This question already has an answer here:
String Concatenation using '+' operator
(1 answer)
Closed 8 years ago.
tell me pls what's problem with this code C#.
string str = string.Empty;
for (var i = 1; i <= 1000; i++)
str += i.ToString();
This was interview question.
actually there is no problem with your code.
inthis case StringBuilder is more appropriate than string.
because StringBuilder is mutable whereas string is immutable.
so whenever you modify the String object using += it creates a new string object so at the end of your loop it creates many string objects.
but if you use StringBuilder: same object will be modified each time you Append the Strings to it.
You can find more info from MSDN: StringBuilder Class
The String object is immutable. Every time you use one of the methods
in the System.String class, you create a new string object in memory,
which requires a new allocation of space for that new object. In
situations where you need to perform repeated modifications to a
string, the overhead associated with creating a new String object can
be costly. The System.Text.StringBuilder class can be used when you
want to modify a string without creating a new object. For example,
using the StringBuilder class can boost performance when concatenating
many strings together in a loop.
Solution :
This
string str = string.Empty;
for (var i = 1; i <= 1000; i++)
str += i.ToString();
Shouldbe this
StringBuilder str =new StringBuilder();
for (var i = 1; i <= 1000; i++)
str.Append(i.ToString());
There is an answer here.
the compiler can't do anything if you concatenate in a loop and this does generate a lot of garbage.
Related
This question already has answers here:
Is there an easy way to change a char in a string in C#?
(8 answers)
Closed 5 years ago.
This is kind of a basic question, but I learned programming in C++ and am just transitioning to C#, so my ignorance of the C# methods are getting in my way.
A client has given me a few fixed length files and they want the 484th character of every odd numbered record, skipping the first one (3, 5, 7, etc...) changed from a space to a 0. In my mind, I should be able to do something like the below:
static void Main(string[] args)
{
List<string> allLines = System.IO.File.ReadAllLines(#"C:\...").ToList();
foreach(string line in allLines)
{
//odd numbered logic here
line[483] = '0';
}
...
//write to new file
}
However, the property or indexer cannot be assigned to because it is read only. All my reading says that I have not set a setter for the variable, and I have tried what was shown at this SO article, but I am doing something wrong every time. Should what is shown in that article work? Should I do something else?
You cannot modify C# strings directly, because they are immutable. You can convert strings to char[], modify it, then make a string again, and write it to file:
File.WriteAllLines(
#"c:\newfile.txt"
, File.ReadAllLines(#"C:\...").Select((s, index) => {
if (index % 2 = 0) {
return s; // Even strings do not change
}
var chars = s.ToCharArray();
chars[483] = '0';
return new string(chars);
})
);
Since strings are immutable, you can't modify a single character by treating it as a char[] and then modify a character at a specific index. However, you can "modify" it by assigning it to a new string.
We can use the Substring() method to return any part of the original string. Combining this with some concatenation, we can take the first part of the string (up to the character you want to replace), add the new character, and then add the rest of the original string.
Also, since we can't directly modify the items in a collection being iterated over in a foreach loop, we can switch your loop to a for loop instead. Now we can access each line by index, and can modify them on the fly:
for(int i = 0; i < allLines.Length; i++)
{
if (allLines[i].Length > 483)
{
allLines[i] = allLines[i].Substring(0, 483) + "0" + allLines[i].Substring(484);
}
}
It's possible that, depending on how many lines you're processing and how many in-line concatenations you end up doing, there is some chance that using a StringBuilder instead of concatenation will perform better. Here is an alternate way to do this using a StringBuilder. I'll leave the perf measuring to you...
var sb = new StringBuilder();
for (int i = 0; i < allLines.Length; i++)
{
if (allLines[i].Length > 483)
{
sb.Clear();
sb.Append(allLines[i].Substring(0, 483));
sb.Append("0");
sb.Append(allLines[i].Substring(484));
allLines[i] = sb.ToString();
}
}
The first item after the foreach (string line in this case) is a local variable that has no scope outside the loop - that’s why you can’t assign a value to it. Try using a regular for loop instead.
Purpose of for each is meant to iterate over a container. It's read only in nature. You should use regular for loop. It will work.
static void Main(string[] args)
{
List<string> allLines = System.IO.File.ReadAllLines(#"C:\...").ToList();
for (int i=0;i<=allLines.Length;++i)
{
if (allLines[i].Length > 483)
{
allLines[i] = allLines[i].Substring(0, 483) + "0";
}
}
...
//write to new file
}
This question already has answers here:
Most efficient way to concatenate strings?
(18 answers)
Make String concatenation faster in C# [duplicate]
(6 answers)
Closed 5 years ago.
What I had tried till Now
string Value ="";
foreach (List<string> val in L1)
{
Value = Value + string.Join(",", val) + " // ";
}
Where L1 is of datatype List <List<strings>>
This Works, But its take almost n half hour to complete
Is there as many fastest and simple way to achieve this.
I'd suggest use StringBuilder instead of concatenations in a loop like that:
StringBuilder builder = new StringBuilder();
foreach (List<string> val in L1)
{
builder.Append(string.Join(",", val) + " // ");
}
string result = builder.ToString();
When concatenating in a loop it needs to copy the string everytime to a new position in memory with the extra allocated memory. StringBuilder prevents that.
You can also refer to:
How to use StringBuilder wisely
How does StringBuilder work?
How the StringBuilder class is implemented? Does it internally create new string objects each time we append?
Still new to the extension methods. According to this guide, it is still unclear to me how I can append an array just like appending a string with Append(). Can anyone provide me with a small example?
int a = 2;
object obj = XXXXX;
StringBuilder sb = new StringBuilder();
sb.Append(a);
sb.Append(obj);
Basically the type of obj is unknown it could be int[], char[]..., I am trying to reuse Append() to make it more generalized to handle array types so the
sb.ToString()
will output the whole thing I have appended
I didn't get the problem - I was trying to do a simple action:
for(i = x.Length-1, j = 0 ; i >= 0 ; i--, j++)
{
backx[j] = x[i];
}
Both are declared:
String x;
String backx;
What is the problem ? It says the error in the title...
If there is a problem - is there another way to do that?
The result (As the name 'backx' hints) is that backx will contain the string X backwards.
P.S. x is not empty - it contains a substring from another string.
Strings are immutable: you can retrieve the character at a certain position, but you cannot change the character to a new one directly.
Instead you'll have to build a new string with the change. There are several ways to do this, but StringBuilder does the job in a similar fashion to what you already have:
StringBuilder sb = new StringBuilder(backx);
sb[j] = x[i];
backx = sb.ToString();
EDIT: If you take a look at the string public facing API, you'll see this indexer:
public char this[int index] { get; }
This shows that you can "get" a value, but because no "set" is available, you cannot assign values to that indexer.
EDITx2: If you're looking for a way to reverse a string, there are a few different ways, but here's one example with an explanation as to how it works: http://www.dotnetperls.com/reverse-string
String is immutable in .NET - this is why you get the error.
You can get a reverse string with LINQ:
string x = "abcd";
string backx = new string(x.Reverse().ToArray());
Console.WriteLine(backx); // output: "dcba"
String are immuatable. You have convert to Char Array and then you would be able to modify.
Or you can use StringBuilder.
for example
char[] wordArray = word.ToCharArray();
In C# strings are immutable. You cannot "set" Xth character to whatever you want. If yo uwant to construct a new string, or be able to "edit" a string, use i.e. StringBuilder class.
Strings are immutable in C#. You can read more about it here: http://msdn.microsoft.com/en-us/library/362314fe.aspx
Both the variables you have are string while you are treating them as if they were arrays (well, they are). Of course it is a valid statement to access characters from a string through this mechanism, you cannot really assign it that way.
Since you are trying to reverse a string, do take a look at this post. It has lot of information.
public static string ReverseName( string theName)
{
string revName = string.Empty;
foreach (char a in theName)
{
revName = a + revName;
}
return revName;
}
This is simple and does not involve arrays directly.
The code below simply swaps the index of each char in the string which enables you to only have to iterate half way through the original string which is pretty efficient if you're dealing with a lot of characters. The result is the original string reversed. I tested this with a string consisting of 100 characters and it executed in 0.0000021 seconds.
private string ReverseString(string testString)
{
int j = testString.Length - 1;
char[] charArray = new char[testString.Length];
for (int i = 0; i <= j; i++)
{
if (i != j)
{
charArray[i] = testString[j];
charArray[j] = testString[i];
}
j--;
}
return new string(charArray);
}
In case you need to replace e.g. index 2 in string use this (it is ugly, but working and is easily maintainbable)
V1 - you know what you want to put their. Here you saying in pseudocode string[2] = 'R';
row3String.Replace(row3String[2], 'R');
V2 - you need to put their char R or char Y. Here string[2] = 'R' if was 'Y' or if was not stay 'Y' (this one line if needs some form of else)
row3String.Replace(row3String[2], row3String[2].Equals('Y') ? 'R' : 'Y');
All,
For the string string s = "abcd", does string w = s.SubString(2) return a new allocated String object i.e. string w = new String ("cd") internally or a String literal?
For StringBuilder, when appending string values and if the size of the StringBuilder needs to be increased, are all the contents copied over to a new memory location or simply the pointers to each of the earlier String value are reassigned to the new location?
String is immutable, so any operation that "changes" the string, will in effect return a new string. This includes SubString and all other operations on String, including those that does not change the length (such as ToLower() or similar).
StringBuilder contains internally a linked list of chunks of characters. When it needs to grow, a new chunk is allocated and inserted at the end of the list, and data is copied here. In other words, the whole StringBuilder buffer will not be copied on an append, only the data you are appending. I double-checked this against the Framework 4 reference sources.
For the string string s = "abcd", does string w = s.SubString(2) return a new allocated String object? Yes
For StringBuilder, when appending string values and if the size of the StringBuilder needs to be increased, are all the contents copied over to a new memory location? Yes
Any change in String small or large results in a new String
If you are going to make large numbers of edits to a string it better to do this via StringBuilder.
From MSDN:
You can use the StringBuilder class instead of the String class for operations that make multiple changes to the value of a string. Unlike instances of the String class, StringBuilder objects are mutable; when you concatenate, append, or delete substrings from a string, the operations are performed on a single string.
Strings are immutable objects so every time you had to make changes you create a new instance of that string. The substring method does not change the value of the original string.
Regards.
Difference between the String and StringBuilder is an important concept which makes the difference when an application has to deal with the editing of a high number of Strings.
String
The String object is a collection of UTF-16 code units represented by a System.Char object which belong to the System namespace. Since the value of this objects are read-only, the entire object String has defined as immutable. The maximum size of a String object in memory is 2 GB, or about 1 billion characters.
Immutable
Being immutable means that every time a methods of the System.String is used, a new sting object is created in memory and this cause a new allocation of space for the new object.
Example:
By using the string concatenation operator += appears that the value of the string variable named test change. In fact, it create a new String object, which has a different value and address from the original and assign it to the test variable.
string test;
test += "red"; // a new object string is created
test += "coding"; // a new object string is created
test += "planet"; // a new object string is created
StringBuilder
The StringBuilder is a dynamic object which belong to the System.Text namespace and allow to modify the number of characters in the string that it encapsulates, this characteristic is called mutability.
Mutability
To be able to append, remove, replace or insert characters, A StringBuilder maintains a buffer to accommodate expansions to the string. If new data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.
StringBuilder sb = new StringBuilder("");
sb.Append("red");
sb.Append("blue");
sb.Append("green ");
string colors = sb.ToString();
Performances
In order to help you better understand the performance difference between String and StringBuilder, I created the following example:
Stopwatch timer = new Stopwatch();
string str = string.Empty;
timer.Start();
for (int i = 0; i < 10000; i++) {
str += i.ToString();
}
timer.Stop();
Console.WriteLine("String : {0}", timer.Elapsed);
timer.Restart();
StringBuilder sbr = new StringBuilder(string.Empty);
for (int i = 0; i < 10000; i++) {
sbr.Append(i.ToString());
}
timer.Stop();
Console.WriteLine("StringBuilder : {0}", timer.Elapsed);
The output is
Output
String : 00:00:00.0706661
StringBuilder : 00:00:00.0012373