but return doesn't work...

Many loop examples with conditionals and return: lispcookbook.github.io/cl-cookbook/iteration.html and functions to search elements in sequences (find, search, elt, index, nth… lispcookbook.github.io/cl-cookbook/…) — Ehvince
– Ehvince, Commented May 2, 2020 at 9:48

Ehvince · Accepted Answer · 2020-05-02 10:17:50Z

Let's rework your function a bit.

First, with better indentation:

(defun chooseBest (TSPs dTSPs)
  (let ((minim (minPos dTSPs 0 0))
        (j 0)
        (best nil))
    (loop for i in TSPs
       do (cond ((= j minim)
                 (progn (setf best i)
                        (setq j (+ j 1))))
                (t 
                 (setq j (+ j 1)))))
    best))

No need of the progn inside the cond:

       do (cond ((= j minim)
                 (setf best i)
                 (setq j (+ j 1)))
                (t 
                 (setq j (+ j 1)))))

Incrementing j can be done with incf:

                 (setq j (+ j 1)))

=>

                 (incf j)

You can move the let variables into the loop.

  (let ((minim (minPos dTSPs 0 0))
        (j 0)
        (best nil))
    (loop for i in TSPs

=>

(loop for it in TSPs
      for j from 0
      with minim = (minPos dTSPs 0 0)
      with best
      do …)

for j from 0 starts a counter, with … = declares a variable which is calculated at the beginning (and not at every iteration).

Returns and conditionals

Inside a loop, we can use if…do, else…do, when and return, and more than once. To do something at the end of the loop, use finally.

(defun chooseBest (TSPs dTSPs)
  (loop for i in TSPs
     for j from 0
     with minim = (minPos dTSPs 0 0)
     with best
     if (= j minim)
     do (progn (setf best i)
               (incf j))
     else
     do (incf j)
     finally (return best)))

Functions to search lists and sequences

Now if you want to find an element inside a list, you have lots of functions for that: find, nth, search, position,… see https://lispcookbook.github.io/cl-cookbook/data-structures.html#functions Since minPos returns an index, you probably want nth index list or elt sequence index.

Rainer Joswig · Accepted Answer · 2020-05-02 08:00:03Z

0

If you want the nth element of a list, you can use the function nth: (nth position list)

CL-USER 15 > (nth 0 '(((1 3600 2300) (2 3100 3300))
                      ((3 4700 5750) (22 6650 2300) (23 5400 1600))))
((1 3600 2300) (2 3100 3300))

CL-USER 16 > (nth 1 '(((1 3600 2300) (2 3100 3300))
                      ((3 4700 5750) (22 6650 2300) (23 5400 1600))))
((3 4700 5750) (22 6650 2300) (23 5400 1600))

If you want to return from a loop, you can use the operator return:

CL-USER > (loop for i from 10
                when (= i (expt (isqrt i) 2))
                do (return i))
16

edited May 2, 2020 at 8:00

answered May 2, 2020 at 7:51

Rainer Joswig

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