# Array#slice! may be too slow and allocate memory too much

**URL:** <https://www.ruby-forum.com/t/array-slice-may-be-too-slow-and-allocate-memory-too-much/131256>\
**Category:** Development\
**Created:** [March 7, 2008, 1:02pm UTC](https://www.ruby-forum.com/t/array-slice-may-be-too-slow-and-allocate-memory-too-much/131256 "2008-03-07T13:02:15Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Yusuke\_ENDOH](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/c89c15/32.png) [@Yusuke\_ENDOH](https://www.ruby-forum.com/u/Yusuke_ENDOH)\
**Post date:** [March 7, 2008, 1:02pm UTC](https://www.ruby-forum.com/t/array-slice-may-be-too-slow-and-allocate-memory-too-much/131256/1 "2008-03-07T13:02:15Z")

</div>

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$ time ./ruby -ve ’

> a = [0] \* 10000  
> (1 … 10000).map { a.slice!(0, 1) }  
> ’  
> ruby 1.9.0 (2008-03-07 revision 15720) [i686-linux]

real 0m10.940s  
user 0m4.200s  
sys 0m6.710s

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t = a[i, n]  
a[i, n] = []  
return t

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2. a[i, n] = [] e$B$,e(B a e$B$re(B rb\_ary\_modify e$B$9$ke(B
3. rb\_ary\_modify e$B$NCf$Ge(B a e$BA4BN$re(B MEMCPY e$B$9$ke(B

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- rb\_ary\_splice e$B$Ge(B rb\_ary\_modify e$B$9$k$H$-!"I,MW$J$H$3$m$@$1e(B  
MEMCPY e$B$9$ke(B
- a.slice!(i, n) e$B$Ge(B n e$B$,e(B a.size / 2 e$B$h$j\>.$5$$\>l9g$Oe(B t  
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±---------±------------±----------------------------------------------+  
| e$B:GE,2=A0e(B | e$B:GE,2=8ee(B | |  
| user sys | user sys | a = [0] \* 10000; b = (1 … 10000).map { … } |  
±---------±------------±----------------------------------------------+  
| 4.2 6.7 | 0.14 0.03 | a.slice!(0, 1) |  
| 0.26 6.6 | 0.050 0.030 | t, a = a, a.slice!(0, a.size - 1); t |  
±---------±------------±----------------------------------------------+

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e$B$3$l$r2?$H$+$9$k$N$O$$$m$$$m$HLLE]$=$&$G$9!#e(B

a = [0] \* 10000  
(1 … 10000).map { t = a[0, 1]; a[0, 1] = []; t }

# Index: array.c

— array.c (revision 15726)  
+++ array.c (working copy)  
@@ -647,6 +647,26 @@  
}

VALUE  
+rb\_ary\_subseq\_copy(VALUE ary, long beg, long len)  
+{

- VALUE klass, ary2;
- 
- if (beg \> RARRAY\_LEN(ary)) return Qnil;
- if (beg \< 0 || len \< 0) return Qnil;
- 
- if (RARRAY\_LEN(ary) \< len || RARRAY\_LEN(ary) \< beg + len) {
- len = RARRAY\_LEN(ary) - beg;
- }
- klass = rb\_obj\_class(ary);
- ary2 = ary\_new(klass, len);
- if (len \> 0) {
- MEMCPY(RARRAY\_PTR(ary2), RARRAY\_PTR(ary) + beg, VALUE, len);
- RARRAY(ary2)-\>len = len;
- }
- return ary2;  
+}
- 

+VALUE  
rb\_ary\_subseq(VALUE ary, long beg, long len)  
{  
VALUE klass, ary2, shared;  
@@ -973,8 +993,8 @@  
rpl = rb\_ary\_to\_ary(rpl);  
rlen = RARRAY\_LEN(rpl);  
}

- rb\_ary\_modify(ary);  
if (beg \>= RARRAY\_LEN(ary)) {

- rb\_ary\_modify(ary);  
len = beg + rlen;  
if (len \>= ARY\_CAPA(ary)) {  
RESIZE\_CAPA(ary, len);  
@@ -993,15 +1013,30 @@  
}

- if (alen \>= ARY\_CAPA(ary)) {
- 

```
 RESIZE_CAPA(ary, alen);

```

- 
- rb\_ary\_modify\_check(ary);
- if (ARY\_SHARED\_P(ary)) {
- 

```
 VALUE *ptr;

```

- 

```
 ptr = ALLOC_N(VALUE, alen);

```

- 

```
 FL_UNSET(ary, ELTS_SHARED);

```

- 

```
 MEMCPY(ptr, RARRAY_PTR(ary), VALUE, beg);

```

- 

```
 MEMCPY(ptr + beg + rlen, RARRAY_PTR(ary) + beg + len,

```

- 

```
  VALUE, RARRAY_LEN(ary) - (beg + len));

```

- 

```
 RARRAY(ary)->aux.capa = RARRAY(ary)->len = alen;

```

- 

```
 RARRAY(ary)->ptr = ptr;

```

}
- else {
- 

```
 if (alen >= ARY_CAPA(ary)) {

```

- RESIZE\_CAPA(ary, alen);
- 

```
 }

```

- 

```
 if (len != rlen) {

```

- MEMMOVE(RARRAY\_PTR(ary) + beg + rlen,
- 

```
 RARRAY_PTR(ary) + beg + len,

```

- 

```
 VALUE, RARRAY_LEN(ary) - (beg + len));

```

- RARRAY(ary)-\>len = alen;
- 

```
 }

```

- }

- if (len != rlen) {
- 

```
 MEMMOVE(RARRAY_PTR(ary) + beg + rlen, RARRAY_PTR(ary) + beg + 

```

len,

- 

```
   VALUE, RARRAY_LEN(ary) - (beg + len));

```

- 

```
 RARRAY(ary)->len = alen;

```

- }  
if (rlen \> 0) {  
MEMMOVE(RARRAY\_PTR(ary) + beg, RARRAY\_PTR(rpl), VALUE, rlen);  
}  
@@ -1790,7 +1825,12 @@  
pos = RARRAY\_LEN(ary) + pos;  
if (pos \< 0) return Qnil;  
}
- arg2 = rb\_ary\_subseq(ary, pos, len);

- if (len \* 2 \< RARRAY\_LEN(ary)) {
- 

```
 arg2 = rb_ary_subseq_copy(ary, pos, len);

```

- }
- else {
- 

```
 arg2 = rb_ary_subseq(ary, pos, len);

```

- }  
rb\_ary\_splice(ary, pos, len, Qundef); /\* Qnil/rb\_ary\_new2(0) \*/  
return arg2;  
}

---

<div class="post-metadata">

**Author:** ![Yusuke\_ENDOH](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/y/c89c15/32.png) [@Yusuke\_ENDOH](https://www.ruby-forum.com/u/Yusuke_ENDOH)\
**Post date:** [March 13, 2008, 5:18pm UTC](https://www.ruby-forum.com/t/array-slice-may-be-too-slow-and-allocate-memory-too-much/131256/2 "2008-03-13T17:18:08Z")

</div>

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In message “Re: [ruby-dev:34005] Array#slice! may be too slow and  
allocate memory too much”  
on Fri, 7 Mar 2008 21:00:53 +0900, “Yusuke ENDOH” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
writes:

|e$B0J2\<$N%W%m%0%i%`$,$d$?$iCY$$$G$9!#$^$?!"%a%b%j$rBgNL$K\>CHq$7$^$9!#e(B  
|  
|$ time ./ruby -ve ’  
|\> a = [0] \* 10000  
|\> (1 … 10000).map { a.slice!(0, 1) }  
|\> ’  
|ruby 1.9.0 (2008-03-07 revision 15720) [i686-linux]  
|  
|real 0m10.940s  
|user 0m4.200s  
|sys 0m6.710s  
|  
|  
|e$BG[Ns$N6&M-$,860x$N$h$&$G$9!#e(B

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|e$B$3$l$r2?$H$+$9$k$N$O$$$m$$$m$HLLE]$=$&$G$9!#e(B  
|  
|a = [0] \* 10000  
|(1 … 10000).map { t = a[0, 1]; a[0, 1] = []; t }

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