CS50 Problem Set 2 文本分级程序结果异常求助

问题现象 #

• 输入文本 "One fish. Two fish. Red fish. Blue fish." 时，程序错误输出 Grade 2，预期输出 Before Grade 1
• 一段需判定为 Grade 16+ 的文本，程序输出 Grade 14

原代码 #

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

int count_letters(string text);
int count_word(string text);
int count_sentences(string text);

int letters;
int words;
int sentences;

int main(void)
{

    string text = get_string("Text: ");

    printf("Text: %s\n", text);
    count_letters(text);
    count_word(text);
    count_sentences(text);

    float L = 100 * (letters / words);
    float S = 100 * (sentences / words);

    int index = round(0.0588 * L - 0.296 * S - 15.8);

    if (index < 1)
    {
        printf("Before Grade 1\n");
    } else if (index > 16)
    {
        printf("Grade 16+\n");
    } else
    {
        printf("Grade: %i\n", index);
    }
}
int count_letters(string text)
{
    letters = 0;

    for (int i = 0; i < strlen(text); i++)
    {
        if ((text[i] >= 65 && text[i] <= 99) || (text[i] >= 97 && text[i] <= 122))
        {
            letters++;
        }
    }
    return letters;}
int count_word(string text)
{
    words = 1;

    for (int i = 0; i < strlen(text); i++)
    {
        if (isspace(text[i]))
        {
            words++;
        }
    }
    return words;}
int count_sentences(string text)
{
    sentences = 0;

    for (int i = 0; i < strlen(text); i++)
    {
        if (text[i] == 33 || text[i] == 46 || text[i] == 63)
        {
            sentences++;
        }
    }
    return sentences;
}

错误分析与修复 #

1. 整数除法导致精度丢失 #

计算L和S时，letters、words、sentences均为整数，执行letters / words会触发整数除法（直接截断小数部分），导致计算结果严重偏离实际值。比如测试案例中21/8会得到2而非正确的2.625，直接导致分级指数计算错误。

修复：将其中一个操作数转为浮点数，触发浮点除法：

float L = 100.0 * letters / words;
float S = 100.0 * sentences / words;

2. 字母判断的ASCII范围错误 #

count_letters函数中，大写字母的判断范围写为65 <= text[i] <= 99，但大写字母A-Z的ASCII范围是65-90，91-99包含了[、\等非字母字符，会导致字母数统计偏多。

修复：修正大写字母的判断范围，也可直接使用字符常量提高可读性：

if ((text[i] >= 'A' && text[i] <= 'Z') || (text[i] >= 'a' && text[i] <= 'z'))

3. 全局变量优化（可选） #

原代码使用全局变量存储统计结果，容易引发意外的变量覆盖问题。建议改为在main中声明局部变量，通过函数返回值获取结果，代码更健壮：

int letters = count_letters(text);
int words = count_words(text);
int sentences = count_sentences(text);

修复后代码 #

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

int count_letters(string text);
int count_words(string text);
int count_sentences(string text);

int main(void)
{
    string text = get_string("Text: ");

    int letters = count_letters(text);
    int words = count_words(text);
    int sentences = count_sentences(text);

    float L = 100.0 * letters / words;
    float S = 100.0 * sentences / words;

    int index = round(0.0588 * L - 0.296 * S - 15.8);

    if (index < 1)
    {
        printf("Before Grade 1\n");
    } 
    else if (index > 16)
    {
        printf("Grade 16+\n");
    } 
    else
    {
        printf("Grade %i\n", index);
    }
}

int count_letters(string text)
{
    int count = 0;
    for (int i = 0; i < strlen(text); i++)
    {
        if ((text[i] >= 'A' && text[i] <= 'Z') || (text[i] >= 'a' && text[i] <= 'z'))
        {
            count++;
        }
    }
    return count;
}

int count_words(string text)
{
    int count = 1;
    for (int i = 0; i < strlen(text); i++)
    {
        if (isspace(text[i]))
        {
            count++;
        }
    }
    return count;
}

int count_sentences(string text)
{
    int count = 0;
    for (int i = 0; i < strlen(text); i++)
    {
        if (text[i] == '!' || text[i] == '.' || text[i] == '?')
        {
            count++;
        }
    }
    return count;
}

验证结果 #

输入 "One fish. Two fish. Red fish. Blue fish." 时，统计结果为：

• letters=21，words=8，sentences=4
  计算得：
  L = (21/8)*100 = 262.5，S = (4/8)*100 = 50
index = round(0.0588*262.5 - 0.296*50 -15.8) = -15，输出 Before Grade 1，符合预期。

内容的提问来源于stack exchange，提问作者Beatriz Martins