How to write first negative number in array?

Question

I am just new in programming so excuse me for this 'easy' question.

I have a array list which have negative and positive numbers. It's in C#. I want to write on console, first negative number in this array list. How can do it? I tried a few things but it's writing all negative numbers or only the last negative number. I can't write the first one. Can anyone help me, please?

Here is my example of code :

int[] numbers = new int[] {13, 22, -5, 94, 66, -38, 41, -79, -1, 53};
int[] firstminus = new int[1];    

for (int i = 0; i < 10; i++)
{
    if (numbers[i] < 0)
        firstminus[0] = numbers[i];
}

Console.WriteLine(firstminus[0]);
Console.ReadLine();

Welcome to StackOverflow! There are a lot of ways to accomplish that, but I think you would learn nothng if someone provides these complete solutions here. It would be better to show what you have tried, so we could spot the mistakes you made. — René Vogt
– René Vogt, Commented Dec 12, 2018 at 19:07
Please post a minimal, complete, and verifiable example of your code. — nicomp
– nicomp, Commented Dec 12, 2018 at 19:07
@nicomp You can use a magic link to automatically link to the MCVE docs. Use [mcve] in your comment. — user47589
– user47589, Commented Dec 12, 2018 at 19:08
@Hogan Because I don't feel like fleshing it out into a good answer. — itsme86
– itsme86, Commented Dec 12, 2018 at 19:20

Gilad Green · Accepted Answer · 2018-12-12 19:16:07Z

To correct your attempt add a break after you found the number. That will terminate the loop and you'll have the first negative number in the array.

for (int i = 0; i < 10; i++)
{
    if (numbers[i] < 0) 
    {
        firstminus[0] = numbers[i];
        break;
    }
}

Notice 2 things:

There is no need for an array firstminus - better just have it a simple int.
The loop runs to 10. This is a magic number. Instead run until array's length. Please take the time to read this: What is a magic number, and why is it bad?

So:

int firstMinus;
for (int i = 0; i < numbers.Length; i++)
{
    if (numbers[i] < 0) 
    {
        firstminus = numbers[i];
        break;
    }
}

And last, if you are familiar with linq then just use .FirstOrDefault which returns the first item in the collection that matches a predicate (or the default of the type if none meet the predicate):

var firstMinus = numbers.FirstOrDefault(i => i < 0);

Thank you so much for that detaily explanation. I understood my fault and I learned news. I am so glad.

Ousmane D. · Accepted Answer · 2018-12-12 19:15:48Z

2

You're close, you'll just need to break as soon as you find the first negative number. i.e.

if (numbers[i] < 0) {
     firstminus[0] = numbers[i];
     break; // terminate the loop
}

btw, I'd use an int variable instead of constructing an array to store the result.

i.e.

int firstminus = 0;
for (int i = 0; i < numbers.Length; i++)
{
   if (numbers[i] < 0) {
      firstminus = numbers[i];
      break; // terminate the loop
   }
}

on another note, the simplest way would be via LINQ:

numbers.FirstOrDefault(x => x < 0);

edited Dec 12, 2018 at 19:15

answered Dec 12, 2018 at 19:10

Ousmane D.

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1 Comment

Jon Vote Over a year ago

IMHO - this is a better answer than mine :).

Hogan · Accepted Answer · 2018-12-12 19:16:09Z

1

I would use a while loop -- it just makes sense for the problem.

Also -- remember the use case where there is no neg -- you have to handle that too.

int[] numbers = new int[] {13, 22, -5, 94, 66, -38, 41, -79, -1, 53};
int index = 0

while (index < numbers.Length & numbers[index] >= 0)
 index++;


if (index < numbers.Length & numbers[index] < 0)
    Console.WriteLine(numbers[index]);
else 
    Console.WriteLing("No neg.");

    Console.ReadLine();

edited Dec 12, 2018 at 19:16

answered Dec 12, 2018 at 19:13

Hogan

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2 Comments

sellimenes Over a year ago

Hmm I didn't even think about while loops, thanks a lot

Hogan Over a year ago

@CPrecius -- next time think of the poor forgotten while loop! It gets to much competition from foreach and linq!

Gilad Green · Accepted Answer · 2018-12-12 19:22:38Z

0

int[] numbers = new int[] {13, 22, -5, 94, 66, -38, 41, -79, -1, 53};

for (int i = 0; i < 10; i++)
{
    if (numbers[i] < 0)
    {
        Console.WriteLine(numbers[i]);
        break;
    }     
}
Console.ReadLine();

edited Dec 12, 2018 at 19:22

Gilad Green

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answered Dec 12, 2018 at 19:11

Miamy

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Comments

Jon Vote · Accepted Answer · 2018-12-12 19:37:36Z

A 'better' solution is to use linq:

int[] numbers = new int[] { 13, 22, -5, 94, 66, -38, 41, -79, -1, 53 };
int[] firstminus = new int[1];
firstminus[0] = numbers.FirstOrDefault(n => n < 0);

Update - in response to Gilad Green's comment about performance - great point! When I run the following, I find that the loop actually performs about 1/2 second faster on my box:

    static void Main(string[] args)
    {
        var stopWatch = new Stopwatch(); 
        Console.WriteLine("Linq method");
        stopWatch.Start();
        for (var i = 0; i < 10000000; i++)
        {
            LinqMethod(); 
        }
        stopWatch.Stop();
        Console.WriteLine($"Elapsed time: {stopWatch.Elapsed}");
        stopWatch.Reset();

        Console.WriteLine("Loop method");
        stopWatch.Start();
        for (var i = 0; i < 10000000; i++)
        {
            LoopMethod();
        }
        stopWatch.Stop();
        Console.WriteLine($"Elapsed time: {stopWatch.Elapsed}");

        Console.ReadLine();
    }

    private static void LinqMethod()
    {
        int[] numbers = new int[] { 13, 22, -5, 94, 66, -38, 41, -79, -1, 53 };
        int[] firstminus = new int[1];
        firstminus[0] = numbers.FirstOrDefault(n => n < 0);
    }

    private static void LoopMethod()
    {
        int[] numbers = new int[] { 13, 22, -5, 94, 66, -38, 41, -79, -1, 53 };
        int[] firstminus = new int[1];
        for (int i = 0; i < 10; i++)
        {
            if (numbers[i] < 0)
            {
                firstminus[0] = numbers[i];
                break; 
            }
        }
    }

Yeah good point. Well it avoids a loop, it's less code and it's more 'C#-ish'. It should perform better.
best to add it properly to the answer. (about performance I wouldn't say so.. it probably compiles to very similar code in this case)
@GiladGreen - You make a great point. In fact - on my box the loop runs faster than linq. See updated code above.

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