计算输入字母的个数，代码如下，求详细解释

int Count(int cou[]) //计算输入的字母的个数
{
char word;
char letter[27]={' ','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o', 'p','q','r','s','t','u','v','w','x','y','z'};
fstream SourceFile;
SourceFile.open("SourceFile.txt",ios::in);
if (!SourceFile)
{
cout<<"Can't open this file"<<endl;
}
while (!SourceFile.eof())

    {
        SourceFile.get(word);
        if (word>='A'&&word<='Z')
        {
            word=word+32;
        }
        int i=word-96;
        cou[i]++;
    }
    SourceFile.close();
    cout<<"letter"<<'\t'<<'\t'<<"频率为"<<endl;
        for (int j=1;j<=26;j++)
        {
            if (cou[j]!=0)
            {
                cout<<letter[j]<<'\t'<<'\t'<<cou[j]<<endl;
            }
        }
        cout<<"读入完事儿！"<<endl;

return 0;

}

 int Count(int cou[]) //计算输入的字母的个数
{
char word;
char letter[27]={' ','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o', 'p','q','r','s','t','u','v','w','x','y','z'}; //纯属多余
fstream SourceFile;
SourceFile.open("SourceFile.txt",ios::in); 从SourceFile.txt读取
if (!SourceFile)
{
cout<<"Can't open this file"<<endl;
}
while (!SourceFile.eof()) 循环直到文件结束
    {
        SourceFile.get(word);
        if (word>='A'&&word<='Z') //如果是大写字母，统一转换为小写
        {
            word=word+32; //32是 'a' - 'A' ascii差
        }
        int i=word-96; // 96是'a'的ascii
        cou[i]++; // cou保存你对应字母的统计数字，+1
    }
    SourceFile.close(); //关闭文件
    cout<<"letter"<<'\t'<<'\t'<<"频率为"<<endl; //输出
        for (int j=1;j<=26;j++) //依次输出a~z字符的出现频率
        {
            if (cou[j]!=0) //如果没有这个字母出现，就不输出
            {
                cout<<letter[j]<<'\t'<<'\t'<<cou[j]<<endl; //letter[j]可以用 (char)('A' + i) 代替
            }
        }
        cout<<"读入完事儿！"<<endl;

return 0;
}