在mapToInt之后调用map是否有任何优势,如果需要的话 [英] Is there any advantage of calling map after mapToInt, where ever required
问题描述
我正在尝试计算列表中值的平方和。
以下是三个变量,它们都计算所需的值。
我想知道哪一个效率最高。我期待第三个
更高效,因为自动装箱只进行一次。
I am trying to calculate the sum of squares of values in the list. Below are three variations which all calculates the required value. I want to know which one is the most efficient. I am expecting the third one to be more efficient as auto boxing is only done once.
// sum of squares
int sum = list.stream().map(x -> x * x).reduce((x, y) -> x + y).get();
System.out.println("sum of squares: " + sum);
sum = list.stream().mapToInt(x -> x * x).sum();
System.out.println("sum of squares: " + sum);
sum = list.stream().mapToInt(x -> x).map(x -> x * x).sum();
System.out.println("sum of squares: " + sum);
推荐答案
如有疑问,请测试!使用jmh,我在100k元素列表中得到以下结果(以微秒为单位,越小越好):
When in doubt, test! Using jmh, I get the following results on a list of 100k elements (in microseconds, smaller is better):
Benchmark Mode Samples Score Error Units
c.a.p.SO32462798.for_loop avgt 10 119.110 0.921 us/op
c.a.p.SO32462798.mapToInt avgt 10 129.702 1.040 us/op
c.a.p.SO32462798.mapToInt_map avgt 10 129.753 1.516 us/op
c.a.p.SO32462798.map_reduce avgt 10 1262.802 12.197 us/op
c.a.p.SO32462798.summingInt avgt 10 134.821 1.203 us/op
所以你有,从更快到更慢:
So you have, from faster to slower:
-
for(int i:list )sum + = i * i;
-
mapToInt(x - > x * x).sum()
和mapToInt(x - > x).map(x - > x * x).sum()
-
collect(Collectors.summingInt(x - > x * x))
-
map(x - > x * x).reduce((x,y) - > x + y)。get()
for(int i : list) sum += i*i;
mapToInt(x -> x * x).sum()
andmapToInt(x -> x).map(x -> x * x).sum()
collect(Collectors.summingInt(x -> x * x))
map(x -> x * x).reduce((x, y) -> x + y).get()
请注意,结果在很大程度上取决于JIT优化。如果映射中的逻辑更复杂,则某些优化可能不可用(更长的代码=更少的内联),在这种情况下,流版本可能比for循环花费多4-5倍的时间 - 但如果该逻辑是CPU重的话差异将再次减少。分析您的实际应用程序将为您提供更多信息。
Note that the results are very much dependent on the JIT optimisations. If the logic in the mapping is more complex, some of the optimisations may be unavailable (longer code = less inlining) in which case the streams versions may take 4-5x more time than the for loop - but if that logic is CPU heavy the difference will reduce again. Profiling your actual application will give you more information.
基准代码供参考:
@State(Scope.Benchmark)
@BenchmarkMode(Mode.AverageTime)
public class SO32462798 {
List<Integer> list;
@Setup public void setup() {
list = new Random().ints(100_000).boxed().collect(toList());
}
@Benchmark public int for_loop() {
int sum = 0;
for (int i : list) sum += i * i;
return sum;
}
@Benchmark public int summingInt() {
return list.stream().collect(Collectors.summingInt(x -> x * x));
}
@Benchmark public int mapToInt() {
return list.stream().mapToInt(x -> x * x).sum();
}
@Benchmark public int mapToInt_map() {
return list.stream().mapToInt(x -> x).map(x -> x * x).sum();
}
@Benchmark public int map_reduce() {
return list.stream().map(x -> x * x).reduce((x, y) -> x + y).get();
}
}
这篇关于在mapToInt之后调用map是否有任何优势,如果需要的话的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!