execution flow

[icefalcn]

hi, i recently came about this piece of code and i am confused as to why it gives the output that it does

here is the code:

#include <iostream>
#include <string>
using namespace std;
 
 
string mysteryRecursive(int counter)
{
     string result = "#";
 
     if (counter == 0)
     {
          return result;
     }
     else
     {
          cout << "Counter is " << counter << endl;
          result = "*" + mysteryRecursive(--counter);
          cout << "Result is " << result << endl;
          return result;
     }
}
 
 
int main( )
{
   cout << mysteryRecursive(5);
   return 0;
}

the output of this code is:

Counter is 5
Counter is 4
Counter is 3
Counter is 2
Counter is 1
Result is *#
Result is **#
Result is ***#
Result is ****#
Result is *****#
*****#

i am a beginner so please excuse my noobie questions

1) what causes the function to loop and cout more than two statements
ie) why was the output not:
Counter is 5
Result is *#

2) why is the output not in chronological order
ie) why was the output not:
Counter is 5
Result is *#
Counter is 4
Result is **#.....ect

thank you for your time =)

[LBEaston]

It's a bit hard to explain without drawing a picture. If you step through the code line by line you should be able to work it out.

[Peter87]

Note that mysteryRecursive calls itself on line 17.

[eypros]

1) The answer is recursion of course. This means that function string mysteryRecursive(int counter) calls itself. Of course there should be a way to stop the recursion (as in other case there would be an infinite recursion loop).

That condition is fulfilled by the argument of the function: counter. See in line 17 that counter is first decreased then used as argument (because of the prefix decrement operator --counter.

When enter inf function mysteryRecursive the argument is examined to 0 (line 10): if (counter == 0). If this condition is true line 12 return result; is executed and the function never goes beyond this line.
The first encounter of return terminates the function.

So that is why string is increased. I hope that answers your first question.

2) Now that I hope you got the picture of recursion examine how it is invoked:
-line 20 is executed first cout << mysteryRecursive(5); where mysteryRecursive() is invoked with argument 5.
-Inside this function now (that is recursive) a recursive loop starts. Since argument is different than 0 the execution continues up to line 17 where a new invocation of the function is found:
result = "*" + mysteryRecursive(--counter);
Consider that up to now we haven't reached line 18 cout << "Result is " << result << endl; but we have reached line 16 cout << "Counter is " << counter << endl;. This will continue until condition in line 10 is fulfilled. Then it will return the string created.

That's why all

Counter is

appear before all

Result is

.

Hope that solved some questions. Be cautious though recursion is powerful but rather subtle point of the language.