How do I concatenate const char to const char? - objective-c

Question

I need to build up a const char string of other const char's ?
const char *sql = "";
const char *sqlBuild = "";
for(int i=0; i < ac_count; ++i) {
if (![sqlBuild isEqualToString:#""]) {
sqlBuild = [sqlBuild stringByAppendingString:
[NSString stringWithUTF8String:#" UNION "]];
}
sql = [[NSString stringWithFormat:
#"select sum(price) from tmp%d where due >= date() and due <= '%#'",
i, strDBDate] cStringUsingEncoding:NSUTF8StringEncoding];
sqlBuild = [sqlBuild stringByAppendingString:
[NSString stringWithUTF8String:sql]];
}
//execute sql
I've had several attempts but can't get it quite right, heres my last attempt. As you can see i'm trying to build up an sql statement.
Where am I going wrong ?
EDIT - I'm using sql lite which doesn't like NSString, see below.
- (NSString*)getCategoryDesc:(int)pintCid {
NSString *ret = #"";
const char *sql = "select category from categories where cid = ?";
sqlite3 *database;
int result = sqlite3_open([[General getDBPath] UTF8String], &database);
if(result != SQLITE_OK)
{
DLog(#"Could not open db.");
}
sqlite3_stmt *statementTMP;
int error_code = sqlite3_prepare_v2(database, sql, -1, &statementTMP, NULL);
if(error_code == SQLITE_OK) {
sqlite3_bind_int(statementTMP, 1, pintCid);
if (sqlite3_step(statementTMP) == SQLITE_ROW) {
ret = [[NSString alloc] initWithUTF8String:
(char *)sqlite3_column_text(statementTMP, 1)];
}
}
sqlite3_finalize(statementTMP);
sqlite3_close(database);
return [ret autorelease];
}

Answer 1

There's no benefit to the conversion to and then from a UTF8 string; it'd be much more efficient to do everything in NSString and convert it to a C-style string at the end. The #"d" syntax also evaluates to an object, not a C-style string.
So you should simplify and correct your code to:
NSMutableString *sqlStatement = [NSMutableString string];
for(int i=0; i < ac_count; ++i) {
if ([sqlStatement length]) [sqlStatement appendString:#" UNION "];
[sqlStatement appendFormat:
#"select sum(price) from tmp%d where due >= date() and due <= '%#'",
i, strDBDate];
}
// execute SQL string [sqlStatement UTF8String]

Answer 2

Declare sqlBuild to be NSMutableString and use that to build up your string:
NSMutableString *sqlBuild = [NSMutableString string];
for (...) {
[sqlBuild appendString:...];
}