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; MODEL5.PRO           ; The name of this program, should I forget it.
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PRO MYFIRSTSUBROUTINE,T,F,V         ; INPUT: T and F, OUTPUT: V      ;
V=0.D0*F                            ; v(f) and f have the same length;
FOR A=0,N_ELEMENTS(F)-1 DO BEGIN    ; Loop calculating v(f) begins 
V(A)=TOTAL(COS(2.D0*!PI*F(A)*T))    ;
ENDFOR & PRINT,V                    ; Loop -"-              ends
ABC='I do not exist in main program'; Not INPUT or OUTPUT !
RETURN & END                        ; Subroutine ends
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;                                   ; MAIN PROGRAM BEGINS
X=2.D0*!PI*FINDGEN(50)              ; Input T and F names irrelevant!
Y=11.+FINDGEN(10)                   ; Output V name irrelevant!
MYFIRSTSUBROUTINE,X,Y,Z             ; 1st use of subroutine
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A=COS(Z*7.D0) & B=SIN(X/90.D0)      ;  
MYFIRSTSUBROUTINE,A,B,C             ; 2nd use of subroutine
PRINT,ABC                           ; Here the program fails. Why?
END
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