13 Strings and Text Processing

Strings and Text Processing
Processing and Manipulating Text
Using the .NET String Class
SoftUni Team
Technical Trainers
Software University
http://softuni.bg

Table of Contents
1. What is a String?
2. Manipulating Strings
 Comparing, Concatenating, Searching
 Extracting Substrings, Splitting
3. Building and Modifying Strings
 Why Is the + Operator Slow?
 Using the StringBuilder Class
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Questions
sli.do
#fund-softuni

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 Strings are sequences of characters (texts)
 The string data type in C#
 Declared by the string keyword
 Maps to System.String .NET data type
 Strings are enclosed in quotes:
 Concatenated using the "+" operator:
Strings
string s = "Hello, C#";
string s = "Hello" + " " + "C#";

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 Strings are immutable (read-only) sequences of characters
 Accessible by index (read-only)
 Strings use Unicode (can use most alphabets, e.g. Arabic)
In C# Strings are Immutable, use Unicode
string str =
"Hello, C#";
let ch = str[2]; // OK
str[2] = 'a'; // Error!
string greeting = "ْ‫م‬ُ‫ك‬ْ‫ي‬‫ا‬‫ل‬‫ا‬‫ع‬ ُ‫م‬‫ا‬‫َل‬َّ‫;"الس‬ // As-salamu alaykum
0 1 2 3 4 5 6 7 8
H e l l o , C #
index =
str[index] =

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 Initializing from a string literal:
 Reading a string from the console:
 Converting a string from and to a char array:
Initializing a String
string str = "Hello, C#";
string name = Console.ReadLine();
Console.WriteLine("Hi, " + name);
string str = new String(new char[] {'s', 't', 'r'});
char[] charArr = str.ToCharArray(); // ['s', 't', 'r']
0 1 2 3 4 5 6 7 8
H e l l o , C #

Manipulating Strings
Comparing, Concatenating, Searching,
Extracting Substrings, Splitting

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 Ordinal (exact binary) string comparison
 Case-insensitive string comparison
 Case-sensitive string comparison
Comparing Strings
int result = string.Compare(str1, str2, true);
// result == 0 if str1 equals str2
// result < 0 if str1 is before str2
// result > 0 if str1 is after str2
int result = string.Compare(str1, str2, false);
int eq = (str1 == str2); // uses String.Equals(…)

Concatenating (Combining) Strings
 Use the Concat() method
 Use the + or the += operators
 Any object can be appended to a string
string str = string.Concat(str1, str2);
string str = str1 + str2 + str3;
string str += str1;
string name = "Peter"; int age = 22;
string s = name + " " + age; //  "Peter 22"
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 Finding a substring within a given string
 str.IndexOf(string term) – returns the first index or -1
 str.LastIndexOf(string term) – finds the last occurence
Searching in Strings
string email = "vasko@gmail.org";
int firstIndex = email.IndexOf("@"); // 5
int secondIndex = email.IndexOf("a", 2); // 8
int notFound = email.IndexOf("/"); // -1
string verse = "To be or not to be…";
int lastIndex = verse.LastIndexOf("be"); // 16

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Problem: Count Substring Occurrences
 You are given a text and a pattern
 Find how many times that pattern occurs in the text
 Overlapping is allowed
Welcome to SoftUni
Java
0
ababa caba
aba
3
aaaaaa
aa
5
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/320#1

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Solution: Count Substring Occurrences
string input = Console.ReadLine().ToLower();
string pattern = Console.ReadLine().ToLower();
int counter = 0;
int index = input.IndexOf(pattern);
while (index != -1)
{
counter++;
index = input.IndexOf(pattern, index + 1)
}
Console.WriteLine(counter);

Extracting Substrings
 str.Substring(int startIndex, int length)
 str.Substring(int startIndex)
string filename = @"C:PicsRila2017.jpg";
string name = filename.Substring(8, 8);
// name == "Rila2017"
string filename = @"C:PicsRila2017.jpg";
string nameAndExtension = filename.Substring(8);
// nameAndExtension == "Rila2017.jpg"
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
C : P i c s R i l a 2 0 1 7 . j p g
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Splitting Strings
 Split a string by given separator(s) :
 Example:
string[] Split(params char[] separator)
string listOfBeers = "Amstel, Zagorka, Tuborg, Becks.";
string[] beers = listOfBeers.Split(' ', ',', '.');
Console.WriteLine("Available beers are:");
foreach (string beer in beers)
Console.WriteLine(beer);
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Other String Operations
Replacing and Deleting Substrings,
Changing Character Casing, Trimming

Replacing and Deleting Substrings
 str.Replace(match, replacement) – replaces all occurrences
 The result is a new string (strings are immutable)
 str.Remove(int index, int length) – deletes part of a string
 Produces a new string as result
string cocktail = "Vodka + Martini + Cherry";
string replaced = cocktail.Replace("+", "and");
// Vodka and Martini and Cherry
string price = "$ 1234567";
string lowPrice = price.Remove(2, 3);
// $ 4567
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Problem: Text Filter (Banned Words)
 You are given a text and a string of banned words
 Replace all banned words in the text with asterisks
 Replace with asterisks (*), whose count is equal to the word's length
Linux, Windows
It is not Linux, it is GNU/Linux. Linux is merely
the kernel, while GNU adds the functionality...
Check your solution here: https://judge.softuni.bg/Contests/Practice/Index/320#2
It is not *****, it is GNU/*****. ***** is merely
the kernel, while GNU adds the functionality...

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Solution: Text Filter
string[] banWords = Console.ReadLine()
.Split(…); // TODO: add separators
string text = Console.ReadLine();
foreach (var banWord in banWords)
{
if (text.Contains(banWord))
{
text = text.Replace(banWord,
new string('*', banWord.Length));
}
}
Console.WriteLine(text);
Contains(…) checks
if string contains
another string
Replace a word with a sequence
of asterisks of the same length

Changing Character Casing
 Using the method ToLower()
 Using the method ToUpper()
string alpha = "aBcDeFg";
string lowerAlpha = alpha.ToLower(); // abcdefg
Console.WriteLine(lowerAlpha);
string alpha = "aBcDeFg";
string upperAlpha = alpha.ToUpper(); // ABCDEFG
Console.WriteLine(upperAlpha);
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 str.Trim() – trims whitespaces at start and end of string
 str.Trim(params char[] chars)
 str.TrimStart() and str.TrimEnd()
Trimming White Space
string s = " example of white space ";
string clean = s.Trim();
Console.WriteLine(clean); // example of white space
string s = " tnHello!!! n";
string clean = s.Trim(' ', ',' ,'!', 'n','t');
Console.WriteLine(clean); // Hello
string s = " C# ";
string clean = s.TrimStart(); // clean = "C# "

Live Exercises in Class (Lab)
String Operations

Building and Modifying Strings
Using the StringBuilder Class

StringBuilder: How It Works?
 StringBuilder keeps a buffer space, allocated in advance
 Do not allocate memory for most operations  performance
H e l l o , C # !StringBuilder:
Length = 9
Capacity = 15
Capacity
used buffer
(Length)
unused
buffer
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 Use the System.Text.StringBuilder to build / modify strings:
Changing the Contents of a String
public static string ReverseString(string str)
{
StringBuilder sb = new StringBuilder();
for (int i = str.Length - 1; i >= 0; i--)
{
sb.Append(str[i]);
}
return sb.ToString();
}

The StringBuilder Class
 StringBuilder(int capacity) constructor allocates in
advance buffer of size capacity
 Capacity holds the currently allocated space (in characters)
 Length holds the length of the string in the buffer
 this[int index] (indexer) access the char at given position
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H e l l o , C # !
Capacity
used buffer (Length) unused buffer

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StringBuilder Operations – Examples
var builder = new StringBuilder(100);
builder.Append("Hello Maria, how are you?");
Console.WriteLine(builder); // Hello Maria, how are you?
builder[6] = 'D';
Console.WriteLine(builder); // Hello Daria, how are you?
builder.Remove(5, 6);
Console.WriteLine(builder); // Hello, how are you?
builder.Insert(5, " Peter");
Console.WriteLine(builder); // Hello Peter, how are you?
builder.Replace("Peter", "George");
Console.WriteLine(builder); // Hello George, how are you?

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Problem: String Concatenation
 Given the code below try to optimize it to go under a second
 Do not change the loop or the Convert.ToString() method
var timer = new Stopwatch();
timer.Start();
string result = "";
for (int i = 0; i < 50000; i++)
result += Convert.ToString(i, 2);
Console.WriteLine(result.Length);
Console.WriteLine(timer.Elapsed);

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Solution: String Concatenation
var timer = new Stopwatch();
timer.Start();
var result = new StringBuilder();
for (int i = 0; i < 50000; i++)
result.Append(Convert.ToString(i, 2));
Console.WriteLine(result.Length);
Console.WriteLine(timer.Elapsed);

Summary
 Strings are immutable sequences of Unicode characters
 String processing methods
 IndexOf(), Compare(), Substring(),
Remove(), ToUpper / ToLower(), Replace(), …
 StringBuilder efficiently builds / modifies strings
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var result = new StringBuilder();
for (int i = 0; i < 500000; i++) result.Append(i.ToString());
string str = "Hello, C#";
str[2] = 'a'; // Error!

?
Strings and Text Processing
https://softuni.bg/courses/programming-fundamentals

License
 This course (slides, examples, demos, videos, homework, etc.)
is licensed under the "Creative Commons Attribution-
NonCommercial-ShareAlike 4.0 International" license
 Attribution: this work may contain portions from
 "Fundamentals of Computer Programming with C#" book by Svetlin Nakov & Co. under CC-BY-SA license
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 Software University Forums – forum.softuni.bg

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