“IDL中的系统变量”的版本间差异
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无编辑摘要 |
（以“*特殊值 !Values”替换内容） |
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* !Values |
*[[特殊值 !Values]] |
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:NAN and Infinity value |
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IDL> help,!Values,/str |
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** Structure !VALUES, 4 tags, length=24, data length=24: |
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F_INFINITY FLOAT Inf |
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F_NAN FLOAT NaN |
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D_INFINITY DOUBLE Infinity |
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D_NAN DOUBLE NaN |
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Inf还可取负 |
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IDL> print,-1*!Values.F_INFINITY |
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-Inf |
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问题：如何给一个整数变量赋一个NAN 或者 INFINITY |
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:F_INFINITY可以和浮点数做比较 F_NAN不能 |
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B = [1.0, 2.0, !VALUES.F_NAN] |
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IDL> PRINT, WHERE(B EQ !VALUES.F_NAN |
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-1 |
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:在求平均等计算时，设置/NAN开关，可将这些数忽略 |
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IDL> print,mean(B) |
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NaN |
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IDL> print,mean(B,/NAN) |
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1.50000 |
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*!EXCEPT |
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:数学错误 |
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!EXCEPT=0 不报错. |
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!EXCEPT=1 （默认值，在程序结束后回到交互控制台报错） |
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!EXCEPT=2 （在出错的地方报错） |
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在涉及到NaN的数据运算时，容易出现数学错误，比如 |
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A = [1.0, 2.0, !VALUES.F_NAN] |
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PRINT, WHERE(A GT 1.0) |
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IDL prints: |
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1 |
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% Program caused arithmetic error: Floating illegal operand |
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:该问题可利用FINITE函数加以解决。如 |
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PRINT, WHERE(FINITE(A) EQ 1) |
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在程序中如果设置！EXCEPT＝0，可以用CHECK_MATH加以监控 |
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PRINT, CHECK_MATH() |
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0 |
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No errors detected since the last interactive prompt or call to CHECK_MATH |
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1 |
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Integer divided by zero |
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2 |
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Integer overflow |
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16 |
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Floating-point divided by zero |
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32 |
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Floating-point underflow |
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64 |
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Floating-point overflow |
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128 |
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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 |
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