我正在尝试使用向量中的相应值来缩放矩阵中的所有列 . 如果此值为0,我想用一个由常量缩放的其他矩阵中的列替换该列 . 听起来很复杂,但在Matlab中它非常简单(但可能没有完全优化):
a(:,b ~= 0) = a(:,b ~= 0)./b(b ~= 0);
a(:,b == 0) = c(:,b == 0)*x;
使用C中的 for loop
执行此操作也非常简单:
RowVectorXf b;
Matrix3Xf a, c;
float x;
for (int i = 0; i < b.size(); i++) {
if (b(i) != 0) {
a.col(i) = a.col(i) / b(i);
} else {
a.col(i) = c.col(i) * x;
}
}
是否有可能使用Eigen内在函数(如 colwise
和 select
)执行此操作(更快)?
附:我试图将if条件缩短到表单
a.col(i) = (b(i) != 0) ? (a.col(i) / b(i)) : (c.col(i) * x);
但这不会编译错误 error: operands to ?: have different types ...(long listing of the types)
编辑:我添加了测试答案的代码,这里是:
#include <Eigen/Dense>
#include <stdlib.h>
#include <chrono>
#include <iostream>
using namespace std;
using namespace Eigen;
void flushCache()
{
const int size = 20 * 1024 * 1024; // Allocate 20M. Set much larger than L2
volatile char *c = (char *) malloc(size);
volatile int i = 8;
for (volatile int j = 0; j < size; j++)
c[j] = i * j;
free((void*) c);
}
int main()
{
Matrix3Xf a(3, 1000000);
RowVectorXf b(1000000);
Matrix3Xf c(3, 1000000);
float x = 0.4;
a.setRandom();
b.setRandom();
c.setRandom();
for (int testNumber = 0; testNumber < 4; testNumber++) {
flushCache();
chrono::high_resolution_clock::time_point t1 = chrono::high_resolution_clock::now();
for (int repetition = 0; repetition < 1000; repetition++) {
switch (testNumber) {
case 0:
for (int i = 0; i < b.size(); i++) {
if (b(i) != 0) {
a.col(i) = a.col(i) / b(i);
} else {
a.col(i) = c.col(i) * x;
}
}
break;
case 1:
for (int i = 0; i < b.size(); i++) {
a.col(i) = (b(i) != 0) ? (a.col(i) / b(i)).eval() : (c.col(i) * x).eval();
}
break;
case 2:
for (int i = 0; i < b.size(); i++) {
a.col(i) = (b(i) != 0) ? (a.col(i) * (1.0f / b(i))) : (c.col(i) * x);
}
break;
case 3:
a = b.cwiseEqual(0.0f).replicate< 3, 1 >().select(c * x, a.cwiseQuotient(b.replicate< 3, 1 >()));
break;
default:
break;
}
}
chrono::high_resolution_clock::time_point t2 = chrono::high_resolution_clock::now();
auto duration = chrono::duration_cast< chrono::milliseconds >(t2 - t1).count();
cout << "duration: " << duration << "ms" << endl;
}
return 0;
}
示例输出是:
duration: 14391ms
duration: 15219ms
duration: 9148ms
duration: 13513ms
顺便说一句,不使用setRandom来初始化变量,输出完全不同:
duration: 10255ms
duration: 11076ms
duration: 8250ms
duration: 5198ms
由于分支预测,@ chtz建议's because of denormalized values, but I think it' . 由于分支预测的原因在于,初始化 b.setZero();
导致与未初始化相同的时序 .
1 回答
会工作,但只是因为表达式是相同的类型,并且它可能在任何时候都不安全(
? :
表达式基本上被转换为与if
-else
分支相同 . )如果您更喜欢将其写入一行,则以下表达式应该起作用:
同样,我怀疑它会产生任何显着的性能差异 .