Can I use for loop insert SQL? - c++

Question

#include <iostream>
#include "libpq-fe.h"
using namespace std;
void CloseConn(PGconn *conn)
{
PQfinish(conn);
getchar();
}
PGconn *ConnectDB()
{
PGconn *conn = NULL;
// Make a connection to the database
conn = PQconnectdb("user=postgres password=password dbname=postgres hostaddr=192.168.xxx.xxx port=5432");
// Check to see that the backend connection was successfully made
if (PQstatus(conn) != CONNECTION_OK)
{
cout << "Connection to database failed.\n";
CloseConn(conn);
}
cout << "Connection to database - OK\n";
return conn;
}
void InsertEmployeeRec(PGconn *conn)
{
int nFields;
// Append the SQL statment
std::string sSQL;
// problem start-------------------------
for(int i=0;i<10;i++)
{
sSQL.append("INSERT INTO test(lat,lng) VALUES (i,20)");
}
//problem end----------------------------
// Execute with sql statement
PGresult *res = PQexec(conn, sSQL.c_str());
if (PQresultStatus(res) != PGRES_COMMAND_OK)
{
cout << "Insert test record failed\n";
PQclear(res);
CloseConn(conn);
}
cout << "Insert test record - OK";
// Clear result
PQclear(res);
}
int main(int argc, char *argv[ ])
{
PGconn *conn = NULL;
conn = ConnectDB();
if (conn != NULL)
{
InsertEmployeeRec(conn);
CloseConn(conn);
}
return 0;
}
and in terminal
alan#alan-virtual-machine:~/文件$ g++ ex6.cpp -I /usr/include/postgresql -l pq -o ex6
alan#alan-virtual-machine:~/文件$ ./ex6
Connection to database - OK
Insert test record failed
*** Error in `./ex6': corrupted double-linked list: 0x08eadad0 ***
But if I remove for loop and modify
INSERT INTO test(lat,lng) VALUES (i,20)
to
INSERT INTO test(lat,lng) VALUES (20,20)
it can work.
Linux Ubuntu 13.04 g++ compiler
I refer to http://www.askyb.com/cpp/c-postgresql-example/

Answer 1

The problem resides in the fact that 'i' is not been evaluated inside the string. To solve that, you can create the string with correct value and send it to the insert command.
for(int i=0;i<10;i++)
{
char buffer[256];
snprintf(buffer, 256, "INSERT INTO test(lat,lng) VALUES (%d,20)", i);
sSQL.append(buffer);
}
Of course, you need to adjust your buffer adequately

Answer 2

The problem is that you're not sending i. It looks like PQexecParams is the function you want:
PGresult *PQexecParams(PGconn *conn,
const char *command,
int nParams,
const Oid *paramTypes,
const char * const *paramValues,
const int *paramLengths,
const int *paramFormats,
int resultFormat);