The D Programming Language's Unofficial Docs
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The D Programming Language's Unofficial Docs
beta
No-seed version. The first element of r is used as the seed's value.
Seed version. The seed should be a single value if fun is a single function. If fun is multiple functions, then seed should be a std.typecons.Tuple, with one field per function in f.
one or more functions
the accumulated result
Many aggregate range operations turn out to be solved with reduce quickly and easily. The example below illustrates reduce's remarkable power and flexibility.
1 import std.algorithm.comparison : max, min; 2 import std.math : approxEqual; 3 import std.range; 4 5 int[] arr = [ 1, 2, 3, 4, 5 ]; 6 // Sum all elements 7 auto sum = reduce!((a,b) => a + b)(0, arr); 8 assert(sum == 15); 9 10 // Sum again, using a string predicate with "a" and "b" 11 sum = reduce!"a + b"(0, arr); 12 assert(sum == 15); 13 14 // Compute the maximum of all elements 15 auto largest = reduce!(max)(arr); 16 assert(largest == 5); 17 18 // Max again, but with Uniform Function Call Syntax (UFCS) 19 largest = arr.reduce!(max); 20 assert(largest == 5); 21 22 // Compute the number of odd elements 23 auto odds = reduce!((a,b) => a + (b & 1))(0, arr); 24 assert(odds == 3); 25 26 // Compute the sum of squares 27 auto ssquares = reduce!((a,b) => a + b * b)(0, arr); 28 assert(ssquares == 55); 29 30 // Chain multiple ranges into seed 31 int[] a = [ 3, 4 ]; 32 int[] b = [ 100 ]; 33 auto r = reduce!("a + b")(chain(a, b)); 34 assert(r == 107); 35 36 // Mixing convertible types is fair game, too 37 double[] c = [ 2.5, 3.0 ]; 38 auto r1 = reduce!("a + b")(chain(a, b, c)); 39 assert(approxEqual(r1, 112.5)); 40 41 // To minimize nesting of parentheses, Uniform Function Call Syntax can be used 42 auto r2 = chain(a, b, c).reduce!("a + b"); 43 assert(approxEqual(r2, 112.5));
Sometimes it is very useful to compute multiple aggregates in one pass. One advantage is that the computation is faster because the looping overhead is shared. That's why reduce accepts multiple functions. If two or more functions are passed, reduce returns a std.typecons.Tuple object with one member per passed-in function. The number of seeds must be correspondingly increased.
1 import std.algorithm.comparison : max, min; 2 import std.math : approxEqual, sqrt; 3 import std.typecons : tuple, Tuple; 4 5 double[] a = [ 3.0, 4, 7, 11, 3, 2, 5 ]; 6 // Compute minimum and maximum in one pass 7 auto r = reduce!(min, max)(a); 8 // The type of r is Tuple!(int, int) 9 assert(approxEqual(r[0], 2)); // minimum 10 assert(approxEqual(r[1], 11)); // maximum 11 12 // Compute sum and sum of squares in one pass 13 r = reduce!("a + b", "a + b * b")(tuple(0.0, 0.0), a); 14 assert(approxEqual(r[0], 35)); // sum 15 assert(approxEqual(r[1], 233)); // sum of squares 16 // Compute average and standard deviation from the above 17 auto avg = r[0] / a.length; 18 assert(avg == 5); 19 auto stdev = sqrt(r[1] / a.length - avg * avg); 20 assert(cast(int) stdev == 2);
Implements the homonym function (also known as accumulate, compress, inject, or foldl) present in various programming languages of functional flavor. There is also fold which does the same thing but with the opposite parameter order. The call reduce!(fun)(seed, range) first assigns seed to an internal variable result, also called the accumulator. Then, for each element x in range, result = fun(result, x) gets evaluated. Finally, result is returned. The one-argument version reduce!(fun)(range) works similarly, but it uses the first element of the range as the seed (the range must be non-empty).