My find-biggest-number Recursive function returns a discremented value

Question

I have a problem in this recursive function that basically takes two numbers and returns the biggest one of them without using comparison (> || < ) operators, thing is, it returns dicremented values even though I held the starting values in a variable.

Here's my code:

#include <stdio.h>

int WhoBig(int A, int B) {
    int TrueA=A, TrueB=B;
    if(A==0)
    {
        return TrueB;
    }
    else if(B==0)
    {
        return TrueA;
    }
    else 
    {
        return WhoBig(A-1,B-1);
    } 
}

void main() {
    printf("%d",WhoBig(9,2));
    //Output:7 
}

by making the variables static it may work (but it's ugly). Otherwise the variables are "updated" at each recursive call (they're local variables). Just add 2 parameters propagating the original values — Jean-François Fabre, Nov 24 '22 at 20:26

Vlad from Moscow · Accepted Answer · 2022-11-24T20:42:27.870

The type of the function parameters and of its return type should be an unsigned integer type. Otherwise the function can invoke undefined behavior if negative values will be passed to the function.

What you need is to add 1 to the returned value of each recursive call. Otherwise you are returning a decremented value.

The function can look the following way

unsigned int WhoBig( unsigned int x, unsigned int y ) 
{
    if ( x == 0 )
    {
        return y;
    }
    else if ( y == 0 )
    {
        return x;
    }
    else 
    {
        return 1 + WhoBig( x - 1, y - 1 );
    } 
}

Here is your updated program.

#include <stdio.h>

unsigned int WhoBig( unsigned int x, unsigned int y )
{
    if (x == 0)
    {
        return y;
    }
    else if (y == 0)
    {
        return x;
    }
    else
    {
        return 1 + WhoBig( x - 1, y - 1 );
    }
}

int main( void )
{
    printf( "%u\n", WhoBig( 9, 2 ) );
}

Its output is

Pay attention to that the function main without parameters shall be declared like

int main( void )

cruelly missing an explanation about the OP mistake. Why is that working? — Jean-François Fabre, Nov 24 '22 at 20:42

score 1 · Answer 2 · answered Nov 24 '22 at 20:33

you are not really saving the true value of A or B you are holding them in variables then you return the new value returned by WhoBig function i would go with something like this:

   #include <stdio.h>

int WhoBig(int A, int B) {
    int TrueA=A, TrueB=B;
    if(A==0)
    {
        return TrueB;
    }
    else if(B==0)
    {
        return TrueA;
    }
    else 
    {
        return 1 + WhoBig(A-1,B-1);
    } 
}

void main() {
    printf("%d",WhoBig(9,2));
    
}

cruelly missing an explanation about the OP mistake. Why is that working? — Jean-François Fabre, Nov 24 '22 at 20:42

My find-biggest-number Recursive function returns a discremented value

2 Answers2