请问这个函数调用(floattostr())毛病出在那里？

Question

x1,x2: string;x1:=floattostr(abs(-0.5*strtofloat(stingdrid1.cells[2,i-1])-0.5*strtofloat(stingdrid1.cells[3,i-1])));
x2:=floattostr(abs(-1*strtofloat(stingdrid1.cells[1,i])-strtofloat(stingdrid1.cells[3,i-1])+3));
运行提示：
"is not a valid pointer value"

Answer 1

你所操作的cells 中的某一個數值為空。"is not a valid floating pointer value"try
  x1 := ...;
  x2 := ...;
except
  ...
end;

Answer 2

x1,x2: string;x1:=floattostr(abs(-0.5*strtofloatDef(stingdrid1.cells[2,i-1],0)-0.5*strtofloatDef(stingdrid1.cells[3,i-1]),0));
x2:=floattostr(abs(-1*strtofloatDef(stingdrid1.cells[1,i],0)-strtofloatDef(stingdrid1.cells[3,i-1])+3),0);