我想将从数据库读取的数据写入二进制文件。在查询中,我需要读取大量数据,其中一些是group_concat
。现在我想把它们写成二进制文件。问题是如何定义它们的大小。起初我使用了put()
,但结果是错误的,因为put
需要char
类型而row[]
是char*
。然后我尝试了write
并按照sizeof(int)
*组中的元素数量定义了大小,但它仍然是错误的。请告诉我一个方法。
MYSQL_RES* res = GetDBManager()->Query("select ls.time, count(ws.id) , group_concat(wc.id) , group_concat(ws.SIGNAL_STRENGTH) , ul.LONGITUDE , ul.LATITUDE , ul.ALTITUDE from user_location_scan ls, wifi_scan ws, wifi_cell wc, user_location ul where ls.id = ws.user_scan and ws.wifi_cell = wc.id and ls.time = ul.time group by ls.id order by ls.id");
if (res == NULL) return false;
MYSQL_ROW row;
ofstream myFile("data.bin", ios::out | ios::binary | ios::app);
while (row = mysql_fetch_row(res)) {
myFile.write(row[0], sizeof(char));
myFile.write(row[1], sizeof(int));
myFile.write(row[2], sizeof(int)*atoi(row[1]));
myFile.write(row[3], sizeof(int)*atoi(row[1]));
myFile.write(row[4], sizeof(double));
myFile.write(row[5], sizeof(double));
myFile.write(row[6], sizeof(double));
//myFile.write(row[0], sizeof(row[0]));
//myFile.write(row[1], sizeof(row[1]));
//myFile.write(row[2], sizeof(row[2]));
//myFile.write(row[3], sizeof(row[3]));
//myFile.write(row[4], sizeof(row[4]));
//myFile.write(row[5], sizeof(row[5]));
//myFile.write(row[6], sizeof(row[6]));
//myFile.put((char)row[0]);
//myFile.put((char)row[1]);
//myFile.put((char)row[2]);
//myFile.put((char)row[3]);
//myFile.put((char)row[4]);
//myFile.put((char)row[5]);
//myFile.put((char)row[6]);
//myFile << row[0] << row[1] << row[2] << row[3] << row[4] << row[5] << row[6];
}
mysql_free_result(res);
答案 0 :(得分:1)
MySql函数mysql_fetch_row()
返回一个以null结尾的字符串数组(字符数组)。您可以使用std::strlen()
来获取这些以null结尾的字符数组的长度。
如果你想把所有东西都写成文件中的字符串,那么做这样的事就足够了:
while((row = mysql_fetch_row(res)))
for(unsigned col = 0; col < 7; ++col)
myFile.write(row[col], std::strlen(row[col]) + 1);
如果你想把数字列写为二进制数(更难以便携),那么你需要转换字符串并以不同的方式写出值。
更少推荐以二进制形式写出每条记录。这非常不便携。
struct record
{
// choose better column names
char col_0[32]; // largest possible string
int col_1;
int col_2;
int col_3;
double col_4;
double col_5;
double col_6;
};
while((row = mysql_fetch_row(res)))
{
record r; // store our binary values here
//copy the string
std::strncpy(r.col_0, row[0], sizeof(r.col_0));
r.col_1 = std::atoi(row[1]); // convert the integers
r.col_2 = std::atoi(row[2]);
r.col_3 = std::atoi(row[3]);
r.col_4 = std::atof(row[4]); // convert the floats
r.col_5 = std::atof(row[5]);
r.col_6 = std::atof(row[6]);
// write the whole object out in binary (very non-portable)
myFile.write(reinterpret_cast<char*>(&r), sizeof(r));
}
再次阅读记录:
// reading back
record r; // store our binary values here
if(myFile.read(reinterpret_cast<char*>(&r), sizeof(r)))
{
// success, use the record here
std::cout << r.col_0 << '\n';
std::cout << r.col_1 << '\n';
std::cout << r.col_2 << '\n';
// ... etc
}
注意:使用二进制文件更复杂,更难以正确使用,不可移植,除非是绝对要求,否则通常要避免使用。