For a few years, I have been teaching a programming class to first-year electrical engineering students -- thus, this is their first contact with computer programming. The language used is C (although this year I started using Logo for the first few weeks as it presents a smaller barrier to entry for a gentler introduction to the practice of programming).
Although the students have issues with many topics of the course, many of them are with more complicated concepts such as recursions and data structures. However, one point that I feel should be easy, yet I'm somehow not getting across to them, is the importance of returning a value in a function.
Concretely, say I want to calculate the norm of a two-dimensional vector, and break it down into the smallest possible functions. So rather than writing something like this:
double norm(double x, double y)
{
return sqrt(x*x + y*y);
}
I might write something like this:
double add(double x, double y)
{
return x + y;
}
double square(double x)
{
return x*x;
}
double norm(double x, double y)
{
return sqrt(add(square(x),square(y)));
}
Although my students have no problem understanding this breakdown of functions, if I left it for them to write the code, they would probably write something along the following lines, which obviously wouldn't work, and they would be unable to figure out why:
double add(double x, double y)
{
double sum = x + y;
printf("the sum of x and y is %lf\n", sum);
}
double square(double x)
{
double sq = x*x;
printf("the square of x is %lf\n", sq);
}
double norm(double x, double y)
{
double result = sqrt(add(square(x),square(y)));
printf("the norm of (x,y) is %lf\n", result);
}
Note: this isn't actual code that any of my students have written. This is just how I imagine they'd write it if I just sticked to their suggestions during the coding session.
What kind of analogies, objections, examples, counterexamples or strategies could I try to get them to understand that, by printing from a function, the value is not "routed" to the rest of the program, and thus the remainder of the computation will have no access to it?
Extra details
I try to introduce the concept of a CS function by analogy with a mathematical function, say $f(x) = x^2$ (I understand there is the issue of side effects in a CS function, which don't exist in a mathematical function, but I do try to mention this point). I show them how a parameter is passed (in this case $x$) and an operation is performed on it. Passing a parameter is a concept that they grasp easily enough, however most of my students appear somehow unable to understand the seemingly symmetrical issue of returning a value from that function.
Usually, by the point I start talking about functions, I have already introduced the printf() function and shown how it can print the value of variables to the screen.
Then, whenever I propose a problem which uses a function and try to engage the class to help me write code for it, they always get stuck when it comes the point of returning the value from the function. They do understand the importance of getting the value out of the function, but they always suggest I do it using printf() rather than return. Clearly this introduces a problem for the program we are writing because although we can see the intermediate results, there is no (sensible) way to consume the output stream in the rest of the program.
Although they do understand the importance of getting the computed value out of the function, they seem to think it's OK if it's just printed with printf() rather than properly returned from the function. I try to object that once something is printed to the screen, although it does show you, the programmer, the value in question, the computer can't "read" a printed value back from the screen gives that it's now just an assorted bunch of pixels on the screen. I even tried to make some kind of analogy with trying to present a picture of a written word to Google (say "programming") and having Google search for that word. However, somehow they don't just get this point.
