Populating array from select query in iOS

cocoa-touch, ios, ios7, nsmutablearray, sqlite

Solution

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);

Problem

Original source