CODEWITHSUNDEEP
c#
NZ_DJ
NZ_DJ

Reputation: 341

Nested for-loop: giving concatenated result

I am currently finding it hard to grasp the output from a simple nested for-loop

int result = 0;

for (int i = 1; i <= 3; i++)
{
    for (int j = 1; j <= 2; j++)
    {
        result = result +i;
    }
}
Console.WriteLine(result);

Which gives the output of 12. I understand that the result for j is 1 and 2, but just cannot grasp why the final result is 12 instead of 3?

Can someone explain please?

Upvotes: 1

Views: 53

Answers (3)

Leon Barkan
Leon Barkan

Reputation: 2703

run that test and you will see:

class Program
{
    public static void test()
    {
        int result = 0;

        for (int i = 1; i <= 3; i++)
        {
            Console.WriteLine("i["+i+"] : ");
            for (int j = 1; j <= 2; j++)
            {
                Console.Write(" when : j["+j+"]");
                Console.WriteLine("Adding " + i + "to" + result);
                result = result + i;
            }
        }
        Console.WriteLine(result);
        Console.ReadLine();
    }

    static void Main(string[] args)
    {
        test();
    }
}

Upvotes: 1

Jon Skeet
Jon Skeet

Reputation: 1500135

Your outer loop has three iterations. On each of those three iterations, you're running the inner loop:

for (int j = 1; j <= 2; j++)
{
    result = result +i;
}

Given that the body of the loop doesn't depend on j, and the inner loop will always run twice, we can replace the whole of the inner loop with:

result = result + i;
result = result + i;

Or to simplify it:

result = result + (i * 2);

So now the whole code is equivalent to:

int result = 0;

for (int i = 1; i <= 3; i++)
{
    result = result + (i * 2);
}
Console.WriteLine(result);

So after the first iteration of the outer loop, result is 2 (we've added 2 to 0).

After the second iteration of the outer loop, result is 6 (we've added 4 to 2).

After the third iteration of the outer loop, result is 12 (we've added 6 to 6).

Upvotes: 2

Iridium
Iridium

Reputation: 23721

You are adding the value of i to result each iteration of the inner loop, and the inner loop runs 3 times (the 3 iterations of the outer loop). Therefore the first time the inner loop runs, you add 1 twice, the second time the inner loop runs you add 2 twice, and the final time it runs you add 3 twice:

result = 1 + 1 + 2 + 2 + 3 + 3 = 12

Upvotes: 2

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