|
|
Why do we use sort before compare ?
|
|
|
|
lsort(n) Tcl Built-In Commands lsort(n) __________________________________________________________________________________________________________________________________________________ NAME
lsort - Sort the elements of a list SYNOPSIS
lsort ?options? list _________________________________________________________________ DESCRIPTION
This command sorts the elements of list, returning a new list in sorted order. The implementation of the lsort command uses the merge-sort algorithm which is a stable sort that has O(n log n) performance characteristics. By default ASCII sorting is used with the result returned in increasing order. However, any of the following options may be specified before list to control the sorting process (unique abbreviations are accepted): -ascii Use string comparison with ASCII collation order. This is the default. -dictionary Use dictionary-style comparison. This is the same as -ascii except (a) case is ignored except as a tie-breaker and (b) if two strings contain embedded numbers, the numbers compare as integers, not characters. For example, in -dictionary mode, bigBoy sorts between bigbang and bigboy, and x10y sorts between x9y and x11y. -integer Convert list elements to integers and use integer comparison. -real Convert list elements to floating-point values and use floating comparison. -command command Use command as a comparison command. To compare two elements, evaluate a Tcl script consisting of command with the two elements appended as additional arguments. The script should return an integer less than, equal to, or greater than zero if the first element is to be considered less than, equal to, or greater than the second, respectively. -increasing Sort the list in increasing order (``smallest'' items first). This is the default. -decreasing Sort the list in decreasing order (``largest'' items first). -index index If this option is specified, each of the elements of list must itself be a proper Tcl sublist. Instead of sorting based on whole sublists, lsort will extract the index'th element from each sublist and sort based on the given element. The keyword end is allowed for the index to sort on the last sublist element, and end-index sorts on a sublist element | offset from the end. For example, lsort -integer -index 1 {{First 24} {Second 18} {Third 30}} returns {Second 18} {First 24} {Third 30}, and | lsort -index end-1 {{a 1 e i} {b 2 3 f g} {c 4 5 6 d h}} | returns {c 4 5 6 d h} {a 1 e i} {b 2 3 f g}. This option is much more efficient than using -command to achieve the same effect. -unique If this option is specified, then only the last set of duplicate elements found in the list will be retained. Note that duplicates are determined relative to the comparison used in the sort. Thus if -index 0 is used, {1 a} and {1 b} would be considered duplicates and only the second element, {1 b}, would be retained. NOTES
The options to lsort only control what sort of comparison is used, and do not necessarily constrain what the values themselves actually are. This distinction is only noticeable when the list to be sorted has fewer than two elements. The lsort command is reentrant, meaning it is safe to use as part of the implementation of a command used in the -command option. EXAMPLES
Sorting a list using ASCII sorting: % lsort {a10 B2 b1 a1 a2} B2 a1 a10 a2 b1 Sorting a list using Dictionary sorting: % lsort -dictionary {a10 B2 b1 a1 a2} a1 a2 a10 b1 B2 Sorting lists of integers: % lsort -integer {5 3 1 2 11 4} 1 2 3 4 5 11 % lsort -integer {1 2 0x5 7 0 4 -1} -1 0 1 2 4 0x5 7 Sorting lists of floating-point numbers: % lsort -real {5 3 1 2 11 4} 1 2 3 4 5 11 % lsort -real {.5 0.07e1 0.4 6e-1} 0.4 .5 6e-1 0.07e1 Sorting using indices: % # Note the space character before the c % lsort {{a 5} { c 3} {b 4} {e 1} {d 2}} { c 3} {a 5} {b 4} {d 2} {e 1} % lsort -index 0 {{a 5} { c 3} {b 4} {e 1} {d 2}} {a 5} {b 4} { c 3} {d 2} {e 1} % lsort -index 1 {{a 5} { c 3} {b 4} {e 1} {d 2}} {e 1} {d 2} { c 3} {b 4} {a 5} Stripping duplicate values using sorting: % lsort -unique {a b c a b c a b c} a b c More complex sorting using a comparison function: % proc compare {a b} { set a0 [lindex $a 0] set b0 [lindex $b 0] if {$a0 < $b0} { return -1 } elseif {$a0 > $b0} { return 1 } return [string compare [lindex $a 1] [lindex $b 1]] } % lsort -command compare {{3 apple} {0x2 carrot} {1 dingo} {2 banana}} {1 dingo} {2 banana} {0x2 carrot} {3 apple} SEE ALSO
lappend(n), lindex(n), linsert(n), list(n), llength(n), lrange(n), lreplace(n), lsearch(n) KEYWORDS
element, list, order, sort Tcl 8.3 lsort(n)
