Search for the first true value in a List c#

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So I have a bool list and I want to find where the first true value lies (index of first true value) how can I achieve this?

Thanks in advance.

Posted 26-Nov-15 1:39am

Member 11971544

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phil.o 26-Nov-15 7:44am

What have you tried? Do you have problems iterating over a collection?

2 solutions

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Manas_Kumar · Accepted Answer · 2015-11-26T01:56:00

Solution 2

Try with below code:

C#

List<bool> myList = new List<bool>();
myList.Add(false);
myList.Add(false);
myList.Add(true);
myList.Add(false);
myList.Add(true);

// Get index of first element
int p = myList.IndexOf(true); // Result: 2
// Get index of last element
int p1 = myList.LastIndexOf(true); // Result: 4

// Get index of first element
int index = myList.FindIndex(a => a == true); // Result: 2
// Get index of last element
int index1 = myList.FindLastIndex(a => a == true); // Result: 4

If it helps please accept the answer.

Posted 26-Nov-15 1:56am

Manas_Kumar

v2

Comments

Member 11971544 26-Nov-15 8:24am

Thanks this helped. But in case I have found first true condition which is: myList[2]/(int p) in your case and I want to find the next true condition. How can I go about this?

[no name] 26-Nov-15 8:28am

Then you have to do it manually by looping over the list and find the index.

Member 11971544 26-Nov-15 8:40am

so just in case I want to loop from the [third] item (because I have find true in the [second] item how can I do this? Thanks for the help so far.

Afzaal Ahmad Zeeshan 26-Nov-15 11:01am

5ed.

[no name] 26-Nov-15 11:21am

:)

F. Xaver · Accepted Answer · 2015-11-26T04:20:00

Solution 3

In addition to Manas_Kumar's answer

C#

List<int> true_indexes = myList.Select((value, index) => value ? index : -1).Where(o => o >= 0).ToList();

gets you a list with all indexes of true elements.
for here: {2,4}

Posted 26-Nov-15 4:20am

F. Xaver

Comments

BillWoodruff 26-Nov-15 20:50pm

+5 nice use of Linq and ?: operator !

You could also write this solution using the 'projection' aspect of Linq Select where you use the 'new operator:

var trueIndexes = myList
.Select((value, index) => new {value, index})
.Where(x => x.value)
.ToList();