How can I find a certain letter in a String in C#?

Question

How can I search a certain letter (char) in a String?

I have to code a little riddle. You basicalley have to guess the right letters of an unknown word. Only the first letter is shown.

Example: "Apple"

A____ --> that's what you actually see. Now, the player has to input a letter/char and if it is inculded in the String "Apple", the correct letter will be added.

Example: input = e A___e

Thx.

Your choice of tags would seem to indicate you already know one answer: **loop** thru the **chars** in the **string** to see if it matches the **letter** entered. — Ňɏssa Pøngjǣrdenlarp, Nov 23 '19 at 23:02
Possible duplicate of [Finding all positions of substring in a larger string in C#](https://stackoverflow.com/questions/2641326/finding-all-positions-of-substring-in-a-larger-string-in-c-sharp) — devNull, Nov 23 '19 at 23:07
Thank you. I have tried it so many times on my own using loops but I just was not able to get the correct result. That's why I am here. — mxy, Nov 23 '19 at 23:17
Next time, if you post the code you have tried and are working with, you are much less likely to collect downvotes. As posted this is asking 'how to write a program' which is too broad. Visit the [help] and study some of the topic there like [ask] — Ňɏssa Pøngjǣrdenlarp, Nov 23 '19 at 23:30
Well, for me votes are not that important. I am satisfied with the most simple answers. Like "You can use IndexOf" or soemthing like that. If people are kind enough to show me the actual code, that is a bonus for me. My code was not even something you could really work with. The whole char-searching part was missing because I did not know what to add and the rest was just a loop. Thx. — mxy, Nov 24 '19 at 12:09

José Pedro · Answer 1 · 2019-11-23T23:10:54.990

4

You can use String.IndexOf.

Example:

var str = "Apple";
var c = 'p';
var i = str.IndexOf(c);
// i will be the index of the first occurrence of 'p' in str, or -1 if not found.

if (i == -1)
{
    // not found
}
else
{
    do
    {
        // do something with index i, which is != -1
        i = str.IndexOf(c, i + 1);
    } while (i != -1);
}

edited Nov 23 '19 at 23:10

answered Nov 23 '19 at 23:01

José Pedro

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What about the character 'p' in "Apple"? – devNull Nov 23 '19 at 23:05
You will get the first index (1), then as it's not -1, you can start on i + 1: `str.IndexOf('p', i + 1)`, and you can repeat this until you get -1. – José Pedro Nov 23 '19 at 23:07

RoadRunner · Answer 2 · 2019-11-24T03:25:08.327

If you want to find all letter indices, you can try this LINQ solution:

var str = "Apple";
var letter = 'p';

var charIndexMap = str
    .Select((ch, idx) => (ch, idx))
    .GroupBy(pair => pair.ch)
    .ToDictionary(entry => entry.Key, 
                  entry => entry.Select(pair => pair.idx));

if (charIndexMap.TryGetValue(letter, out var value))
{
    Console.WriteLine("[" + string.Join(", ", value) + "]");
} else
{
    // not found
}

Output:

[1, 2]

Explanation:

Use Enumerable.Select to map over the characters and indices and convert them to (character, index) tuples.
Use Enumerable.GroupBy to group tuples by character.
Convert grouped results to a dictionary using Enumerable.ToDictionary, which maps each character to an IEnumerable<int> of indices.
Use Dictionary<TKey,TValue>.TryGetValue(TKey, TValue) to get the associated indices for the letter.

How can I find a certain letter in a String in C#?

2 Answers2