Aprenda Frequency Analysis | Text Analysis & Simple Ciphers

Frequency analysis is a technique used to determine how often different elements appear within a piece of text. In the context of strings, this usually means counting how many times each character or word occurs. This method is widely applied in cryptography, where analyzing the frequency of letters can help break simple ciphers, and in data analysis, where understanding word usage can reveal patterns or trends in large bodies of text.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string sentence = "Frequency analysis reveals patterns.";
            Dictionary<char, int> letterCounts = new Dictionary<char, int>();

            foreach (char c in sentence)
            {
                if (char.IsLetter(c))
                {
                    char lower = char.ToLower(c);
                    if (letterCounts.ContainsKey(lower))
                    {
                        letterCounts[lower]++;
                    }
                    else
                    {
                        letterCounts[lower] = 1;
                    }
                }
            }

            Console.WriteLine("Letter frequencies:");
            foreach (var pair in letterCounts)
            {
                Console.WriteLine($"'{pair.Key}': {pair.Value}");
            }
        }
    }
}

To perform frequency analysis, you need a way to store the number of times each letter or word appears. Arrays can be used for simple cases, like counting letters from 'a' to 'z', but dictionaries (Dictionary<char, int> or Dictionary<string, int>) are more flexible and powerful. The process involves iterating through the string, examining each character or word, and updating the count in your chosen data structure.

Program.cs


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using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string paragraph = "This is a test. This test is only a test.";
            Dictionary<string, int> wordCounts = new Dictionary<string, int>();
            string[] words = paragraph.ToLower().Split(new char[] { ' ', '.', ',', '!', '?' }, StringSplitOptions.RemoveEmptyEntries);

            foreach (string word in words)
            {
                if (wordCounts.ContainsKey(word))
                {
                    wordCounts[word]++;
                }
                else
                {
                    wordCounts[word] = 1;
                }
            }

            Console.WriteLine("Word frequencies:");
            foreach (var pair in wordCounts)
            {
                Console.WriteLine($"\"{pair.Key}\": {pair.Value}");
            }
        }
    }
}

Once you have counted the frequencies, it is often helpful to sort and display the results for easier interpretation. Sorting by frequency can highlight the most common letters or words, making patterns more visible, especially when dealing with large amounts of text. This is particularly useful in cryptography, where the most frequent letters in a cipher text may correspond to common letters in the language.

Study More

Study more: Frequency analysis has played a key role in code breaking and cryptanalysis for centuries. Classic ciphers like the Caesar cipher and substitution ciphers can often be cracked by comparing letter frequencies in encrypted messages to those in the target language. For further reading, consider studying the history of cryptography and the role of frequency analysis in famous code-breaking efforts.


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// General pattern for frequency counting
Dictionary<T, int> counts = new Dictionary<T, int>();
foreach (T item in collection)
{
    if (counts.ContainsKey(item))
    {
        counts[item]++;
    }
    else
    {
        counts[item] = 1;
    }
}

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Obrigado pelo seu feedback!

Seção 3. Capítulo 3

Pergunte à IA

Pergunte o que quiser ou experimente uma das perguntas sugeridas para iniciar nosso bate-papo

Deslize para mostrar o menu

Program.cs


              12345678910111213141516171819202122232425262728293031323334353637
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string sentence = "Frequency analysis reveals patterns.";
            Dictionary<char, int> letterCounts = new Dictionary<char, int>();

            foreach (char c in sentence)
            {
                if (char.IsLetter(c))
                {
                    char lower = char.ToLower(c);
                    if (letterCounts.ContainsKey(lower))
                    {
                        letterCounts[lower]++;
                    }
                    else
                    {
                        letterCounts[lower] = 1;
                    }
                }
            }

            Console.WriteLine("Letter frequencies:");
            foreach (var pair in letterCounts)
            {
                Console.WriteLine($"'{pair.Key}': {pair.Value}");
            }
        }
    }
}

Program.cs


              12345678910111213141516171819202122232425262728293031323334
            
using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string paragraph = "This is a test. This test is only a test.";
            Dictionary<string, int> wordCounts = new Dictionary<string, int>();
            string[] words = paragraph.ToLower().Split(new char[] { ' ', '.', ',', '!', '?' }, StringSplitOptions.RemoveEmptyEntries);

            foreach (string word in words)
            {
                if (wordCounts.ContainsKey(word))
                {
                    wordCounts[word]++;
                }
                else
                {
                    wordCounts[word] = 1;
                }
            }

            Console.WriteLine("Word frequencies:");
            foreach (var pair in wordCounts)
            {
                Console.WriteLine($"\"{pair.Key}\": {pair.Value}");
            }
        }
    }
}

Study More


              12345678910111213
            
// General pattern for frequency counting
Dictionary<T, int> counts = new Dictionary<T, int>();
foreach (T item in collection)
{
    if (counts.ContainsKey(item))
    {
        counts[item]++;
    }
    else
    {
        counts[item] = 1;
    }
}

Tudo estava claro?

Obrigado pelo seu feedback!

Seção 3. Capítulo 3

Frequency Analysis

1. What data structure is commonly used for frequency analysis?

2. Why is frequency analysis important in cryptography?

3. What is the first step in word frequency analysis?

Frequency Analysis

1. What data structure is commonly used for frequency analysis?

2. Why is frequency analysis important in cryptography?

3. What is the first step in word frequency analysis?