如何取得前一日"十大法人未沖銷買賣口"數

//程式碼 Begin

value1 = GetSymbolField("Future*1","十大法人未沖銷買口","D");
value2 = GetSymbolField("Future*1","十大法人未沖銷賣口","D");
value3 = value1 - value2; //當日
value4 = value1[1] - value2[1]; //前日 執行為0
print("當日:",value3);
print("前日:",value4);

//End

執行時Value4 的值是0 , 請問為什麼value1[1] 會是 0  ?

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