Issue when replacing strings

Question

Asking for best practice for doing the following, having this string in C#:

string sentence = "I was watching TV last night while she was reading a book and you were playing video games";

I'm trying to get it automatically replaced by something like:

"you were watching TV last night while she was reading a book and I was playing video games"

Problem here is I can't use

sentence.Replace("I was", "you were").Replace("you were", "I was")

because final sentence would look bad: "I was watching tv while she was reading a book and I was playing video games".

It might look simple but it is not as simple as it seems, can I get some advice please?

One of hte overloads of Replace allows you to defined the first & last Index it should cover. | You could also expand the replace strings to include "watching" and "playing" respeectively. Expanding the Search string to filler stuff is a common way to avoid fake positives. | However overall this belongs into the area of Language Science and Artificial Intelligence, not normal programming. — Christopher
– Christopher, Commented Nov 19, 2019 at 22:07
When replacing multiple item in a string and want to make sure your replacements are unique. So the following will work : sentence.Replace("I was watching", "you were watching").Replace("you were playing", "I was playing") — jdweng
– jdweng, Commented Nov 19, 2019 at 22:16
This is a very interesting question. With code you introduce a third variable, but this is a string. — ctwheels
– ctwheels, Commented Nov 19, 2019 at 22:20
You could get the indices for each replacement location and then substring it. — ctwheels
– ctwheels, Commented Nov 19, 2019 at 22:23

Wiktor Stribiżew · Accepted Answer · 2019-11-19 22:13:37Z

2

You may leverage a MatchEvaluator as the replacement argument in Regex.Replace:

var sentence = "I was watching TV last night while she was reading a book and you were playing video games";
var result = Regex.Replace(sentence, @"\b(?:(I was)|you were)\b", m =>
    m.Groups[1].Success ? "you were" : "I was");
Console.WriteLine(result); 
// => you were watching TV last night while she was reading a book and I was playing video games

See the C# demo.

Here, \b(?:(I was)|you were)\b matches either I was (while capturing the match into Group 1) or you were as whole words (\b is a word boundary). If m.Groups[1].Success is true, the replacement is you were, else, it is I was.

answered Nov 19, 2019 at 22:13

Wiktor Stribiżew

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1 Comment

LordCaos Over a year ago

awesome! a simple and elegant solution

FrqSalah · Accepted Answer · 2019-11-19 22:19:39Z

0

Another solution is to create two functions, one to replace the first occurence and the second to replace the last occurence :

public static string ReplaceFirstOccurrence (string Source, string Find, string Replace)
{
    int Place = Source.IndexOf(Find);
    string result = Source.Remove(Place, Find.Length).Insert(Place, Replace);
    return result;
}

public static string ReplaceLastOccurrence(string Source, string Find, string Replace)
{
    int Place = Source.LastIndexOf(Find);
    string result = Source.Remove(Place, Find.Length).Insert(Place, Replace);
    return result;
}
ReplaceFirstOccurrence(sentence, "I was", "you were");
ReplaceLastOccurrence(sentence, "you were", "I was");

answered Nov 19, 2019 at 22:19

FrqSalah

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Comments

tmaj · Accepted Answer · 2019-11-19 22:22:06Z

This is very crude, would be slow for large strings, makes many memory allocations, but it's easy to explain and does the job.

var input = "I was watching TV last night while she was reading a book and you were playing video games";

var pairs = new [] { 
    (From: "I was", To: "you were"),
    (From: "you were", To:"I was")
    };

var s = input;
for(int i = 0; i < pairs.Length; i++) s = s.Replace(pairs[i].From, $"{{TOKEN{i}}}", ignoreCase: true, CultureInfo.InvariantCulture);
for(int i = 0; i < pairs.Length; i++) s = s.Replace($"{{TOKEN{i}}}", pairs[i].To);

Console.WriteLine(input);
Console.WriteLine(s);
s = char.ToUpper(s[0]) + s.Substring(1);
Console.WriteLine(s);

Output

I was watching TV last night while she was reading a book and you were playing video games
you were watching TV last night while she was reading a book and I was playing video games
You were watching TV last night while she was reading a book and I was playing video games

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