CODEWITHSUNDEEP
c++recursion
Thomas
Thomas

Reputation: 315

C++ recursion example explanation

I have the following code example and I cannot figure out why it displays 123. Since these are integers I understand the decimals are not displayed. But I expected it to show 3, 2(.3), 1(.23) (in the opposite order). When n goes below 10 everything stops after the final cout... right?

#include <iostream>
using namespace std;
void recursion(int n)  {
    if (n < 10)  cout << n;
    else  {
        recursion(n / 10);
        cout << n % 10;
    }
}

int main()  {   
  recursion(123);
  return 0;
}

Upvotes: 1

Views: 526

Answers (1)

Christophe
Christophe

Reputation: 73627

Well, you call with 123 as n, the function executes the statement: 

if (n < 10)      // its false, so it continues with else:  
else {
   recursion ( n /10 )  // recursive call n/10 = 123/10 = 12 (as it's int) 
...

It will continue like this, recursively calling with n being 12 

    recursion (n/10)      // second recursion call n=12, so n/10 = 1

then the function is executed, with n being 1 so smaller than 10

        if (n < 10)      // its true 
            cout << n;   // 1 gets printed
        else             // no, the rest is skiped  
        ...

then it returns from recursion. So we're back in the context where n was 12. The next statement to be executed in that context is : 

    cout << n %10;    //  prints 12 % 10, which is 2

then, continuing like this, similarly it will print 123%10, which is 3. In conclusion, the 123 that is printed has nothing to do with the 123 entered as input.

I think you wanted to do : 

...
else  {
    cout << n % 10;       // first print to see the count down
    recursion(n / 10);    // then recurse 
}

But you have to learn using a debugger. As long as you don't, put some extra cout to visualize what's happening.

Upvotes: 5

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