Var/reference parameters in C

Consider the problem of calculating statistics about a collection of data. Suppose the data is in an array and you want to calculate the mean and maximum values. These could be calculated by separate methods but it would be more efficient to calculate both with one pass through the data.

 void meanMax( double a[], int size, double mean, double max);
The problem here is that assignments to mean and max inside the method are to the copies and the originals in the calling method remain unchanged.

One solution would be to make the mean and max global variables declared outside of any function. This works very well for small programs but does not "scale up" to larger programs where different programmers work on various parts and tend to want to use the same global variables.

If only one variable needs to be set then the function/method can return that value.

Another solution is to pass pointers to the variables which need to be set and remember to allocate space for them in the calling method. Allocating space for strings is particularly an issue that C programmers need pay attention to.
 void meanMax( double a[], int size,
    double* ptrMean, double* ptrMax);
and inside the method set *ptrMean = and similarly for the other var/reference parameters.

Most C programmers first encounter this problem with the scanf input function.
 scanf("%f %f", &mean, &max);
It is very common to forget the ampersands.

Consider the problem of reading a name and a list of attributes associated with that name. If these both make sense to be part of one object (struct in C) then we can allocate that struct in the calling function and pass a pointer to it and the parts of the object are initialized in the function. An alternative would be to allocate space for the object in the function and return a pointer to it. This requires dynamic space allocation which is more expensive than "automatic" allocation in the caller.

Note that strings, arrays and lists in C are represented by pointers. Passing one of these as a var-parameter to a function to be set means that you pass a "pointer to a pointer" (** in C). One way to do this is to pass an array to the function:

  double stats[2];
  calcStats( stats );
  // now stats[0] is mean, and stats[1] is the max

Getting back to reading a name and a list of attributes.

    char name[SIZE];
    Node * head;
    getNameAndAttributes( &name, &head);

    char* name;
    Node * head;
    getNameAndAttributes( &name, &head);