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IDL programming: control variables in IDL across routines

IDL, unlike C++, has its own approach and principle when handling variables across routines. These features are often very different with the variable scope within C++.
I will try to answer several key questions and prove them using simple demos.
Question 1: Will the value of a variable be changed after it’s passed into a function and changed inside?
Demo:
;;.....................................................................................
PRO variable_change
a=1.0
b=2.0
PRINT,'Before the operation, a is ',a
c=plus(a,b)
PRINT,'After the operation, a is ',a
END
FUNCTION plus,a,b
a=a+b
RETURN,a
END
And the IDL Console output:
Before the operation, a is 1.00000
After the operation, a is 3.00000
;;.....................................................................................
Conclusion:
If the value of a variable has been changed inside a routine, it will remains changed outside. Question 2: What if this variable is not even passed into any routine, but the routine has a variable which has the same name?
Demo:
;;.....................................................................................
FUNCTION plus, m, n
a= m+n
print, a
RETURN, a
END
PRO variable_test
a = 1
PRINT,a
b =2
c= 3
d= plus(b,c)
PRINT,d
PRINT,a
END
a is 1 a is 5 d is 5 a is 1
;;.....................................................................................
Conclusion: If it's not passed, it won't change even other routines have the same variable name.
Tips:
Whenever you have many variables which may be passed to other routines, try not to use the same variable name as much as possible. A simple solution is to get a copy of the variable before changing it.
Question 3: Then how to define a global variable?Answer: IDL provides common and Defsysv to handle global variables.
Demo:
;;.....................................................................................
PRO demo
COMMON exponent,m
m=2.0
result=QROMO('integration', 1.0, 2.0)
PRINT,'The integration result is', result
END
FUNCTION integration,x
;To integrate the expression: y=x^m+1
;However variable m is uncertain.
COMMON exponent,m
result=x^m+1
RETURN, result
END
And the IDL Console output:
The integration result is 3.33333
;;.....................................................................................
Discussion:
By reviewing the IDL Helper, as we could see, the COMMON defines a shared block to store the variables. And there are more than one way to refer the shared block. In the above demo, you can also simply use: COMMON exponent in the integration function without specify the m.
Besides, if a shared block is defined, it can’t be changed in number and type in the other functions. Also, the sequence of all variables could be exactly the same as you refer.
Tips:
Though the COMMON is a traditional way to share variables, it could also cause conflicts if not configured well.
In the Defsysv case:
The IDL provides system variable way to store global variables as well. The defsysv procedure  could create a new system variable with initial value if configured.
Demo:
;;.....................................................................................
PRO demo
DEFSYSV, '!m', 2.0
result=QROMO('integration', 1.0, 2.0)
PRINT,'The integration result is', result
END
FUNCTION integration,x
;To integrate the expression: y=x^m+1
;However variable m is uncertain.
result=x^!m+1
RETURN, result
END
The console output as:
The integration result is 3.33333
;;.....................................................................................
So it is the same with the common case. And the only thing I did was ‘DEFSYSV, '!m', 2.0  ’. It seems that this approach is somewhat easier than ‘common’ way.
However, once defined , the system variable can not be destroyed until the whole procedure ends.
What if the variable we want to configure is a vector?
The demo will indicate that the ‘defsysv’ won’t allow it. But ‘common’ could!
Of course, other approaches like uvalue and sav file could also provide similar function.
Especially the uvalue method is frequently used along with defsysv.
Hope it can help!
I will present a specific example to solve a complex nonlinear equation using the above method in other posts.

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