样本：英文世界名著（1000部）

样本容量：1.4亿（包含6918个文件，366M）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int CountL[26];

int CheckL(char c)
{
    int index;
    char i, j;
    int flag = 0;

    for(i = 'A', j = 'a'; i <= 'Z', j <= 'z'; i++, j++)
    {
        if(i == c)
        {
            CountL[i - 65]++;
            flag = 1;
            break;
        }
        if(j == c)
        {
            CountL[j - 97]++;
            flag = 1;
            break;
        }
    }

    return flag;
}

long LetterScan(char *filepath)
{
    FILE *fp;
    long AllCount = 0;
    char c;

    if((fp = fopen(filepath, "r")) == NULL)
    {
        perror("Can't open!");
        exit(1);
    }

    while(!feof(fp))
    {
        c = fgetc(fp);
        if(CheckL(c))
        {
            AllCount++;
        }
    }

    fclose(fp);
    return AllCount;
}

char *SortL()
{
    int i, j, t;
    char c;
    char *Letter;

    Letter = (char *)calloc(26, sizeof(char));

    for(i = 0; i < 26; i++)
    {
        Letter[i] = i + 97;
    }

    for(i = 0; i < 25; i++)
    {
        for(j = 0; j < 25 - i; j++)
        {
            if(CountL[j] < CountL[j + 1])
            {
                t = CountL[j];
                c = Letter[j];
                CountL[j] = CountL[j + 1];
                Letter[j] = Letter[j + 1];
                CountL[j + 1] = t;
                Letter[j + 1] = c;
            }
        }
    }

    return Letter;
}

int main()
{
    char *FilePath = "/share/test/doc/test.txt";
    int i;
    long count = 0;
    char *Letter;

    Letter = (char *)calloc(26, sizeof(char));
    system("rm -rf doc/test.txt");
    system("cat doc/*txt > doc/test.txt");

    count = LetterScan(FilePath);
    Letter = SortL();

    printf("测试文本包含有效英文字母总数：%ld\n", count);
    for(i = 0; i < 26; i++)
    {
        printf("%c共出现%d次，概率是%f\n", Letter[i], CountL[i], (float)CountL[i] / count);
    }
    free(Letter);
    return 0;
}