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  • 【217】◀▶ IDL 控制语句说明

    参考：Statements Routines —— 控制语句关键字

    01   FOR 循环语句。
    02   FOREACH 循环语句。
    03   WHILE...DO 循环语句。
    04   IF...THEN...ELSE 选择语句。
    05   CASE 只执行符合条件的一个语句块。 
    06   SWITCH
    		 执行符合条件后面的所有语句块。
    07   REPEAT...UNTIL 循环语句。 
    08   BEGIN...END 用于显示多条语句。
    09   CONTINUE  执行循环语句中的下一条循环。
    10   BREAK 从循环、CASE 或 SWITCH 语句中跳出。
    序号 名称  

    功能说明 & 语法 & 举例
    01 FOR  

    循环语句。
    -----------------------------------------------------------------------------------------------
     FOR variable = init, limit [, Increment] DO statement 
    -----------------------------------------------------------------------------------------------
     FOR variable = init, limit [, Increment] DO BEGIN 
        statements                                     
     ENDFOR                                           
    -----------------------------------------------------------------------------------------------
    ◈ init：开始的数
    ◈ limit：结束的数
    ◈ Increment：增量值
    ◈ statement(s)：循环语句内容
      
    IDL> FOR i = 1, 10, 4 DO PRINT, i
       1
       5
       9

  FOR i = 1, 10, 4 DO BEGIN
    PRINT, i
  ENDFOR
     02 FOREACH  

    循环语句。
    -----------------------------------------------------------------------------------------------
         FOREACH Element, Variable [, Index] DO Statement 
    -----------------------------------------------------------------------------------------------
     FOREACH Element, Variable [, Index] DO BEGIN 
        statements                                
     ENDFOREACH                  
    -----------------------------------------------------------------------------------------------
    ? Element：每一个元素
    ◈ Variable：数组变量
    ◈ Index：元素的索引值
    ◈ Statement(s)：循环语句内容
      
    IDL> strs = ["alex", "bn", "lee"]
IDL> FOREACH e, strs, i DO PRINT, i, "--", e
                     0--alex
                     1--bn
                     2--lee

  strs = ["alex", "bn", "lee"]
  FOREACH e, strs, i DO BEGIN
    PRINT, i, "--", e
  ENDFOREACH
    03 WHILE...DO  

      循环语句。
    -----------------------------------------------------------------------------------------------
     WHILE     expression DO statement 
    -----------------------------------------------------------------------------------------------
     WHILE     expression DO BEGIN 
        statements             
     ENDWHILE                  
    -----------------------------------------------------------------------------------------------
    ◈ expression：判断表达式
    ◈ statement(s)：循环语句
      
    IDL> i = 3
IDL> WHILE(i gt 0) DO PRINT, i--
       3
       2
       1

  i = 3
  WHILE(i gt 0) DO BEGIN
    PRINT, i--
  ENDWHILE
    04 IF...THEN...ELSE  

    选择语句
    -----------------------------------------------------------------------------------------------
     IF     expression THEN statement 
    -----------------------------------------------------------------------------------------------
     IF     expression THEN statement ELSE statement 
    -----------------------------------------------------------------------------------------------
     IF     expression THEN BEGIN 
       statements             
     ENDIF                    
    -----------------------------------------------------------------------------------------------
     IF     expression THEN BEGIN 
       statements             
     ENDIF ELSE BEGIN         
       statements             
     ENDELSE                  
    -----------------------------------------------------------------------------------------------
    ◈ expression：判断表达式
    ◈ statement(s)：语句内容
      
    IDL> A = 2
IDL> B = 3
IDL> IF(A EQ 2) AND (B EQ 3) THEN PRINT, 'A = ', A, STRING(13B), 'B = ', B
A =        2
B =        3

IDL> A = 2
IDL> IF(A EQ 2) THEN PRINT, 'A = ', A ELSE PRINT, 'A ≠ ', A
A =        2

  A = 2
  B = 3
  IF(A EQ 2) AND (B EQ 3) THEN BEGIN
    PRINT, 'A = ', A
    PRINT, 'B = ', B
  ENDIF

  A = 2
  IF(A EQ 2) THEN BEGIN
    PRINT, 'A = ', A
  ENDIF ELSE BEGIN
    PRINT, 'A ≠ ', A
  ENDELSE
    05 CASE  

    只执行符合条件的一个语句块。(多条语句需要使用 BEGIN...END）
    ------------------------------------------------------------
     CASE expression OF           
         expression: statement(s) 
         ...                      
         expression: statement(s) 
     ENDCASE                      
    ------------------------------------------------------------
     CASE         expression OF           
         expression: statement(s) 
         ...                      
         expression: statement(s) 
       ELSE: satement(s)          
     ENDCASE                      
    ------------------------------------------------------------
     CASE     expression OF           
         expression: statement(s) 
         ...                      
         expression: statement(s) 
       ELSE: BEGIN                
         satement(s)              
       END                        
     ENDCASE                      
    ------------------------------------------------------------
    ◈ expression：判断语句
    ◈ statement(s)：执行语句
      
      x=1
  CASE x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
  ENDCASE

IDL Prints:
one

  x=2
  CASE x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
     ELSE: PRINT, 'You entered: ', x
  ENDCASE

IDL Prints:
two

  x=3
  CASE x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
     ELSE: BEGIN
        PRINT, 'You entered: ', x
        PRINT, 'Please enter a value between 1 and 2'
     END
  ENDCASE

IDL Prints:
You entered: 3
Please enter a value between 1 and 2
    06
    		 SWITCH
    		  

    执行符合条件后面的所有语句块。       
    -----------------------------------------------------------------------------------------------
     SWITCH     expression OF         
         expression: statement(s) 
         ...                      
         expression: statement(s) 
     ENDSWITCH                    
    -----------------------------------------------------------------------------------------------
         SWITCH expression OF         
         expression: statement(s) 
         ...                      
         expression: statement(s) 
       ELSE: satement(s)          
     ENDSWITCH                    
    -----------------------------------------------------------------------------------------------
     SWITCH         expression         OF         
         expression: statement(s) 
         ...                      
         expression: statement(s) 
       ELSE: BEGIN                
         satement(s)              
       END                               
     ENDSWITCH                   
    -----------------------------------------------------------------------------------------------
    ◈ expression：判断语句
    ◈ statement(s)：执行语句
      
      x=1
  SWITCH x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
  ENDSWITCH

IDL Prints:
one
two

  x=2
  SWITCH x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
     ELSE: PRINT, 'You entered: ', x
  ENDSWITCH

IDL Prints:
two
You entered:        2

  x=2
  SWITCH x OF
     1: PRINT, 'one'
     2: PRINT, 'two'
     ELSE: BEGIN
        PRINT, 'You entered: ', x
        PRINT, 'Please enter a value between 1 and 2'
     END
  ENDSWITCH

IDL Prints:
two
You entered:        2
Please enter a value between 1 and 2
    07 REPEAT...UNTIL  

    循环语句（与 WHILE...DO 语句类似）
    -----------------------------------------------------------------------------------------------
    REPEAT statement UNTIL expression
    -----------------------------------------------------------------------------------------------
    REPEAT BEGIN
       statements
    ENDREP UNTIL expression
    -----------------------------------------------------------------------------------------------
    ◈ statement(s)：执行语句
    ◈ expression：判断语句
      
    IDL> i = 3
IDL> REPEAT PRINT, i-- UNTIL(i lt 0)
       3
       2
       1
       0

  i = 3
  REPEAT BEGIN
    PRINT, i--
  ENDREP UNTIL(i lt 0)
    08 BEGIN...END  

    用于显示多条语句。
    -----------------------------------------------------------------------------------------------
    BEGIN
       statements
    END | ENDIF | ENDELSE | ENDFOR | ENDFOREACH | ENDREP | ENDWHILE
    -----------------------------------------------------------------------------------------------
      
      CASE name OF
    'Moe': BEGIN
      PRINT, 'Stooge'
    END
  ENDCASE

  SWITCH name OF
    'Moe': BEGIN
      PRINT, 'Stooge'
    END
  ENDSWITCH
    09  CONTINUE  

    执行FOR、WHILE 或 REPEAT 循环语句中下一条循环。
      
      FOR I = 1,10 DO BEGIN
   ; If odd, start next iteration:
   IF (I AND 1) THEN CONTINUE
   PRINT, I
  ENDFOR

IDL Prints:
       2
       4
       6
       8
      10
    10  BREAK  

    从 FOR、WHILE、REPEAT 循环、CASE 或 SWITCH 语句中跳出。
      
      i = 0
  PRINT, 'Initial value: ', i
  WHILE (1) DO BEGIN
    i = i + 1
    IF (i eq 5) THEN BREAK
    PRINT, 'Loop value: ', i
  ENDWHILE
  PRINT, 'END VALUE: ', i

IDL Prints:
Initial value:        0
Loop value:        1
Loop value:        2
Loop value:        3
Loop value:        4
END VALUE:        5
           
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  • 原文地址：https://www.cnblogs.com/alex-bn-lee/p/5801627.html
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