NCL有一个小问题不知道怎么回事

空城不空

我刚学NCL，然后看到个例2 有一句 asciiwrite(ascii_file,temp(7:3:2,0:4))  为什么是temp(7:3:2,0:4)呢，按照数组下标的规则不是应该为temp(7:3:-2,0:4)吗

xiaocaoqiqiao

可能是个例写错了

xuebiz

能把例子贴全么，或者给个链接

空城不空

xuebiz 发表于 2016-9-12 16:09
能把例子贴全么，或者给个链接

http://www.ncl.ucar.edu/Document/Manuals/Getting_Started/Examples/gsun02n.shtml
94行~

xuebiz

我没用过ascii文件的功能，所以具体不是很清楚。但是这些例子都有详细注解，而且运行过没有报错。
等候ascii达人解释下~~
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这个网页下面有注释，可以仔细看下。部分原文如下：
Lines 92-94:

  ascii_file = "data.asc"
  system("/bin/rm -f " + ascii_file)
  asciiwrite(ascii_file,temp(7:3:2,0:4))

Write a subset of the temperature data to an ASCII file using asciiwrite. The first argument of asciiwrite is the file name to which the data will be written, and the second argument is the data to write. The syntax "7:3:2" selects indices 7 to 3 with a stride of 2, so only indices "7", "5", and "3" are selected for the first dimension (in that order).

The system call takes a string as an argument and executes a UNIX system command from NCL. In this case, it is being used to remove the file before writing to it. The "+" symbol, in addition to being used as an arithmetic operator, is also used in NCL to concatenate strings.

For more information on asciiwrite, system, and other NCL functions and procedures, see the list of built-in NCL functions and procedures.
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这个例子我运行了下，没有错误。运行结果如下：
#ncl gsun02n.ncl

Copyright (C) 1995-2014 - All Rights Reserved
University Corporation for Atmospheric Research
NCAR Command Language Version 6.2.0
The use of this software is governed by a License Agreement.
See http://www.ncl.ucar.edu/ for more details.


Variable: temp (subsection)
Type: float
Total Size: 48 bytes
            12 values
Number of Dimensions: 2
Dimensions and sizes:   [lat | 4] x [lon | 3]
Coordinates:
            lat: [22.5..26.25]
            lon: [-122.5..-117.5]
Number Of Attributes: 6
  _FillValue :  -9999
  valid_range : (  0, 400 )
  units :       (C)
  long_name :   temperature
  frtime :      0
  level :       1000
(0,0)   -30.31982
(0,1)   -30.31982
(0,2)   -30.31982
(1,0)   -31.31982
(1,1)   -31.81982
(1,2)   -31.81982
(2,0)   -31.31982
(2,1)   -31.81982
(2,2)   -31.81982
(3,0)   -28.31982
(3,1)   -28.81982
(3,2)   -28.81982
(0)     lat
(0)     lon
(0)     temperature
(0)     (C)


Variable: lat (coordinate)
Type: float
Total Size: 132 bytes
            33 values
Number of Dimensions: 1
Dimensions and sizes:   [lat | 33]
Coordinates:
Number Of Attributes: 2
  long_name :   latitude
  units :       degrees_north
(0)     20
(1)     21.25
(2)     22.5
(3)     23.75
(4)     25
(5)     26.25
(6)     27.5
(7)     28.75
(8)     30
(9)     31.25
(10)    32.5
(11)    33.75
(12)    35
(13)    36.25
(14)    37.5
(15)    38.75
(16)    40
(17)    41.25
(18)    42.5
(19)    43.75
(20)    45
(21)    46.25
(22)    47.5
(23)    48.75
(24)    50
(25)    51.25
(26)    52.5
(27)    53.75
(28)    55
(29)    56.25
(30)    57.5
(31)    58.75
(32)    60


Variable: lon (coordinate)
Type: float
Total Size: 144 bytes
            36 values
Number of Dimensions: 1
Dimensions and sizes:   [lon | 36]
Coordinates:
Number Of Attributes: 2
  long_name :   longitude
  units :       degrees_east
(0)     -140
(1)     -137.5
(2)     -135
(3)     -132.5
(4)     -130
(5)     -127.5
(6)     -125
(7)     -122.5
(8)     -120
(9)     -117.5
(10)    -115
(11)    -112.5
(12)    -110
(13)    -107.5
(14)    -105
(15)    -102.5
(16)    -100
(17)    -97.5
(18)    -95
(19)    -92.5
(20)    -90
(21)    -87.5
(22)    -85
(23)    -82.5
(24)    -80
(25)    -77.5
(26)    -75
(27)    -72.5
(28)    -70
(29)    -67.5
(30)    -65
(31)    -62.5
(32)    -60
(33)    -57.5
(34)    -55
(35)    -52.5

xuebiz

The syntax "7:3:2" selects indices 7 to 3 with a stride of 2, so only indices "7", "5", and "3" are selected for the first dimension (in that order).

我理解的是，7到3，幅度2，系统会根据给出的数值自动加2或者减2

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我把例子中的2，改成-2，运行也不报错，运行结果跟之前一样一样的。。。
SO，2，-2 都可以的吧