Replace string on loop

Question

I'm working on a razor page where I need to drop a series of custom fields from a variable-attribute tables. My purpose is to replace a specific pairing of symbols ('{}') with an htmlstring. To simplify however lets say i wanted to replace it with an incremental number.

This is pseudo-code for what i'm looking for.

string s = "This sample will count: {}, {}, {}."
int i = 0;
while(*string not finished*)//?
{ 
   i ++;
   s.Replace("{}", i);  
}

Output:

"This sample will count 1, 2, 3."

Is this something I need to use regex on? Any other thoughts?

EDIT

I should clarify: I am not aware of how many '{}'s until run time. The numbers may be throwing people off, I'm liking going to do something more akin to:

 s.replace("{}", stringArray[i]);

Strings are immutable in .NET. Can only transform from one string to another. — John Alexiou
– John Alexiou, Commented Jan 13, 2014 at 20:32
@ja72 Yes I know, but even if I swapped it into StringBuilder, which is what replace likely does behind the scenes, my question remains valid i think. The objective isn't to count, the objective it to replace each sub-string fitting the pattern with a different substring. In my actual instance the source is probably going to be strings from an array or list. If I did s.replace(1) Then the output would be "This sample will count 1, 1, 1" — Seth
– Seth, Commented Jan 13, 2014 at 20:36
So, you're trying to replicate string.Format(), but without numbers inside the braces? — Magus
– Magus, Commented Jan 13, 2014 at 20:42
There's an overload of string.Format which takes a param (you could pass your list direcrly as an array), if you could get your input with numbers in the braces it would be perfect. — Pierre-Luc Pineault
– Pierre-Luc Pineault, Commented Jan 13, 2014 at 20:43
@Magus That's not an unfair assessment. In my instance programmers don't get to write the strings. Users do in real time, string s is coming from a database. I was not aware of string.format(str, array). Given Pierre's comment, all I really need is to add the numbers in, which ironically makes my dummy example not so dummy. — Seth
– Seth, Commented Jan 13, 2014 at 21:02

RJ Lohan · Accepted Answer · 2014-01-13 21:14:02Z

3

A simple sample using Split to tokenise your input string;

string s = "This sample will count: {}, {}, {}.";
string[] tokens = s.Split(new[] { "{}"}, StringSplitOptions.None);
StringBuilder sb = new StringBuilder();

int counter = 1;
for (int i = 0; i < tokens.Length; i++ )
{
    sb.Append(tokens[i]);
    if (i < tokens.Length - 1)
        sb.Append(counter++);
}

Console.WriteLine(sb.ToString());

Solves your posted example, but I imagine your real requirement is going to be subtly more complicated.

edited Jan 13, 2014 at 21:14

answered Jan 13, 2014 at 20:32

RJ Lohan

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2 Comments

Tim S. Over a year ago

This leaves out the last token. (in this case, the . at the end)

RJ Lohan Over a year ago

Corrected (but not tested)

Wagner DosAnjos · Accepted Answer · 2014-01-13 20:50:43Z

2

You can use a regular expression match evaluator as follows:

var re = new Regex(@"\{\}");

string s = "This sample will count: {}, {}, {}.";

string[] replacementStrings = new string[]{"r1", "r2", "r3"};

int i = 0;

string n = re.Replace(s, m => 
    {
        return replacementStrings[i++];
    }); 

Console.WriteLine(n); // outputs: This sample will count: r1, r2, r3.

answered Jan 13, 2014 at 20:50

Wagner DosAnjos

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4 Comments

Seth Over a year ago

Thanks, though I won't have the option to hard code the number of replacement strings.

Wagner DosAnjos Over a year ago

Please elaborate. The above solution is based on your EDIT regarding s.replace("{}", stringArray[i]);

Seth Over a year ago

Your code takes into account there are specifically three bracket pairs. Otherwise it functions quite well.

Magus Over a year ago

@Seth: This solution will work regardless of the value of replacementStrings. That just needs to be a parameter. It definitely does not constrain it to only 3.

Haney · Accepted Answer · 2014-01-13 20:32:35Z

1

If performance is important to you, traverse the string manually and write its contents to a StringBuilder (to prevent tons of string instance creations while concatenating and replacing strings) and then find and replace instances of the {}. string.IndexOf would be good to use for this. Once you're done, cache the result and output the string builder via StringBuilder.ToString()

answered Jan 13, 2014 at 20:32

Haney

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Comments

p.s.w.g · Accepted Answer · 2014-01-13 21:10:00Z

0

You could do this with plain old string manupulation:

string s = "This sample will count: {}, {}, {}.";
string[] stringArray = new[] { "1", "2", "3" };
int i = 0, c = s.IndexOf("{}");
while(c >= 0)
{
    s = s.Remove(c) + stringArray[i++] + s.Substring(c + 2);  
    c = s.IndexOf("{}");
}
// s == "This sample will count: 1, 2, 3."

Or using regular expressions:

string s = "This sample will count: {}, {}, {}.";
string[] stringArray = new[] { "1", "2", "3" };
int i = 0;
s = Regex.Replace(s, "{}", m => stringArray[i++]);
// s == "This sample will count: 1, 2, 3."

Or using Linq:

string s = "This sample will count: {}, {}, {}.";
string[] stringArray = new[] { "1", "2", "3" };
s = String.Join("", s.Split(new[]{ "{}" }, StringSplitOptions.None)
                     .Select((x, i) => i < stringArray.Length ? x + stringArray[i] : x));
// s == "This sample will count: 1, 2, 3."

edited Jan 13, 2014 at 21:10

answered Jan 13, 2014 at 20:27

p.s.w.g

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6 Comments

Haney Over a year ago

These examples will have poor performance due to the number of strings being created.

BartoszKP Over a year ago

@DavidHaney Poor performance as compared to what? Do you have any proof actually that this "loss" of performance will be meaningful?

Haney Over a year ago

Yes, the strings will be instantiated, and thus need to be garbage collected. You'll create a lot more Gen 0 or 1, possibly 2 GC events, which will slow down your application. StringBuilder exists SOLELY to circumvent the overhead of creating string instances as by-products of operations that repeatedly manipulate them.

Haney Over a year ago

MSDN mentions performance issues, unfortunately without much detail, here: msdn.microsoft.com/en-us/library/ms228504.aspx

Wagner DosAnjos Over a year ago

Regex.Replace(string, MatchEvaluator) method uses StringBuilder internally to build the resulting string.

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