[?]Nobody hasn't add column?

I'm first time in postgresql...
Um....
I made little program... with C and postgres on the web.
Everything is OK.
But I want to add a column.
So
.......
sprintf(rest, "alter table danga add (a%s float)",entries[0].val);
res = PQexec(conn, rest);
if (PQresultStatus(res) != PGRES_COMMAND_OK) {
fprintf(stderr,"command failed\n");
PQclear(res);
printf("###");
exit_nicely(conn);
}
PQclear(res);
.........

Excute this program print JUST

###

WHY?
Maybe Nobody hasn't create column.... I don't know...

My English write level is very low.
so my message reading thank you so much.