Substring search in string using linq

Question

I am looking in a string for operators. I need the actual operator and its index in the string

For example: x>10&y>=10

Operators

>
&
>=
=

So I need results like

>  1
&  4
>= 6

So I wrote the code like this

string substr= "x>10&y>=10";
List<string> substringList = new List<string>{">", "&", ">=", "="};

 var orderedOccurances = substringList
      .Where((substr) => str.IndexOf(substr, StringComparison.Ordinal) >= 0)
      .Select((substr, inx) => new 
          { substr, inx = str.IndexOf(substr, StringComparison.Ordinal) })
      .OrderBy(x => x.inx).ToList();

However I got results like this(obviously)

 > 1
 & 4
 > 6
 = 7

I can use a for loop for the search and cover this error scenario. But I like the linq short hand code. Is there anyway that I can cover the error condition using lambdas/linq?

Where is this monstreous LINQ-statement shorter and in particular better readable than a simple foreach-loop? — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Feb 21, 2017 at 14:46
I sort of agree to that point. I still would like to know if linq can be done for this scenario. — Jimmy
– Jimmy, Commented Feb 21, 2017 at 14:49
this is the sort of thing regular expressions handle best, see this question stackoverflow.com/questions/1851795/… — David Culp
– David Culp, Commented Feb 21, 2017 at 14:53
@David sure, now add support for parenthesis. Still convinced regular expresisons are best? — CodeCaster
– CodeCaster, Commented Feb 21, 2017 at 14:57

Slai · Accepted Answer · 2017-02-21 17:16:40Z

1

Here is more general alternative:

string str = "x>10&y>=10";

var result = Regex.Matches(str, @">=|>|&|=").Cast<Match>()
    .Select(m => new { s = m.Value, i = m.Index }).ToList();

Result:

>   1
&   4
>=  6

or a bit shorter if there aren't any other operators in the string:

var d = Regex.Matches(str, @"\W+").Cast<Match>().ToDictionary(m => m.Index, m => m.Value);

edited Feb 21, 2017 at 17:16

answered Feb 21, 2017 at 16:51

Slai

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Comments

Harald Coppoolse · Accepted Answer · 2017-02-22 08:08:41Z

So basically what you want is to scan your sequence for the characters '<', '>', '=' and '&', and if any of them found remember the index and the found character, if '<' or '>' is found you want to know if '=' is after it, and if so, the next search should start after the '='.

Note that you didn't specify what you want with &= or ==.

Whenever you have to scan strings for some syntax, it is always wise to at least consider the use of regular expressions.

According to the specification above you want a regular expression that matches if you find any of the following:

'<='
'>='
'='
'&'
'<' followed by something else than '='
'>' followed by something else than '='

Code would be simple:

using System.Text.RegularExpressions;

string expression = ...;
var regex = new RegularExpression("&|<=|>=|[<>][^=]");
var matches = regex.Matches(expression);

Object matches is an array of Match objects. Every match object has properties Index, Length and Value; exactly the properties you want.

foreach (var match in matches)
{
    Console.WriteLine($"Match {match.Value} found"
        + " at index {match.Index} with length {match.Length}");
}

The vertical bar | in the regular expression means an OR; the [ ] means any of the characters between the brackets,; the [^ ] means NOT any of the characters between the brackets.

So a match is found if either & or <= or >= or any character in <> which is not followed by =.

If you also want to find &= and ==, then your reguilar expression would be even easier:

find any <>&= that is followed by =
or find any <>&= that is not followed by =

Code:

var regex = new Regex("[<>&=]|[<>&=][^=]");

A good online regex tester where you can check your regular expression can be found here. This shows also which matches are found and a description of the syntax of regular expressions.

InBetween · Accepted Answer · 2017-02-21 16:17:54Z

Well, if you are bent on using LINQ you could do the following:

public static IEnumerable<(int Index, string Substring)> GetAllIndicees(this string str, IEnumerable<string> subtrings)
{
    IEnumerable<(int Index, string Substring)> GetAllIndicees(string substring)
    {
        if (substring.Length > str.Length)
            return Enumerable.Empty<(int, string)>();

        if (substring.Length == str.Length)
            return Enumerable.Repeat((0, str), 1);

        return from start in Enumerable.Range(0, str.Length - substring.Length + 1)
               where str.Substring(start, substring.Length).Equals(substring)
               select (start, substring);
    }

    var alloperators = subtrings.SelectMany(s => GetAllIndicees(s));
    return alloperators.Where(o => !alloperators.Except(new[] { o })
                                                .Any(other => o.Index >= other.Index &&
                                                              o.Index < other.Index + other.Substring.Length &&
                                                              other.Substring.Contains(o.Substring)));    
}

using c#7 syntax here becuase code is more concise and readable but its easily translatable to previous versions.

And now if you do:

var substr = "x>10&y>=10";
var operators = new HashSet<string>(new[] { ">", "&", ">=", "=" });
Console.WriteLine(string.Join(", ", filteredOperators.Select(o => $"[{o.Operator}: {o.Index}]")));

You'll get the expected result:

[>: 1], [&: 4], [>=: 6]

Is this "better" than using other tools? I'm not so sure.

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