C# return index of array where value matches an arbitrary string

Question

I need to find the index of an item in an array of strings where that item's value matches a certain pattern.

Example array & values:

string[] stringArray = { "roleName","UserID=000000","OtherID=11111" }

I need to get the index of the item whose value begins with "UserID=". I know I could iterate through the array and match each value to a regex, but that just sounds slow and messy. I was thinking of doing something like this:

int indexIneed = Array.IndexOf(stringArray,"\bUserID=");

But I've never really had to mess with regular expressions before, so I fumbling around like a toddler. Is there I way I can accomplish what I'm tring to do in O(n) or am I going to have to resort to iteration?

What's wrong with the simple iteration? Why do you consider it "slow and messy"? It is O(n) in fact, if you do not take into account the regex matching cost. — DixonD
– DixonD, Commented Feb 3, 2015 at 20:24
@DixonD and Magnus You're both absolutely right. For some reason I was thinking it would be O(n<sup>2</sup>). Deadlines have me jumping to conclusions :) — Jerreck
– Jerreck, Commented Feb 3, 2015 at 20:32

D Stanley · Accepted Answer · 2015-02-03 20:23:29Z

7

FindIndex will give you what you want:

int indexIneed = Array.FindIndex(stringArray,s => s.StartsWith("UserID="));

answered Feb 3, 2015 at 20:23

D Stanley

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3 Comments

Selman Genç Over a year ago

completely forgot about that. +1 :)

DixonD Over a year ago

anyway, it is the same iteration, just one-liner. the same can be done with LINQ also

Selman Genç Over a year ago

@DixonD not without creating unnecessary objects

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