如何將const char連接到const char？

Question

我需要建立其他const char的const char字符串嗎？

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

我已經嘗試過幾次，但是做不到，這是我的最后一次嘗試。 如您所見，我正在嘗試建立一個sql語句。

我要去哪里錯了？

編輯-我正在使用不喜歡NSString的sql lite，請參見下文。

- (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

轉換為UTF8字符串再轉換都沒有任何好處。 在NSString中執行所有操作，最后將其轉換為C樣式字符串，效率會更高。 @"d"語法還求值一個對象，而不是C樣式的字符串。

因此，您應該將代碼簡化並更正為：

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

將sqlBuild聲明為NSMutableString並使用它來構建您的字符串：

NSMutableString *sqlBuild = [NSMutableString string];
for (...) {
    [sqlBuild appendString:...];
}