Binary search with recursion c#

Question

I got stuck with my binary search method and I don't really know what I'm doing.

At this moment I got

1 Error (CS0161) not all code paths return a value

int low = 0;
int high = cities.Length;

static int BinarySearch(City[] cities, int key, int low, int high)
{
    int mid;

    if (low > high)
        return -1; //Sökning EJ hittad

    mid = low + high / 2;

    if (key == cities[mid].temp)
        return mid; //Sökning Hittad!
    else if (key < cities[mid].temp)
        BinarySearch(cities, key, low, mid - 1);
    else
        BinarySearch(cities, key, mid + 1, high);
}

...searching for temperature with input from user.

Console.WriteLine("\n\tBINARY SEARCH\n");
do
{
    Console.Write("Search temp:");
    loop = true;
    str = Console.ReadLine();
    try
    {
        key = Convert.ToInt32(str);
        
         
        index = BinarySearch(cities, key, low, high);
        if (index == -1)
        {
            Console.WriteLine($"Couldn't find any city with temperature: {key}°C");
        }
        else
        {
            Console.WriteLine("Search results: ");
            Console.WriteLine(cities[index].ToString());
            loop= false;
        }
    }
    catch
    {
        Console.WriteLine("Only numbers, please.");
    }
} while (loop);

You need to return the value from the calls to BinarySearch(), i.e. return BinarySearch(cities, key, low, mid - 1); etc — Matthew Watson
– Matthew Watson, Commented May 6, 2022 at 14:48
First of all but unrelated: Always use curly braces even on single-line ifs etc. Especially in the beginning this will save you headaches. — Fildor
– Fildor, Commented May 6, 2022 at 14:48
The BinarySearch method doesn't return anything in the else if or else blocks. Hence, "not all code paths return a value". The method has a return type, so every logical path needs to return something. — David
– David, Commented May 6, 2022 at 14:49
Check out this answer for the simplest implementation of recursive Binary Search using C# 12. — Ash K
– Ash K, Commented Jan 28, 2024 at 21:12

Fildor · Accepted Answer · 2022-05-06 14:54:38Z

0

The error message 'not all code paths return a value' is correct:

if (key == cities[mid].temp)
    return mid; //Sökning Hittad!
else if (key < cities[mid].temp)
    BinarySearch(cities, key, low, mid - 1); // <-- does not return anything
else
    BinarySearch(cities, key, mid + 1, high); // <-- does not return anything

Fix:

if (key == cities[mid].temp)
    return mid; //Sökning Hittad!
else if (key < cities[mid].temp)
    return BinarySearch(cities, key, low, mid - 1); 
else
    return BinarySearch(cities, key, mid + 1, high);

answered May 6, 2022 at 14:54

Fildor

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5 Comments

Jerry Ohlson Over a year ago

This worked! Thank you. But there's still one small problem with this code. When I search for a number that isn't an element in the array, I get "Only numbers, please." instead of "Couldn't find any city with temperature: {key}°C"

Fildor Over a year ago

In that case, actually catch the exception thrown and look into it. But this should be a different issue, then. If you cannot resolve it, feel free to post a new question with the new issue.

Fildor Over a year ago

Not 100% sure, but int high = cities.Length; might cause an ArgumentOutOfRangeException. I think it should be int high = cities.Length-1;.

Jerry Ohlson Over a year ago

int high = cities.Lenght -1 doesn't work well in this case. But I've seen it before.

Jerry Ohlson Over a year ago

my issue is not an issue anymore. I dont know why but sometimes the program fix it self through a simple restart.

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