I would assume this is a common question but I couldn't seem to find anything on SO or google.
Is it possible to only format a single argument. For example, format string foo = "{0} is {1} when {2}"; so that it returns read "{0} is cray when {2}"?
Intentions:
I am trying to format the string while overriding a method before it gets formatted in it's base method
Success
Got it thanks to this answer, all answers were helpful :).
This unit test worked:
string foo = String.Format("{0} is {1} when {2}", "{0}", "cray", "{2}");
Assert.AreEqual("{0} is cray when {2}", foo);
string bar = string.Format(foo, "this", null, "it works");
Assert.AreEqual("this is cray when it works", bar);
Taking your question at face value, I suppose you could do the following:
string foo = String.Format("{0} is {1} when {2}", "{0}", "cray", "{2}");
That is, simply replace each unevaluated format item with itself.
No, this is not possible. String.Format is going to try and replace every bracketed placeholder. If you do not supply the correct number of arguments, an exception will be raised.
I'm not sure why you are trying to do this, but if you want the output to look like that, you'll have to escape the brackets:
var foo = String.Format("{{0}} is {0} when {{1}}", "cray");
// foo is "{0} is cray when {1}"
Perhaps if you tell us what exactly you're trying to do, we'll be able to better help you.
Related
I have looked about everywhere (exaggeration) before I started asking the question and have come up with nothing, I think the question may be confusing but I am trying to pass the variable i to the parameters in the WriteLine(..) method string, here is an example;
Expression<Func<int, int, int>> expression = (a, b) => a + b;
for(int i = 0; i < expression.Parameters.Count; i++)
{
Console.WriteLine("Expression param[{i+1}]: {i}", expression.Parameters[i]);
}
Is this valid in c#? to add the int i into the Console.WriteLine(..) method.
I have also tried:
Console.WriteLine("Expression param[{" + i+1 +"}]: {"+ i +"}", expression.Parameters[i]);
It work's
Console.WriteLine("Expression param[{0}]: {1}", i+1, expression.Parameters[i]);
But if you are using C# 6, string interpolation is better:
Console.WriteLine($"Expression param[{i+1}]: {expression.Parameters[i]}");
String interpolation lets you more easily format strings. String.Format and its cousins are very versatile, but their use is somewhat clunky and error prone. Particularly unfortunate is the use of numbered placeholders like {0} in the format string, which must line up with separately supplied arguments.
ref: https://blogs.msdn.microsoft.com/csharpfaq/2014/11/20/new-features-in-c-6/
Use it in this way:
Console.WriteLine("Expression param[{0}]: {1}", i, expression.Parameters[i]);
{0} refers to the first argument after the format (i).
{1} refers to the second argument after the format (expression.Parameters[i]).
And so on.
I keep running into a problem with my sql:
{
Name.Text = String.Format("{0} {1}", reader.GetString(0), reader.GetString(1));
Aboutme.Text = String.Format("{2}", reader.GetString(0));
}
Index (zero based) must be greater than or equal to zero?
Not sure if its my String.Format?
Yes, the second line should be again 0 for string.Format:
Aboutme.Text = String.Format("{0}", reader.GetString(0));
My guess would be the second line of code
Aboutme.Text = String.Format("{2}", reader.GetString(0));
The String.Format, is looking for 3 parameters in that case, {0}, {1} and finally {2}. You have only one.
You should write it has :
Aboutme.Text = String.Format("{0}", reader.GetString(0));
The problem isn't the index for SQL, it's the index for string.Format. This should be evident in the stack trace, which will point to string.Format rather than reader.GetString. Looking carefully at stack traces can save you all manner of effort :)
Of course you don't really need to format at all here (given that you'll just end up with the input string), and I suspect you want column 2. (My guess is that that's where the error came from, after all.)
Aboutme.Text = reader.GetString(2);
If you really want to use string.Format:
Aboutme.Text = string.Format("{0}", reader.GetString(2));
There is a better solution than that proposed by the accepted answer. Note that the statement ...
Aboutme.Text = String.Format("{0}", reader.GetString(0));
... passes the format string and some string value to String.Format, which analyzes the format string, calls ToString() on the string value, and then inserts that value into the result at the position indicated by the sequence {0}. Then, of course, it returns the constructed string as its result. Because the format string is "{0}", the constructed string will have the same value as the argument.
Both the written code and the executed algorithm will be simpler if you write this:
Aboutme.Text = reader.GetString(0);
I m curious why would i use string formatting while i can use concatenation such as
Console.WriteLine("Hello {0} !", name);
Console.WriteLine("Hello "+ name + " !");
Why to prefer the first one over second?
You picked too simple of an example.
String formatting:
allows you to use the same variable multiple times: ("{0} + {0} = {1}", x, 2*x)
automatically calls ToString on its arguments: ("{0}: {1}", someKeyObj, someValueObj)
allows you to specify formatting: ("The value will be {0:3N} (or {1:P}) on {2:MMMM yyyy gg}", x, y, theDate)
allows you to set padding easily: (">{0,3}<", "hi"); // ">hi <"
You can trade the string for a dynamic string later.
For example:
// In a land far, far away
string nameFormat = "{0} {1}";
// In our function
string firstName = "John";
string lastName = "Doe";
Console.WriteLine(nameFormat, firstName, lastName);
Here, you can change nameFormat to e.g. "{1}, {0}" and you don't need to change any of your other code. With concatination, you would need to edit your code or possibly duplicate your code to handle both cases.
This is useful in localization/internationalization.
There isn't a singular correct answer to this question. There are a few issues you want to address:
Performance
The performance differences in your examples (and in real apps) are minimal. If you start writing MANY concatenations, you will gradually see better memory performance with the formatted string. Refer to Ben's answer
Readability
You will be better off with a formatted string when you have formatting, or have many different variables to stringify:
string formatString = "Hello {0}, today is {1:yyyy-MM-dd}";
Console.WriteLine(formatString, userName, Date.Today);
Extensibility
Your situation will determine what's best. You tell me which is better when you need to add an item between Username and Time in the log:
Console.WriteLine(
#"Error!
Username: " + userName + "
Time: " + time.ToString("HH:mm:ss") + "
Function: " + functionName + "
Action: " + actionName + "
status: " + status + "
---");
or
Console.WriteLine(#"Error!
Username: {0}
Time: {1}
Function: {2}
Action: {3}
status: {4}
---",
username, time.ToString("HH:mm:ss"), functionName, actionName, status);
Conclusion
I would choose the formatted string most of the time... But I wouldn't hesitate at all to use concatenation when it was easier.
I think the main thing here is readability. So I always choose for what have the best readability for each case.
Note:
With string interpolation of C# 6 your code could be simplified to this:
Console.WriteLine($"Hello {name}!");
Which I think better than your two suggested options.
String formatting allows you to keep the format string separate, and use it where it's needed properly without having to worry about concatenation.
string greeting = "Hello {0}!";
Console.WriteLine(greeting, name);
As for why you would use it in the exact example you gave... force of habit, really.
I think a good example is about i18n and l10n
If you have to change a string between different languages, this: "bla "+variable+"bla bla.."
Will give problems to a program used to create sobstitution for your strings if you use a different language
while in this way: "bla {0} blabla" is easily convertible (you will get {0} as part of the string)
Formatting is usually preferred for most of the reasons explained by other members here. There are couple more reasons I want to throw in from my short programming experience:
Formatting will help in generating Culture aware strings.
Formatting is more performant than concatenation. Remember, every concatenation operation will involve creation of temporary intermediate strings. If you have a bunch of strings you need to concatenate, you are better off using String.Join or StringBuilder.
You are using a trivial example where there is not much of a difference. However, if you have a long string with many parameters it is much nicer to be able to use a format string instead of many, many + signs and line breaks.
It also allows you to format numerical data as you wish, i.e., currency, dates, etc.
The first format is recommended. It allows you to specify specific formats for other types, like displaying a hex value, or displaying some specific string format.
e.g.
string displayInHex = String.Format("{0,10:X}", value); // to display in hex
It is also more consistent. You can use the same convention to display your Debug statement.
e.g.
Debug.WriteLine (String.Format("{0,10:X}", value));
Last but not least, it helps in the localisation of your program.
In addition to reasons like Ignacio's, many (including me) find String.Format-based code much easier to read and alter.
string x = String.Format(
"This file was last modified on {0} at {1} by {2}, and was last backed up {3}."
date, time, user, backupDate);
vs.
string x = "This file was last modified on " + date + " at "
+ time + " by " + user + " and was last backed up " + backupDate + ".";
I have found the former approach (using string.format) very useful when overriding ToString() methods in Entity classes.
For example, in my Product class;
public override string ToString()
{
return string.format("{0} : {1} ({2} / {3} / {4}",
this.id,
this.description,
this.brand,
this.model);
}
Then, when users decide they want the product to appear differently it's easy to change the order/contents or layout of the string that is returned.
Of course you still concatentate this string together but I feel string.Format makes the whole thing a bit more readable and maintainable.
I guess that's the short answer I'm giving then isn't it - readability and maintainability for lengthy or complex strings.
Since C# 6 release a few years ago, it is also possible to perform string interpolation. Per example this:
var n = 12.5847;
System.Console.WriteLine(string.Format("Hello World! {0:C2}", n));
Becomes that:
var n = 12.5847;
System.Console.WriteLine($"Hello World! {n:C2}");
And both give you this result:
Hello World! £12.58
How can I format a string like this:
string X = "'{0}','{1}','{2}','{3}','{4}','{5}','{6}','{7}','{8}','{9}','{10}','{11}','{12}','{13}'",????
I remember I used to be able to put a comma at the end and specify the actual data to assign to {0},{1}, etc.
Any help?
Use string.Format method such as in:
string X = string.Format("'{0}','{1}','{2}'", foo, bar, baz);
An alternative is to use Join, if you have the values in a string array:
string x = "'" + String.Join("','", valueArray) + "'";
(Just wanted to be different from the 89724362 users who will show you how to use String.Format... ;)
String.Format("'{0}', '{1}'", arg0, arg1);
The String.Format method accepts a format string followed by one to many variables that are to be formatted. The format string consists of placeholders, which are essentially locations to place the value of the variables you pass into the function.
Console.WriteLine(String.Format("{0}, {1}, {2}", var1, var2, var3));
Your question is a bit vague, but do you mean:
// declare and set variables val1 and val2 up here somewhere
string X = string.Format("'{0}','{1}'", val1, val2);
Or are you asking for something else?
use string.format, and put individual format specifiers inside the braces, after the number and a colon, as in
string s = string.Format(
" {0:d MMM yyyy} --- {1:000} --- {2:#,##0.0} -- {3:f}",
DateTime.Now, 1, 12345.678, 3e-6);
and, as you can see from the example, you don;t need the single quotes to delineate literals, anything not inside braces will be output literally
are you looking for:
String.Format("String with data {0}, {1} I wish to format", "Foo", "Bar");
would result in
"String with data Foo, Bar I wish to format"
Use string.Format as in
var output = string.Format("'{0}', '{1}'", x, y);
Before using String.Format to format a string in C#, I would like to know how many parameters does that string accept?
For eg. if the string was "{0} is not the same as {1}", I would like to know that this string accepts two parameters
For eg. if the string was "{0} is not the same as {1} and {2}", the string accepts 3 parameters
How can I find this efficiently?
String.Format receives a string argument with format value, and an params object[] array, which can deal with an arbitrary large value items.
For every object value, it's .ToString() method will be called to resolve that format pattern
EDIT: Seems I misread your question. If you want to know how many arguments are required to your format, you can discover that by using a regular expression:
string pattern = "{0} {1:00} {{2}}, Failure: {0}{{{1}}}, Failure: {0} ({0})";
int count = Regex.Matches(Regex.Replace(pattern,
#"(\{{2}|\}{2})", ""), // removes escaped curly brackets
#"\{\d+(?:\:?[^}]*)\}").Count; // returns 6
As Benjamin noted in comments, maybe you do need to know number of different references. If you don't using Linq, here you go:
int count = Regex.Matches(Regex.Replace(pattern,
#"(\{{2}|\}{2})", ""), // removes escaped curly brackets
#"\{(\d+)(?:\:?[^}]*)\}").OfType<Match>()
.SelectMany(match => match.Groups.OfType<Group>().Skip(1))
.Select(index => Int32.Parse(index.Value))
.Max() + 1; // returns 2
This also address #280Z28 last problem spotted.
Edit by 280Z28: This will not validate the input, but for any valid input will give the correct answer:
int count2 =
Regex.Matches(
pattern.Replace("{{", string.Empty),
#"\{(\d+)")
.OfType<Match>()
.Select(match => int.Parse(match.Groups[1].Value))
.Union(Enumerable.Repeat(-1, 1))
.Max() + 1;
You'll have to parse through the string and find the highest integer value between the {}'s...then add one.
...or count the number of sets of {}'s.
Either way, it's ugly. I'd be interested to know why you need to be able to figure out this number programatically.
EDIT
As 280Z28 mentioned, you'll have to account for the various idiosyncrasies of what can be included between the {}'s (multiple {}'s, formatting strings, etc.).
I rely on ReSharper to analyze that for me, and it is a pity that Visual Studio does not ship with such a neat feature.