香农熵不打印任何东西

Question

Java, Intellij IDE Coursera, Computer Science: Programming with a Purpose Princeton University

My program is not returning any output because n and f[] aren't returning any value outside the while loop - I checked it using the print statement. However, when I use the same print statement to print the value of n and f[] inside the while loop it prints the value. It seems like n and f[] becomes obsolete outside the while loop.

The question is Shannon entropy. Write a program ShannonEntropy.java that takes a command-line integer m; reads a sequence of integers between 1 and m from standard input; and prints the Shannon entropy to standard output, with 4 digits after the decimal point. The Shannon entropy of a sequence of integers is given by the formula: H=−(p1log2p1+p2log2p2+…+pmlog2pm) where pi denotes the proportion of integers whose value is i. If pi=0, then treat pilog2pi as 0. If the question is unclear please take a look

It will be great if you can help me out. Thanks in advance.

public class ShannonEntropy {

public static void main(String[] args) {

int m = Integer.parseInt(args[0]);

int[] f = new int[m + 1];

int n = 0;



// calculating the frequency by incrementing the array and incrementing n alongside

while (!StdIn.isEmpty()) {

int value = StdIn.readInt();

f[value]++;

n++;

}

double entropy = 0;

for (int i = 1; i <= m; i++) {

double p = (double) f[i] / n;

System.out.println(p);

if (f[i] > 0)

entropy -= p * (Math.log(p) / Math.log(2));



}



// printing the output

StdOut.println((double) Math.round(entropy * 10000) / 10000);



}

}

Answer 1

Hi I have tested your code. There is indeed a problem with the way you wrote your output code.

Instead of:

StdOut.println((double) Math.round(entropy * 10000) / 10000);

You should use formatted printing aka printf() to achieve 4 decimal places instead of using Math.round() as 1.0 will be printed as 1.0 instead of the desired 1.0000

Use this:

StdOut.printf("%.4f\n" ,entropy);

For more regarding printf(), refer to this link: printf() guide