Question

I have some question from the exam in which I need to deduce the output of the following code:

01 int foo(int a) {
02 print 'F';
03 if (a <= 1) return 1;
04 return bar(a, foo(a-1));
05 }
06
07 int bar(int x, int y) {
08 print 'B';
09 if (x > y) return baz(x, y);
10 return baz(y, x);
11 }
12
13 int baz(int x, int y) {
14 print 'Z'
15 if (y == 0) return 0;
16 return baz(x, y-1) + x;
17 }
18
19 void main() {
20 foo(3);
21 }

my question is what tactic will be the best to solve this kind of the questions? I’m not allowed to use PC of course
P.S. You can use eager evaluation as in c++ or normal order evaluation(output will be different of course, but I’m interested in tactics only), I tried to solve it using stack, every time write the function which I call, but anyway it is complicated
thanks in advance for any help

Answer 1

I would use a “bottom-to-top” attempt:

baz is the function that is called, but doesn’t call other functions (except itself). It outputs ‘Z’ exactly y + 1 times, the return code is x*y (you add x after each call).

bar is the “next higher” function, it outputs ‘B’ once and calls baz with its lower argument as the second parameter – the return code is x*y, too.

foo is the “top” function (right after main) and its the most complicated function. It outputs ‘F’, not only once, but a times (because of the foo(a-1) at the end that is evaluated before the bar call. The bar call multiplies a and foo(a-1), which will multiply a-1 and foo(a-2) and so on, until foo(1) is evaluated and returns 1. So the return code is a * (a-1) * ... 2 * 1, so a!.

This is not a complete analysis, f.e. we don’t know in which order the characters will be output, but it is a rough scheme of what happens – and as you and other people in the comments pointed out, this is what you want – tactics instead of a complete answer.