java中的分数背包实现(但不是正确答案)
我在java中使用分数背包实现时遇到问题。我没有得到正确的答案 这是我的密码:java中的分数背包实现(但不是正确答案),java,algorithm,sorting,knapsack-problem,greedy,Java,Algorithm,Sorting,Knapsack Problem,Greedy,我在java中使用分数背包实现时遇到问题。我没有得到正确的答案 这是我的密码: import java.util.Scanner; import java.math.RoundingMode; import java.text.DecimalFormat; import static java.lang.Integer.min; public class FractionalKnapsack { // this method for calculating the maximum i
import java.util.Scanner;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import static java.lang.Integer.min;
public class FractionalKnapsack {
// this method for calculating the maximum index
public static int select_max_index(int[] values, int[] weights, int n) {
int index = -1;
double max = 0;
for (int i = 1; i <= n; i++) {
if (weights[i] > 0 && (double) values[i] / (double) weights[i] > max) {
max = (double) values[i] / (double) weights[i];
index = i;
}
}
return index;
}
private static double getOptimalValue(int capacity, int[] values, int[] weights, int n) {
// fractional knapsack problem
DecimalFormat df = new DecimalFormat("#.####"); // for getting the decimal point upto 4 digits
df.setRoundingMode(RoundingMode.CEILING);
int i;
double value = 0.0000d;
if (capacity == 0)
return value;
for (i = 0; i < n; i++) {
int max_index = select_max_index(values, weights, n);// call the maximum index
if (max_index >= 0) {
int b = min(capacity, (weights[max_index]));
value = value + b * ((double) values[max_index] / (double) weights[max_index]);
weights[i] = (weights[max_index] - b);
capacity = capacity - b;
}
}
return Double.parseDouble(df.format(value));
//return value;
}
public static void main(String args[]) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int capacity = scanner.nextInt();
int[] values = new int[n + 2];
int[] weights = new int[n + 2];
for (int i = 0; i < n; i++) {
values[i] = scanner.nextInt();
weights[i] = scanner.nextInt();
}
System.out.println(getOptimalValue(capacity, values, weights, n));
}
}
正确输出:
180.0000
我的输出:
200.0
我已经遵循了所有的步骤。但有两个问题:
这个背包有替换品吗?通常在分数背包中,一旦你选择了一件物品,你就不能再选择它了。在select_max_索引的实现中,您似乎总是选择最高的值/比率,这意味着您将始终使用它。在您的示例中: 第一个选择是value=120,权重为30,所以剩下20 然后再次选择value=120,但这次取其值的2/3进行拟合,即80 这就是为什么你得到200 您需要修复代码,这样您就不会一直选择同一项。最简单的方法是将优先级队列与deleteMin或deleteMax一起使用
3 50
60 20
100 50
120 30