I am trying to figure out this double recursion problem but I cannot. To visualize, I used the function stack.
if(n==1) {
return 1;
}
else {
return r((n+1)/2) + r(n/2);
}
}
Using the stack can someone help me understand how to get the answer if int x = r(10)?
so I have a program that is running a bunch of different recursive methods, and I cannot get it to compile/run. The error is in this method, according to my computer:
public static int fibo(int n)
// returns the nth Fibonacci number
{
if (n==0)
{
return 0;
}
else if (n==1)
{
return 1;
}
else if (n>1)
{
return fibo(n-1) + fibo(n-2);
}
}
I have this method called correctly in my main method, so the issue is in this bit of code.
I think I can help you in this. Add return n; after your else if. Outside of the code but before the last curlicue.
The code will work as long as n ≥ 0 btw; another poster here is right in that you may want to add something to catch that error.
Make sure all possible paths have a return statement. In your code, if n < 0, there is no return statement, the compiler recognizes this, and throws the error.
public static int fibo(int n)
// returns the nth Fibonacci number
{
if (n<=0)
{
return 0;
}
else if (n==1)
{
return 1;
}
else // All other cases, i.e. n >= 1
{
return fibo(n-1) + fibo(n-2);
}
}
This is a recursive solver to try to solve Euler#60. http://projecteuler.net/problem=60
The solver runs through, but fails to find a solution for the last array member, so backtracks (like I think it's supposed to) but when I get back to the first array member, the loop runs out all the way. Can anybody spot for me why it doesn't stop at the next prime?
I've posted just the solver function below; the other function (Concat check) works properly and returns true for a partially filled array.
int Solver (int primes[5])
{
int i=1;
int x=0;
while (primes[x]!=0) {++x;} //work on the next one
if ((x>5) && Concat_Check(primes)) {return 1;} //solved array
for (i=3; i<=SIZE; i++) //try each value, if successful, return true
{
if (Is_Prime(i)) {primes[x]=i; cout<<"primes["<<x<<"] = "<<i<<endl;}
if ((Concat_Check (primes)) && Solver (primes)) {return 1;}
}
primes[x-1] = 0;
return 0;
}
I can't get the purpose of recursion in your code, seems a loop...
Anyway, maybe you forgot to increment x in loop, and the test seems incomplete.
for (i=3; i<=SIZE; i+=2) //try each value, if successful, return true
{
if (Is_Prime(i)) {
primes[x++]=i; cout<<"primes["<<x<<"] = "<<i<<endl;}
if ((Concat_Check (primes)) && Solver (primes)) {return 1;}
}
}
I've been running through codeacademy's tutorials and projects. On FizzBuzz++ 1.5 they want us to re-write the "Wobble" function as Wob, using ternary operators. I keep getting an error saying "missing operand" with the following code. Also the +1 on the end of the return, how does that work, does javaScript store it as a temp value because it doesn't get assigned to any var. Thanks for the help. Code is below.
var Wibble = {
Wobble: function(a, b) {
if(a===b)
//if the variables are equal return 0
return 0;
else {
//decrement the larger of the 2 values
if (a>b) {
a--;
} else {
b--;
}
//call this function again with the decremented values
//once they are equal the functions will return up the stack
//adding 1 to the return value for each recursion
return Wibble.Wobble(a,b)+1;
}
},
//This is the line of code that doesn't want to function..
Wob: function(a, b) {
(a===b) ? return 0 :(a<b) ? return this.Wob(a--,b)+1 : return this.Wob(a,b--)+1;
}
};
The following expression with the ternary operator:
result = (a) ? x : y;
is equivalent to the following:
if(a)
{
result = x;
}
else
{
result = y;
}
Note the syntactical difference, where in the ternary operator you are switching in the assignment, whereas in the if statement syntax, you are assigning in the switch.
That is to say that:
(a == b) ? return 0 : return 1;
is not equivalent to:
if(a == b)
return 0;
else
return 1;
Instead, you would want to write:
return (a == b) ? 0 : 1;
I have just been studying the concept of recursion and I thought that I would try a simple example. In the following code, I am attempting to take the numbers: 1, 2, 3, 4, 5, and add them together using recursion. I expected the result to be 15, but my code is returning 16.
What am I doing wrong?
Code:
static void Main(string[] args)
{
Console.WriteLine(Sum(5));
Console.Read();
}
static int Sum(int value)
{
if (value > 0)
{
return value + Sum(value - 1);
}
else
{
return 1;
}
}
You're returning 1 in the else clause. You should be returning 0:
else
{
return 0;
}
If the value is not greater than zero, why would you return one in the first place?
Your code executes as follows:
Sum --> 5
Sum --> 4
Sum --> 3
Sum --> 2
Sum --> 1
Sum --> 0
1 <---
2 <---
4 <---
7 <---
11 <---
16 <---
Check your base case.
Others already noted the error, and I will elaborate on recursion.
Although C# does not currently perform tail call optimization (although IL has special tail instruction), it's worth mentioning that tail recursion is generally a good thing.
Tail recursion is a special case of recursion in which the last operation of the function, the tail call, is a recursive call. Since the last call is the recursive call there is no need to preserve stack frame of the calling function and the compiler can easily use this information to generate machine instruction such that the stack doesn't grow at all. So it can basically turn recursive function into an iterative one.
Rewriting your code to support tail recursion can be done as follws:
static int Sum(int result, int value)
{
if(value == 0)
return result;
return Sum(result + 1, value - 1);
}
static int Sum(int value)
{
if (value > 0)
{
return value + Sum(value - 1);
}
else
{
return 0; //Change this.
}
}
That's because, when the value is = 0, you return 1. Then it get's added.
Sum's "else" clause should return 0.
I always prefer to put the terminating case(s) up front so they're obvious, and I have a violent near-psychopathic hatred of "if cond then return a else return b" constructs. My choice would be (making it clear that it won't work properly for negative numbers):
static unsigned int Sum(unsigned int value) {
if (value == 0)
return 0;
return value + Sum(value - 1);
}
I believe that's far more readable than a morass of braces and control flow.
The others have already answered that question, but when I work with recursion, one of the things I like to do to check that it works is to use check the base case and one additional case. I your case I would test it with 1, which would yield 2. Since this is obviously wrong you might want to check for 0 which is not going to use any recursion and so it should be obvious that the error lies in the base class.
In general recursion is easier to reason about, since you can list the limited number of things you need to check, but it does initially require a leap of faith since your intuition will be wrong. Just test the edge cases and trust the math it will never fail.
int summation(int num){
if (num==1)
return 1;
return summation(num-1)+num;
}
I'm pretty sure the problem is because you want your recursion to terminate when value == 1, and it's currently terminating when value == 0.
Your terminating expression is at issue. When value == 0 (or lower), it should return a 0 rather than 1. For sake of efficiency (which, let's admit it here, obviously isn't a concern, otherwise recursion wouldn't have been used for this task), you should terminate the recursion at value == 1 and return a literal 1 to save one unnecessary level of recursion.
using System;
using NUnit.Framework;
namespace Recursion
{
[TestFixture()]
public class Test
{
[Test()]
public void TestSum ()
{
Assert.AreEqual (Sum (new int[] { }), 0);
Assert.AreEqual (Sum (new int[] { 0 }), 0);
Assert.AreEqual (Sum (new int[] { 1 }), 1);
Assert.AreEqual (Sum (new int[] { 1, 2, 3, 4, 5 }), 15);
}
public int Sum(int[] head)
{
if (head.Length == 0) return 0;
int[] tail = new int[head.Length - 1];
for (int i = 1; i < head.Length; i++)
{
tail [i-1] = head [i];
}
return head[0] + Sum (tail);
}
}
}
It could also be written like this:
public static int sum(int n){
int total;
if(n==1){
total =1;
}else{
total = sum(n-1)+n;
}
return total;
}
Actually, I think you don't need to check case else because
public static int sum(int number){
if(number > 0){
return number + sum(--number);
}
return number; // return 0 so that's why you don't need check else condition
}
To begin at the end, a recursive Sum method looks like this:
// version 3
public static int Sum(int startRange, int endRange)
{
if (endRange > startRange)
{
return endRange + Sum(startRange, endRange - 1);
}
if (endRange < startRange)
{
return startRange + Sum(endRange, startRange - 1);
}
return endRange;
}
Hardcoding the startRange to be 0 gives us:
// version 2
public static int Sum(int range)
{
if (range > 0)
{
return range + Sum(0, range - 1);
}
if (range < 0)
{
return Sum(range, -1);
}
return range;
}
...and if you want to limit the method to positive numbers only, there's no need for a sign:
// version 1
public static unsigned int Sum(unsigned int range)
{
if (range > 0)
{
return range + Sum(0, range - 1);
}
return range;
}
I hope this helps give more of an insight into summing number ranges via recursion.
static int Sum(int[] addends)
{
if (addends.Length == 1)
{
return addends[0];
}
else
{
int tailIndex = addends.Length - 1;
var subArray = addends[0..tailIndex];
return addends[tailIndex] + Sum(subArray);
}
}
Try this code:
def sumr(n):
if n==0:
return n
return n+sumr(n-1)