IDL中的系统变量

• !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

• !EXCEPT

数学错误

  !EXCEPT=0 不报错.
  !EXCEPT=1 （默认值，在程序结束后回到交互控制台报错）
  !EXCEPT=2 （在出错的地方报错）

在涉及到NaN的数据运算时，容易出现数学错误，比如

   A = [1.0, 2.0, !VALUES.F_NAN]
   PRINT, WHERE(A GT 1.0)
   IDL prints:
     1
   % Program caused arithmetic error: Floating illegal operand

该问题可利用FINITE函数加以解决。如

PRINT, WHERE(FINITE(A) EQ 1)

在程序中如果设置！EXCEPT＝0，可以用CHECK_MATH加以监控

PRINT, CHECK_MATH() 
     0     
     No errors detected since the last interactive prompt or call to CHECK_MATH
     1     
     Integer divided by zero
     2     
     Integer overflow
     16     
     Floating-point divided by zero
     32     
     Floating-point underflow
     64     
     Floating-point overflow
     128     
     Floating-point operand error. An illegal operand was encountered, such as a negative operand to the SQRT or ALOG functions, or an attempt to convert to integer a number whose absolute value is greater than 231 - 1