IDL遥感应用入门（1）：基本语法练习

初步接触了下IDL在遥感方面的应用，准备写个系列小教程，重点集中在IDL海洋遥感方面。

这个小系列教程是示例化的，不是传统的IDL深入浅出，每个示例用到的数据都提供下载地址，文章写的很仓促，我的水平亦是有限，如有错误欢迎指出，如有建议欢迎提出。

言归正传，本节的主要目的是IDL的语法，整个程序可以正常运行，下面是代码

;尹全超-程序1
;2013-09-29

PRO yinquanchao_1

  ;-----------------------------------------------------
  ;                 PRACTICE WITH SCALARS
  ;-----------------------------------------------------
    
  x=99
  y=100
  print ,"x+y=",(x+y)
  print ,"x-y=",(x-y)
  
  print ,'USE & character'
  x=99&y=100& print ,'x+y=',(x+y)&  print ,'x-y=',(x-y)
  
  ;the result of adding 3 times X to 2 times Y
  
  print ,'3x+2y=',(3*x+2*y)
  
  ;the product of (X-Y) and (X+Y)
  
  print ,'(x+y)*(x-y)=',(x+y)*(x-y)
  
  ;the common logarithm of the absolute value of X+Y
  
  print ,ALOG10(ABS(x+y)) 
  
  ;difference between X = 5/2 and Y = 5./2 
  
  print , 'x=5/2' , (5/2)    ;输出结果是2.       整数
  print , 'x=5./2' , (5./2)  ;输出结果是2.50000  浮点数
  
  ;difference between  X = FLOAT(5/2) and Y = FLOAT(5)/2 
  
  print , 'X = FLOAT(5/2)' , FLOAT(5/2) ;把结果转换成一个浮点数
  print , 'Y = FLOAT(5)/2' , FLOAT(5)/2 ;先把5转换成浮点数，然后进行运算
  
  ;define a string variable for each word in the sentence: 
  ;"This is a concatenation"; then concatenate them and print the result
  
  str1='This' & str2='is' & str3='a' & str4='concatenation'
  print , str1 + ' ' + str2 + ' ' + str3 + ' ' + str4
  
  
  ;-----------------------------------------------------
  ;                 PRACTICE WITH VECTORS
  ;-----------------------------------------------------
  
  x = INDGEN(100)
  
  ;How many elements does X contain
  
  print , 'The elements of X is' , N_ELEMENTS(x)
  
  ;What is the minimum value in X
  
  print , 'The minimum value in X is ' , MIN(x)
  
  ;What is the maximum value in X
  
  print , 'The maximum value in X is ' , MAX(x)
  
  ;What is the sum of all the elements in X
  
  print , 'The sum of all the elements in X is' , TOTAL(x)
  
  ;Print X to your terminal window
  
  print ,x
  
  ;How are values in X related to the corresponding subscripts?
  
  ;->Answer: x[0],x[1]......x[99] => 从0到99，100个元素
  
  ;Is X a floating-point array?
  
  ;->Answer: Of course not
  
  ;Print the fifteenth entry in X to your terminal
  
  print , 'The fifteenth entry in X is' , x[14]
  
  ;print the entry containing the number 15
  
  print , 'the entry containing the number 15' , x[15]
  
  ;Is X a row-vector or a column-vector?
  
  ;->Answer: row-vector
  
  ;PRINT,TRANSPOSE(X) 转换列向量
  
  print ,TRANSPOSE(x)
  
  ;Try the following one-line command and inspect its output:
  
  FOR I=0,99 DO PRINT,I,X(I)
  FOR I=0,99 DO PRINT,I,X[I]  ;测试发现()和[]都可以
  
  ; Using the WHERE function:
  ; Define Q=2*X, then type FIND = WHERE(Q LE 40, COUNT)
  ; Examine the contents of FIND and COUNT so that you understand how the WHERE function operates.
  
  Q=2*X                            ; 复制了一个数组，每个值都*2
  FIND = WHERE(Q LE 40, COUNT)    ; 遍历数组Q,把遍历结果赋值给FIND变量，同时用COUNT变量记录符合条件都个数
  print , 'FIND=' , FIND
  print , 'COUNT=' , COUNT
  
  ;PRINT,Q(FIND)
  
  PRINT , 'Q(FIND):' , Q(FIND)
  
  ;Print to your terminal the values of the vector X/10
  
  print , 'The values of the vector X/10 ' , x/10     ;结果说明是一个整形数组
  
  ;print the values of X/10.0 and compare the results
  
  print , 'The values of X/10.0 ' , x/10.0            ;一个浮点型数组
  
  ;Do the same for FLOAT(X)/10  Do the same for FIX(X/10.0)
  
  print , 'FLOAT(X)/10' , FLOAT(X)/10
  print , 'FIX(X/10.0)' , FIX(X/10.0)                ;表示把小数截去小数部分，只保留整数部分
  
  ;What is the difference between Z = X*0.0 and Z = 0? 
  
  print , 'Z = X*0.0 is ' , (x*0.0) 
  print , 'Z = 0 is ' , 0                             ;一个是数组，一个是整数
  
  ;Print the 11 elements centered on X = 10  
  ;First do this using a FOR loop (on a single line)
  ;Then do this using the standard IDL subscript range notation, e.g. X(2:6).  No FOR loop is needed.
  
  for i=x[5],x[15],1 do print , 'For Loop =>' , i
  
  print , 'IDL subscript => ' , X(5:15)
  
  ;Compare to the following: K=5 & PRINT,X(10-K:10+K)
  
  K=5 & PRINT,X(10-K:10+K)
  
  ;Define Y to be the subarray consisting of those 11 elements, using subscript range notation; print Y to your window as a check.
  
  k=5
  y=x(10-k:10+k)
  print ,' Check y => ', y
  
  ;Verify lengths of X,Y using the N_ELEMENTS utility
  
  print , 'The lengths of x =>' , N_ELEMENTS(x)
  print , 'The lengths of y =>' , N_ELEMENTS(y)
  
  ;Use the SIZE utility to find the sizes of X and Y; what other info does it supply?
  
  print , 'The sizes of x =>' , SIZE(x) ;1 100 2 100 表示一维   一维上有100个元素   整型  100个元素
  print , 'The sizes of y =>' , SIZE(y) 
  
  ;What information does the command HELP,X,Y provide?
  ;->Answer  输出 x y 的基本信息
  
  HELP , x , y
  
  ;Define Q = Y+3 --  Note the values that Q contains
  ;Define Q = Y*3 --  Note the values that Q contains
  ;Define Q = Y^3 --  Note the values that Q contains
  ;Define Q = Y^4 --  Note the values that Q contains  why are they not monotonic?
  
  Q = Y + 3 & print , 'Y+3 =>' , Q
  Q = Y - 3 & print , 'Y-3 =>' , Q
  Q = Y * 3 & print , 'Y*3 =>' , Q
  Q = Y ^ 4 & print , 'Y^4 =>' , Q            ;int型最大到32767,产生溢出
  Q = (long(Y) ^ 4) & print , 'Y^4 =>' , Q    ;正确写法
  
  ;Define Q = FLOAT(Y)^4 --  Note the change.
  
  Q = FLOAT(Y)^4 & print , 'FLOAT(Y)^4 =>' , Q ;浮点数范围正负10^38
  
  ;Print the vector which results from Q = X*Y
  
  Q = x * y
  print , 'Q = x * y =>' , Q

  ;What did IDL actually do to arrive at this result?
  ;->Answer  对x数组进行了截取
  
  ;Using the built-in functions TOTAL and N_ELEMENTS:
  ;Find and print the mean values of X and Y; 
  
  print , 'The mean values of X is' , TOTAL(x)/N_ELEMENTS(x)
  print , 'The mean values of Y is' , TOTAL(y)/N_ELEMENTS(y)
  
  ;Find and print the variances of the two arrays
  
  print , 'The variances of X is' , TOTAL((x-(TOTAL(x)/N_ELEMENTS(x)))^2)/(N_ELEMENTS(x)-1)
  print , 'The variances of Y is' , TOTAL((y-(TOTAL(y)/N_ELEMENTS(y)))^2)/(N_ELEMENTS(y)-1)
  
  ;Do the same using the built-in functions MEAN and VARIANCE.
  
  print , 'The mean values of X is' , MEAN(x)
  print , 'The mean values of Y is' , MEAN(y)
  
  print , 'The variances of X is' , VARIANCE(x)
  print , 'The variances of Y is' , VARIANCE(y)
  
  print , 'Test Moment x' , MOMENT(x)
  print , 'Test Moment y' , MOMENT(y)    ;计算平均值、方差、斜度、峰度
  
  ;write Q = X & Q[2] = Y.  What will happen if you write Q[98] = Y?  Try it.

  Q = X 
  print , 'Q = x :' , Q
  Q[2] = Y
  print , 'Q[2] = y :' , Q 
  ;Q[98] = Y
  ;print , 'Q[98] = y :' , Q ;溢出报错
  
  ;Q = [Y,Y] Q = [Q,Y]
  
  Q = [Y,Y] 
  print , 'Q = [Y,Y]:' , Q 
  Q = [Q,Y]
  print , 'Q = [Q,Y]:' , Q
  
  ;Create a 16-element vector [1.01, 1.02, 1.03...] 
  ;using a simple one-line command employing FINDGEN.  Verify.
  
  x16 = (1 + FINDGEN(16))/100 + 1 
  print , 'x16=>' , x16
  
  ;Create a 16-element vector: [1,2,4,8,16....] using a simple
  ;one-line command employing FINDGEN.  Verify.
  
  y16 = 1 + FINDGEN(16)
  print , 'y16=>' , y16
  
  ;Create the 100-element vector Z = 10*X - 0.1*X^2, where 
  ;X contains the integers between 0 and 99.
  
  x = INDGEN(100)
  z = 10 * x - 0.1 * x ^ 2
  print , 'z=>' , z
  
  ;Use MAX to determine the maximum value of Z.
  
  print , 'The maximum value of Z' , MAX(Z)
  
  ;Use WHERE to locate the X value for which this maximum occurs.
  
  FIND = WHERE(Z EQ MAX(Z))
  print , ' The X value for which this maximum occurs is ' , FIND
  
  ;X was an integer vector but Z is not.  Why?
  ;->Answer z在进行赋值的时候做了自动转换
  
  ;Using the SHIFT function, shift the elements of the array you
  ;just created three entries to the left.  Verify that the
  ;maximum is now in the expected location.
  
  z = SHIFT(z,-3)
  print , ' The X value for which this maximum occurs is ' , WHERE(Z EQ MAX(Z))
  
  
END

这只是一个简单的介绍，用到的是很基本的函数，一些函数我在本站3S探秘版块也做过介绍，需要的请自行搜索，也可以留言。