Lisp for the C++ programmer: Function definitions and function calls

Here is the simple Common Lisp example, in which a sum and an ArithmaticMean functions defined. Both C++ and Common Lisp code here calculate the arithmetic mean of 1,2,3,4,5 and 6 which is 3.5. C++ code is commented as in the Common Lisp file.

 ; double sum (double *d, int len){  
 ;    double mysum = 0.0;  
 ;    for (int i = 0; i < len; i++){  
 ;        mysum += d[i];  
 ;    }  
 ;    return(mysum);  
 ; }  
 ;  
 ; double ArithmeticMean (double *d, int len){  
 ;    return ( sum(d) / len );  
 ; }  
   
 ; int main(){  
 ;    double *mylist = (double*) malloc(sizeof(double) * 6);  
 ;    for (int i = 0; i < 6; i++){  
 ;        d[i] = (double)i;  
 ;    }  
 ;    printf("%f\n", ArithmeticMean(d, 6);  
 ;    return(0); 
 ; }  
   
   
   
 (defun sum (aList)  
     (setq mysum 0.0)  
     (dotimes (i (length aList))  
         (setq mysum (+ mysum (nth i aList )))  
     )  
     mysum  
 )  
   
 (defun ArithmeticMean (aList)  
     (/ (sum aList) (length aList))  
 )  
   
 (print (ArithmeticMean '(1 2 3 4 5 6)))