GetField的頻率可否用變數

請問GetField的"頻率"可否用儲存有字串的變數？例如以下是一個函數程式，但編譯無法辨識Frequency，表示其非字串：

SetBarMode(1);

input:

Frequency(string);

Length(string);

Var:HV(0), LV(0), _Index(0);

HV = GetField("High", Frequency)[1];

LV = GetField("Low", Frequency)[1];

If Length>1 then

Begin

  For _Index = 2 to (Length + 1)

  Begin

    If HV <= GetField("High", Frequency)[_Index] then HV = Frequency[_Index];

    If LV >= GetField("Low", Frequency)[_Index] then LV = Frequency[_Index];

  End;

End;

If GetField("Close", Frequency) > HV then Tower = 1

Else if GetField("Close", Frequency) < LV then Tower = -1;