find if an integer exists in a list of integers

Question

i have this code:

  List<T> apps = getApps();

        List<int> ids;

        List<SelectListItem> dropdown = apps.ConvertAll(c => new SelectListItem
        {
            Selected = ids.Contains(c.Id),
            Text = c.Name,
            Value = c.Id.ToString()
        }).ToList();


ids.Contains

seems to always return false even though the numbers do match

any ideas?

mquander is right - perhaps you can try a small example like List foo = new List(); for (int i = 0; i < 10; ++i) { foo.Add(i); } Console.WriteLine(foo.Contains(2)); — NG., Oct 13 '10 at 13:40
I guess that you have a bug elsewhere. That approach SHOULD work. — Daniel Mošmondor, Oct 13 '10 at 13:40
Like mquander said that's how you do it. There must be something else wrong. — RonaldV, Oct 13 '10 at 13:41
the List ids is not initialized to anything. try adding with some known values ~ var ids = new List() {2,4,6,8}; — Bobby Borszich, Oct 13 '10 at 13:46
see my answer below that has been updated 2x in response to your edits. — Woot4Moo, Oct 13 '10 at 13:47

Bobby Borszich · Accepted Answer · 2010-10-13T13:44:21.203

103

If you just need a true/false result

bool isInList = intList.IndexOf(intVariable) != -1;

if the intVariable does not exist in the List it will return -1

edited Oct 13 '10 at 13:44

answered Oct 13 '10 at 13:39

Bobby Borszich

11,639
9
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35

1

`IndexOf` is zero-based. That should be `>= 0` :) – Frédéric Hamidi Oct 13 '10 at 13:40
9

Or really it should just check `!= -1` since the -1 return value is defined as the negative result. – Daniel DiPaolo Oct 13 '10 at 13:43

score 88 · Answer 2 · answered Oct 13 '10 at 13:40

88

As long as your list is initialized with values and that value actually exists in the list, then Contains should return true.

I tried the following:

var list = new List<int> {1,2,3,4,5};
var intVar = 4;
var exists = list.Contains(intVar);

And exists is indeed set to true.

answered Oct 13 '10 at 13:40

Rune Grimstad

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OP updated his question which you answer. his ids list does not appear to be initialized. Shouldn't that be a compiler catch? – NG. Oct 13 '10 at 13:46
Yeah. An uninitialized list should cause a compiler error. But not if the list is defined at class level, then it will be initialized to null and the compiler will be happy. – Rune Grimstad Oct 13 '10 at 14:16

score 9 · Answer 3 · answered Oct 13 '10 at 13:55

Here is a extension method, this allows coding like the SQL IN command.

public static bool In<T>(this T o, params T[] values)
{
    if (values == null) return false;

    return values.Contains(o);
}
public static bool In<T>(this T o, IEnumerable<T> values)
{
    if (values == null) return false;

    return values.Contains(o);
}

This allows stuff like that:

List<int> ints = new List<int>( new[] {1,5,7});
int i = 5;
bool isIn = i.In(ints);

Or:

int i = 5;
bool isIn = i.In(1,2,3,4,5);

score 6 · Answer 4 · edited Mar 21 '19 at 12:10

6

The way you did is correct. It works fine with that code: x is true. probably you made a mistake somewhere else.

List<int> ints = new List<int>( new[] {1,5,7}); // 1

List<int> intlist=new List<int>() { 0,2,3,4,1}; // 2

var i = 5;
var x = ints.Contains(i);   // return true or false

edited Mar 21 '19 at 12:10

Siva rama krishna N

5
2

answered Oct 13 '10 at 13:42

gsharp

27,557
22
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score 3 · Answer 5 · answered May 31 '18 at 21:52

The best of code and complete is here:

NumbersList.Exists(p => p.Equals(Input)

Use:

List<int> NumbersList = new List<int>();
private void button1_Click(object sender, EventArgs e)
{
    int Input = Convert.ToInt32(textBox1.Text);
    if (!NumbersList.Exists(p => p.Equals(Input)))
    {
       NumbersList.Add(Input);
    }
    else
    {
        MessageBox.Show("The number entered is in the list","Error");
    }
}

eriksv88 · Answer 6 · 2010-10-13T14:33:30.050

1

bool vExist = false;
int vSelectValue = 1;

List<int> vList = new List<int>();
vList.Add(1);
vList.Add(2);

IEnumerable vRes = (from n in vListwhere n == vSelectValue);
if (vRes.Count > 0) {
    vExist = true;
}

edited Oct 13 '10 at 14:33

answered Oct 13 '10 at 14:24

eriksv88

3,482
3
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score 0 · Answer 7 · answered Oct 13 '10 at 13:43

You should be referencing Selected not ids.Contains as the last line.

I just realized this is a formatting issue, from the OP. Regardless you should be referencing the value in Selected. I recommend adding some Console.WriteLine calls to see exactly what is being printed out on each line and also what each value is.

After your update: ids is an empty list, how is this not throwing a NullReferenceException? As it was never initialized in that code block

score 0 · Answer 8 · answered Mar 02 '20 at 13:52

string name= "abc";
IList<string> strList = new List<string>() { "abc",  "def", "ghi", "jkl", "mno" };
if (strList.Contains(name))
{
  Console.WriteLine("Got It");
}

/////////////////   OR ////////////////////////

IList<int> num = new List<int>();
num.Add(10);
num.Add(20);
num.Add(30);
num.Add(40);

Console.WriteLine(num.Count);   // to count the total numbers in the list

if(num.Contains(20)) {
    Console.WriteLine("Got It");    // if condition to find the number from list
}

find if an integer exists in a list of integers

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