java - 类未返回预期的输出

Question

我正在编写一个Guitar37类，其中将37个键映射到从110Hz到880Hz的色标。

public static final String KEYBOARD =
        "q2we4r5ty7u8i9op-[=zxdcfvgbnjmk,.;/' ";  // keyboard layout

给定此字符串，字符串的第i个字符对应于440 * 2（i-24）/ 12的频率。例如：字符'q'为110Hz，'i'为220Hz，'v'为440Hz， “”（空格键）为880Hz。

吉他37课

public class Guitar37 implements Guitar {
   public static final String KEYBOARD =
      "q2we4r5ty7u8i9op-[=zxdcfvgbnjmk,.;/' ";  // keyboard layout

   private GuitarString[] strings;
   private int currentTime;

   public Guitar37() {
      currentTime = 0;
      strings = new GuitarString[37];

      for (int i = 0; i < 37; i++) {
         strings[i] = new GuitarString(440 * Math.pow(2, (double) (i - 24) / 12));
      }
   }

   public void playNote(int pitch) {
      int indexPitch = pitch + 24;

      if (indexPitch < 37 && indexPitch > 0) {
         strings[indexPitch].pluck();
      }
   }

   public boolean hasString(char key) {
      return KEYBOARD.indexOf(key) != -1;
   }

   public void pluck(char key) {
      if(!hasString(key)) {
         throw new IllegalArgumentException();
      }

      strings[KEYBOARD.indexOf(key)].pluck();
   }

   public double sample() {
      double sum = 0.0;

      for (int i  = 0; i < 37; i++) {
         sum += strings[i].sample();
      }
      return sum;
   }

   public void tic() {
      for (int i = 0; i < 37; i++) {
         strings[i].tic();
      }
      currentTime++;
   }

   public int time() {
      return currentTime;
   }
}

运行Test37类以测试Guitar37类是否返回正确的输出时，输出中第一个音符的声音样本应为：

Playing note -24 (initially 110.25)
time 0 sample = 110.25
time 1 sample = 100.25
time 2 sample = 90.25
time 3 sample = 80.25
Plucking string 'q'
time 4 sample = 110.25
time 5 sample = 100.25
time 6 sample = 90.25
time 7 sample = 80.25
making an extra call on tic

相反，我的Guitar37类返回：

Playing note -24 (initially 110.25)
time 0 sample = 0.0
time 1 sample = 0.0
time 2 sample = 0.0
time 3 sample = 0.0
Plucking string 'q'
time 4 sample = 110.25
time 5 sample = 100.25
time 6 sample = 90.25
time 7 sample = 80.25
making an extra call on tic

所有其他注释产生正确的输出，但该类仅在0到4的时间内返回0.0，而不是4到7的值。

Test37类

// This is a program that can be used to test the basic functionality of the
// Guitar37 class.  It does not test all of the functionality.
//
// The main method has an array of 37 frequency values that should be produced
// by a correctly working Guitar37 class.  The actual frequencies don't all end
// in 0.25, but this version of GuitarString converts them to that form so that
// it is easier to understand the output produced by the program.  In
// particular, it is easier to see if it is properly adding new values as notes
// are played.

import java.util.*;

public class Test37 {
    public static final String KEYBOARD =
        "q2we4r5ty7u8i9op-[=zxdcfvgbnjmk,.;/' ";  // keyboard layout

    public static void main(String[] args) {
        double[] frequencies = 
            {110.25, 116.25, 123.25, 130.25, 138.25, 146.25, 155.25, 164.25,
             174.25, 184.25, 195.25, 207.25, 220.25, 233.25, 246.25, 261.25,
             277.25, 293.25, 311.25, 329.25, 349.25, 369.25, 391.25, 415.25,
             440.25, 466.25, 493.25, 523.25, 554.25, 587.25, 622.25, 659.25,
             698.25, 739.25, 783.25, 830.25, 880.25};

        Guitar g = new Guitar37();
        compare(frequencies, GuitarString.nums);
        for (int i = 0; i < KEYBOARD.length(); i++) {
            int note = i - 24;
            System.out.println("Playing note " + note + " (initially " + 
                               frequencies[i] + ")");
            g.playNote(note);
            advance(g, 4);
            char key = KEYBOARD.charAt(i);
            System.out.println("Plucking string '" + key + "'");
            if (g.hasString(key)) {
                g.pluck(key);
                advance(g, 4);
            } else {
                System.out.println("ERROR: not recognizing key '" + key + "'");
            }
            System.out.println("making an extra call on tic");
            // throw in an extra call on tic without calling time and sample
            g.tic();
            System.out.println();
        }

        // now test a few unsupported values of pitch which should be ignored
        // but should not throw an exception
        int[] unsupportedPitch = {-32, -25, 13, 18};
        for (int n : unsupportedPitch) {
            System.out.println("testing playNote for unsupported pitch " + n);
            g.playNote(n);
            advance(g, 4);
            System.out.println();
        }
    }

    // This method advances the simulation the given number of tics reporting
    // the time reading and sample values from the given guitar.
    public static void advance(Guitar g, int tics) {
        for (int i = 0; i < tics; i++) {
            System.out.println("time " + g.time() + " sample = " + g.sample());
            g.tic();
        }
    }

    // This method compares the array nums with the set of numbers in nums2,
    // suspending program execution if the two sets of numbers differ.
    public static void compare(double[] nums, Set<Integer> nums1) {
        Set<Integer> nums2 = new TreeSet<>();
        for (double n : nums) {
            nums2.add((int) n);
        }
        if (!nums1.equals(nums2)) {
            System.out.println("Wrong frequencies for Guitar37 construction.");
            System.out.println("should be approximately: " + nums2);
            System.out.println("yours are approximately: " + nums1);
            System.exit(1);
        }
    }
}

GuitarString类

// This class is used for debugging the Guitar37 class.  It is not an example
// to be emulated.  When a string is plucked, it is set to the integer part of
// the string's frequency plus 0.25.  It goes down by 10 each time tic is
// called until it becomes less than or equal to 10 when it is set to 0.

import java.util.*;

public class GuitarString {
    static Set<Integer> nums = new TreeSet<>();  // observed frequency values
    double value;
    double freq;

    public GuitarString(double frequency) {
        freq = frequency;
        nums.add((int) frequency);
    }

    public void pluck() {
        value = (int) freq + 0.25;
    }

    public void tic() {
        if (value <= 10) {
            value = 0.0;
        } else {
            value = value - 10;
        }
    }

    public double sample() {
        return value;
    }
}

Answer 1

我在您的代码中看到的错误在这里：

strings[i] = new GuitarString(440 * Math.pow(2, (double) (i - 24) / 12));

您正在做 (i - 24) / 12（这是int除法），然后强制转换为double。

代替：

strings[i] = new GuitarString(440 * Math.pow(2, (i - 24) / 12.0));

这将执行双重除法而不是整数除法。