特殊值 !Values

NAN and Infinity value

IDL> help,!Values,/str
** Structure !VALUES, 4 tags, length=24, data length=24:
  F_INFINITY      FLOAT               Inf
  F_NAN           FLOAT               NaN
  D_INFINITY      DOUBLE            Infinity
  D_NAN           DOUBLE                 NaN
Inf还可取负
IDL> print,-1*!Values.F_INFINITY
        -Inf
问题：如何给一个整数变量赋一个NAN 或者 INFINITY

F_INFINITY可以和浮点数做比较 F_NAN不能

B = [1.0, 2.0, !VALUES.F_NAN]
IDL> PRINT, WHERE(B EQ !VALUES.F_NAN
         -1

在求平均等计算时，设置/NAN开关，可将这些数忽略

IDL> print,mean(B)
         NaN
IDL> print,mean(B,/NAN)
     1.50000