Subtract: Difference between revisions
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:''This page describes the dyadic arithmetic function. For negation of a single argument, see [[Negate]]. For subtraction of sets, see [[Without]].''  :''This page describes the dyadic arithmetic function. For negation of a single argument, see [[Negate]]. For subtraction of sets, see [[Without]].''  
{{BuiltinSubtract}}, '''Minus''', '''Subtraction''', or '''Difference''' is a [[dyadic]] [[scalar function]] which gives the arithmetic [[wikipedia:Subtractiondifference]] of its [[argument]]s. Subtract shares the [[glyph]] <  {{BuiltinSubtract}}, '''Minus''', '''Subtraction''', or '''Difference''' is a [[dyadic]] [[scalar function]] which gives the arithmetic [[wikipedia:Subtractiondifference]] of its [[argument]]s. Subtract shares the [[glyph]] <syntaxhighlight lang=apl inline></syntaxhighlight> with the arithmetic function [[Negate]], and its result is the left argument [[plus]] the negation of the right.  
== Examples ==  == Examples ==  
<  <syntaxhighlight lang=apl>  
¯2 9 5  3 ¯4 6  ¯2 9 5  3 ¯4 6  
¯5 13 ¯1  ¯5 13 ¯1  
× 3  2 3 4.5 ⍝ Sign of difference  × 3  2 3 4.5 ⍝ Sign of difference  
1 0 ¯1  1 0 ¯1  
</  </syntaxhighlight>  
The second example computes a [[wikipedia:Threeway comparisonthreeway comparison]] of each pair of arguments, with a result of <  The second example computes a [[wikipedia:Threeway comparisonthreeway comparison]] of each pair of arguments, with a result of <syntaxhighlight lang=apl inline>1</syntaxhighlight> to indicate the left argument was greater, <syntaxhighlight lang=apl inline>0</syntaxhighlight> to indicate the arguments are [[Comparison toleranceintolerantly]] equal, and <syntaxhighlight lang=apl inline>¯1</syntaxhighlight> to indicate the right argument was greater.  
== Properties ==  == Properties ==  
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See also [[Add#Properties]].  See also [[Add#Properties]].  
Subtraction is anticommutative: swapping the arguments negates the result, or using [[Commute]], <  Subtraction is anticommutative: swapping the arguments negates the result, or using [[Commute]], <syntaxhighlight lang=apl inline>⍨</syntaxhighlight> {{←→}} <syntaxhighlight lang=apl inline></syntaxhighlight>.  
=== Reduction and scan ===  === Reduction and scan ===  
[[Reduction]] with Subtract gives an ''alternating sum'' of the argument array, that is, elements are alternately added and subtracted to the result. The first element, third element, and so on are added to the final result while the second, fourth, and so on are subtracted.  [[Reduction]] with Subtract gives an ''alternating sum'' of the argument array, that is, elements are alternately added and subtracted to the result. The first element, third element, and so on are added to the final result while the second, fourth, and so on are subtracted.  
<  <syntaxhighlight lang=apl>  
6  1  2  6  1  2  
7  7  
/ 6 1 2  / 6 1 2  
7  7  
</  </syntaxhighlight>  
In the absense of rounding error we have <  In the absense of rounding error we have <syntaxhighlight lang=apl inline>/v</syntaxhighlight> {{←→}} <syntaxhighlight lang=apl inline>+/v×(⍴v)⍴1 ¯1</syntaxhighlight> for a [[vector]] <syntaxhighlight lang=apl inline>v</syntaxhighlight>.  
An interesting property of the alternating difference is that it can be used as a [[wikipedia:Divisibility ruledivisibility test]] for division by 11, a counterpart to the betterknown test for divisibility by 9. A number is divisible by 11 if and only if the sum of its digits is divisible by 11, so repeatedly taking the alternating sum of the digits determines divisibility by 11.  An interesting property of the alternating difference is that it can be used as a [[wikipedia:Divisibility ruledivisibility test]] for division by 11, a counterpart to the betterknown test for divisibility by 9. A number is divisible by 11 if and only if the sum of its digits is divisible by 11, so repeatedly taking the alternating sum of the digits determines divisibility by 11.  
<  <syntaxhighlight lang=apl>  
11  946 943  11  946 943  
0 8  0 8  
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(⌿ 10 ⊥⍣¯1 ⊢)⍣≡ 946 943  (⌿ 10 ⊥⍣¯1 ⊢)⍣≡ 946 943  
0 2  0 2  
</  </syntaxhighlight>  
[[Scan]] with subtraction produces a prefix of an [[wikipedia:Alternating seriesalternating series]]. This property is one of the reasons why scan was designed to reduce [[prefix]]es rather than [[suffix]]es of the argument array. As an example, we can see that an alternating series using the [[power]]s of two begins to converge to a third:  [[Scan]] with subtraction produces a prefix of an [[wikipedia:Alternating seriesalternating series]]. This property is one of the reasons why scan was designed to reduce [[prefix]]es rather than [[suffix]]es of the argument array. As an example, we can see that an alternating series using the [[power]]s of two begins to converge to a third:  
<  <syntaxhighlight lang=apl>  
\ ÷2*⍳6  \ ÷2*⍳6  
0.5 0.25 0.375 0.3125 0.34375 0.328125  0.5 0.25 0.375 0.3125 0.34375 0.328125  
</  </syntaxhighlight>  
== See also ==  == See also ==  
* [[Divide]]  * [[Divide]] 
Latest revision as of 10:58, 11 September 2022
 This page describes the dyadic arithmetic function. For negation of a single argument, see Negate. For subtraction of sets, see Without.


Subtract (
), Minus, Subtraction, or Difference is a dyadic scalar function which gives the arithmetic difference of its arguments. Subtract shares the glyph 
with the arithmetic function Negate, and its result is the left argument plus the negation of the right.
Examples
¯2 9 5  3 ¯4 6 ¯5 13 ¯1 × 3  2 3 4.5 ⍝ Sign of difference 1 0 ¯1
The second example computes a threeway comparison of each pair of arguments, with a result of 1
to indicate the left argument was greater, 0
to indicate the arguments are intolerantly equal, and ¯1
to indicate the right argument was greater.
Properties
See also Add#Properties.
Subtraction is anticommutative: swapping the arguments negates the result, or using Commute, ⍨

.
Reduction and scan
Reduction with Subtract gives an alternating sum of the argument array, that is, elements are alternately added and subtracted to the result. The first element, third element, and so on are added to the final result while the second, fourth, and so on are subtracted.
6  1  2 7 / 6 1 2 7
In the absense of rounding error we have /v
+/v×(⍴v)⍴1 ¯1
for a vector v
.
An interesting property of the alternating difference is that it can be used as a divisibility test for division by 11, a counterpart to the betterknown test for divisibility by 9. A number is divisible by 11 if and only if the sum of its digits is divisible by 11, so repeatedly taking the alternating sum of the digits determines divisibility by 11.
11  946 943 0 8 ⌿ 10 (⊥⍣¯1) 946 943 11 8 (⌿ 10 ⊥⍣¯1 ⊢)⍣≡ 946 943 0 2
Scan with subtraction produces a prefix of an alternating series. This property is one of the reasons why scan was designed to reduce prefixes rather than suffixes of the argument array. As an example, we can see that an alternating series using the powers of two begins to converge to a third:
\ ÷2*⍳6 0.5 0.25 0.375 0.3125 0.34375 0.328125
See also
External links
Documentation
 Dyalog
 APLX
 J Dictionary, NuVoc
 BQN