Populating array from select query in iOS - ios

Question

So I'm fairly new to iOS development and I'm having problems with my select function. I made a function that should take in a select query and the table name and return an array of results where each array entry is a dictionary with a row of results. Somehow my query for column names is deleting my columnNames variable and returning crazy results. I'm just trying to figure out an easy way to store, access, manipulate query results
Here is the function that converts results into array:
-(NSMutableArray *)selectQuery:(NSString*)query
table:(NSString*)table
{
NSMutableArray *returnArray = [NSMutableArray new];
if(sqlite3_open([databasePath UTF8String], &database) == SQLITE_OK)
{
NSMutableArray *columnNames;
NSString *tableQuery = [NSString stringWithFormat:#"PRAGMA table_info('%#')", table];
if (sqlite3_prepare_v2(database, [tableQuery UTF8String], -1, &statement, nil) == SQLITE_OK)
{
while (sqlite3_step(statement) == SQLITE_ROW)
{
[columnNames addObject:[NSString stringWithUTF8String:(const char*)sqlite3_column_text(statement, 1)]];
}
}
else
{
NSLog(#"Error preparing table query:");
NSLog(tableQuery);
}
if (sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, nil) == SQLITE_OK)
{
while(sqlite3_step(statement)==SQLITE_ROW)
{
NSMutableDictionary *temp= [NSMutableDictionary new];
for (int i = 0; i < [columnNames count]; i++)
{
[temp setObject:[NSString stringWithUTF8String:(const char*)sqlite3_column_text(statement, i)] forKey:columnNames[i]];
}
if (temp != nil)
{
[returnArray addObject:temp];
temp = nil;
}
}
sqlite3_reset(statement);
sqlite3_close(database);
}
else
{
NSLog(#"Error preparing select statement with query:");
NSLog(query);
}
}
else
{
NSLog(#"Could not open database");
}
return returnArray;
}
and heres the call to it
NSMutableArray *queryResults = [dbInstance selectQuery:[NSString stringWithFormat:#"SELECT gallons, mileage FROM fillups WHERE carId = \"%d\" ORDER BY date asc",
carId]
table:#"fillups"];

Answer 1

You are never instantiating columnNames. Thus, your attempt to add column names to that array will not succeed. To remedy this, when you declare it, you want to instantiate the mutable array object, too:
NSMutableArray *columnNames = [NSMutableArray array];
Unrelated to this problem, when you're done retrieving the column names, before you prepare your second statement, don't forget to release the memory associated with the first prepared statement:
sqlite3_finalize(statement);
Finally, when you're done retrieving the second prepared SQL statement, rather than calling sqlite3_reset, you want to call sqlite3_finalize again for that second prepared statement. The sqlite3_reset is used to reset a statement when you want to bind new values to ? placeholders in a statement, which is not applicable here, so no sqlite3_reset is needed. But if you don't call sqlite3_finalize, you're not releasing the memory associated with the prepared statement.
By the way, if you wanted to dynamically retrieve the column names and column types (without having to do PRAGMA table_info), you could do something like:
int rc;
if ((rc = sqlite3_prepare_v2(database, [query UTF8String], -1, &statement, NULL)) != SQLITE_OK) {
NSLog(#"select failed %d: %s", rc, sqlite3_errmsg(database));
}
NSMutableArray *returnArray = [NSMutableArray array];
NSInteger columnCount = sqlite3_column_count(statement);
id value;
while ((rc = sqlite3_step(statement)) == SQLITE_ROW) {
NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
for (NSInteger i = 0; i < columnCount; i++) {
NSString *columnName = [NSString stringWithUTF8String:sqlite3_column_name(statement, i)];
switch (sqlite3_column_type(statement, i)) {
case SQLITE_NULL:
value = [NSNull null];
break;
case SQLITE_TEXT:
value = [NSString stringWithUTF8String:(const char *)sqlite3_column_text(statement, i)];
break;
case SQLITE_INTEGER:
value = #(sqlite3_column_int64(statement, i));
break;
case SQLITE_FLOAT:
value = #(sqlite3_column_double(statement, i));
break;
case SQLITE_BLOB:
{
NSInteger length = sqlite3_column_bytes(statement, i);
const void *bytes = sqlite3_column_blob(statement, i);
value = [NSData dataWithBytes:bytes length:length];
break;
}
default:
NSLog(#"unknown column type");
value = [NSNull null];
break;
}
dictionary[columnName] = value;
}
[returnArray addObject:dictionary];
}
if (rc != SQLITE_DONE) {
NSLog(#"error returning results %d %s", rc, sqlite3_errmsg(database));
}
sqlite3_finalize(statement);