【217】◀▶ IDL 控制语句说明

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

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

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

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

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

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

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

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)

BEGIN...END

CASE name OF
  'Moe': BEGIN
    PRINT, 'Stooge'
  END
ENDCASE
 
SWITCH name OF
  'Moe': BEGIN
    PRINT, 'Stooge'
  END
ENDSWITCH

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

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