[英]Populating array from select query in iOS
所以我對iOS開發相當新,我的select函數出現問題。 我創建了一個函數,它應該接受一個select查詢和表名並返回一個結果數組,其中每個數組條目都是一個帶有一行結果的字典。 不知怎的,我對列名的查詢是刪除我的columnNames
變量並返回瘋狂的結果。 我只是想找出一種存儲,訪問,操作查詢結果的簡單方法
這是將結果轉換為數組的函數:
-(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;
}
並且繼續呼吁它
NSMutableArray *queryResults = [dbInstance selectQuery:[NSString stringWithFormat:@"SELECT gallons, mileage FROM fillups WHERE carId = \"%d\" ORDER BY date asc",
carId]
table:@"fillups"];
您永遠不會實例化columnNames
。 因此,您嘗試將列名添加到該數組將不會成功。 為了解決這個問題,當你聲明它時,你也希望實例化可變數組對象:
NSMutableArray *columnNames = [NSMutableArray array];
與此問題無關,當您完成檢索列名稱時,在准備第二個語句之前,請不要忘記釋放與第一個預准備語句關聯的內存:
sqlite3_finalize(statement);
最后,當您完成檢索第二個准備好的SQL語句,而不是調用sqlite3_reset
,您希望再次為第二個sqlite3_finalize
准備語句調用sqlite3_finalize
。 當您想要將新值綁定到時, sqlite3_reset
用於重置語句?
語句中的占位符在此處不適用,因此不需要sqlite3_reset
。 但是,如果不調用sqlite3_finalize
,則不會釋放與sqlite3_finalize
准備語句關聯的內存。
順便說一下,如果你想動態檢索列名和列類型(不必做PRAGMA table_info
),你可以這樣做:
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);
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