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Removing the last element of a list

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6

Is there a simpler way to remove the last element of a list than this?

(setq list (reverse (cdr (reverse list))))

list deletion

share|improve this question

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If doing that, you would likely want to be using nreverse (destructive) rather than reverse (copying), for the sake of efficiency.
  
  – phils
  May 22 at 4:16
  

  
  
  
  

add a comment | 

6

Is there a simpler way to remove the last element of a list than this?

(setq list (reverse (cdr (reverse list))))

list deletion

share|improve this question

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If doing that, you would likely want to be using nreverse (destructive) rather than reverse (copying), for the sake of efficiency.
  
  – phils
  May 22 at 4:16
  

  
  
  
  

add a comment | 

Is there a simpler way to remove the last element of a list than this?

(setq list (reverse (cdr (reverse list))))

list deletion

share|improve this question

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

(setq list (reverse (cdr (reverse list))))

share|improve this question

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

share|improve this question

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

edited May 21 at 15:12

Drew

49.4k465111

edited May 21 at 15:12

Drew

49.4k465111

asked May 21 at 14:14

ToothrotToothrot

1,126414

asked May 21 at 14:14

ToothrotToothrot

1,126414

1 Answer
1

active

oldest

votes

10

Yes there is:

(setq list (butlast list))

That is a function from subr.el. (Loaded by default. No need to load anything.)

You can also cut a tail with N elements by

(setq list (butlast list N))

---

A word about phils' comment:

If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).

The comment is right and useful but it has to be considered very cautious.

For an exmple the application of nbutlast is safe if the following three conditions are fulfilled:

• if you are in a pure function (you have local non-leaking bindings)


• you generate the list from its elements within the function


• you want to split off the last element at the end of the function (i.e. in the last evaluated form before exiting the function)

Be aware that nbutlast modifies the argument list in the calls of the following functions!

(defun queue (el list)
 "Prepend el and remove last element."
 (nbutlast (cons el list)))

(defun foo (head list)
 "Prepend head and remove last element."
 (nbutlast (append head list)))

(setq list (list 2 3 4))

(queue 1 list)

list ;; -> (2 3)

(foo '(1) list)

list ;; -> (2)

share|improve this answer

edited May 22 at 12:16

answered May 21 at 14:18

TobiasTobias

16.1k11137

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).
  
  – phils
  May 22 at 4:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that behind the scenes, these functions are using setcdr to modify the nthcdr where N is based on the length of the list.
  
  – phils
  May 22 at 4:13
  
  

  
  
  
  

add a comment | 

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10

Yes there is:

(setq list (butlast list))

That is a function from subr.el. (Loaded by default. No need to load anything.)

You can also cut a tail with N elements by

(setq list (butlast list N))

---

A word about phils' comment:

If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).

The comment is right and useful but it has to be considered very cautious.

For an exmple the application of nbutlast is safe if the following three conditions are fulfilled:

• if you are in a pure function (you have local non-leaking bindings)


• you generate the list from its elements within the function


• you want to split off the last element at the end of the function (i.e. in the last evaluated form before exiting the function)

Be aware that nbutlast modifies the argument list in the calls of the following functions!

(defun queue (el list)
 "Prepend el and remove last element."
 (nbutlast (cons el list)))

(defun foo (head list)
 "Prepend head and remove last element."
 (nbutlast (append head list)))

(setq list (list 2 3 4))

(queue 1 list)

list ;; -> (2 3)

(foo '(1) list)

list ;; -> (2)

share|improve this answer

edited May 22 at 12:16

answered May 21 at 14:18

TobiasTobias

16.1k11137

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).
  
  – phils
  May 22 at 4:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that behind the scenes, these functions are using setcdr to modify the nthcdr where N is based on the length of the list.
  
  – phils
  May 22 at 4:13
  
  

  
  
  
  

add a comment | 

10

Yes there is:

(setq list (butlast list))

That is a function from subr.el. (Loaded by default. No need to load anything.)

You can also cut a tail with N elements by

(setq list (butlast list N))

---

A word about phils' comment:

If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).

The comment is right and useful but it has to be considered very cautious.

For an exmple the application of nbutlast is safe if the following three conditions are fulfilled:

• if you are in a pure function (you have local non-leaking bindings)


• you generate the list from its elements within the function


• you want to split off the last element at the end of the function (i.e. in the last evaluated form before exiting the function)

Be aware that nbutlast modifies the argument list in the calls of the following functions!

(defun queue (el list)
 "Prepend el and remove last element."
 (nbutlast (cons el list)))

(defun foo (head list)
 "Prepend head and remove last element."
 (nbutlast (append head list)))

(setq list (list 2 3 4))

(queue 1 list)

list ;; -> (2 3)

(foo '(1) list)

list ;; -> (2)

share|improve this answer

edited May 22 at 12:16

answered May 21 at 14:18

TobiasTobias

16.1k11137

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).
  
  – phils
  May 22 at 4:09
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that behind the scenes, these functions are using setcdr to modify the nthcdr where N is based on the length of the list.
  
  – phils
  May 22 at 4:13
  
  

  
  
  
  

add a comment | 

Yes there is:

(setq list (butlast list))

That is a function from subr.el. (Loaded by default. No need to load anything.)

You can also cut a tail with N elements by

(setq list (butlast list N))

---

A word about phils' comment:

If it's safe to modify the original list structure, then nbutlast will be slightly more efficient (n.b. you still need to assign the result back to the variable).

The comment is right and useful but it has to be considered very cautious.

For an exmple the application of nbutlast is safe if the following three conditions are fulfilled:

• if you are in a pure function (you have local non-leaking bindings)


• you generate the list from its elements within the function


• you want to split off the last element at the end of the function (i.e. in the last evaluated form before exiting the function)

Be aware that nbutlast modifies the argument list in the calls of the following functions!

(defun queue (el list)
 "Prepend el and remove last element."
 (nbutlast (cons el list)))

(defun foo (head list)
 "Prepend head and remove last element."
 (nbutlast (append head list)))

(setq list (list 2 3 4))

(queue 1 list)

list ;; -> (2 3)

(foo '(1) list)

list ;; -> (2)

share|improve this answer

edited May 22 at 12:16

answered May 21 at 14:18

TobiasTobias

16.1k11137

(defun queue (el list)
 "Prepend el and remove last element."
 (nbutlast (cons el list)))

(defun foo (head list)
 "Prepend head and remove last element."
 (nbutlast (append head list)))

(setq list (list 2 3 4))

(queue 1 list)

list ;; -> (2 3)

(foo '(1) list)

list ;; -> (2)

share|improve this answer

edited May 22 at 12:16

answered May 21 at 14:18

TobiasTobias

16.1k11137

answered May 21 at 14:18

TobiasTobias

16.1k11137

answered May 21 at 14:18

TobiasTobias

16.1k11137