How to I search and change a line in a text file in c#

Question

I am wanting to search for a person and change there interest in the file via user input then print it out to the console.

public static void List_Contestants()
{
Console.Clear();
Contestants[] Person = new Contestants[45];

        StreamReader File = new StreamReader(@"C:\Users\tom\OneDrive - Otago Polytechnic\Desktop\DealOrNoDeal.txt");
        int pos = 0;
        while (!File.EndOfStream)
        {
            Person[pos].First_Name = File.ReadLine();
            Person[pos].Last_Name = File.ReadLine();        //making it so we can distinguish between First name last name and Intrest
            Person[pos].Interests = File.ReadLine();
            pos++;
        }

        foreach (var contestants in Person)
        {
            Console.WriteLine(contestants.Last_Name);  //Write out The Last Names of eveyone with on the file.
        }
    }

Answer 1

Hello,

The following first reads all people, search for a particular person, if found change part of their phone number and writes people back to the original file.

If say one of the lines per person is not correct in any way you need to deal with that.

Code sample expects people.txt in the executable folder

Ken
Sánchez
697-555-0142
Terri
Duffy
819-555-0175
Roberto
Tamburello
212-555-0187
Rob
Walters
612-555-0100
Gail
Erickson
849-555-0139
Jossef
Goldberg
122-555-0189

Person class has Contents property for writing data back to the file.

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Phone { get; set; }
    public string Contents => $"{FirstName}\n{LastName}\n{Phone}";
    public override string ToString() => $"{FirstName} {LastName} {Phone}";

}

File operations

public class FileOperations
    {
        public static string _fileName = "people.txt";
        /// <summary>
        /// Write people back to same file as read from
        /// </summary>
        /// <param name="peopleList">valid list of <seealso cref="Person"/></param>
        public static void Write(List<Person> peopleList)
        {
            File.WriteAllLines(_fileName, peopleList
                .Select(item => item.Contents)
                .ToArray());

        }

        /// <summary>
        /// Assumes file exists and there are three line per person
        /// </summary>
        /// <returns>List of <see cref="Person"/></returns>
        public static List<Person> Read()
        {
            var contents = File.ReadAllLines(_fileName);

            string[][] chunks = contents
                .Select((line, index) => new
                {
                    Value = line, 
                    Index = index
                })
                .GroupBy(x => x.Index / 3)
                .Select(grp => 
                    grp.Select(x => x.Value).ToArray())
                .ToArray();

            return chunks.Select(item => new Person()
            {
                FirstName = item[0], 
                LastName = item[1], 
                Phone = item[2]
            }).ToList();

        }
    }

Read file, search for person, change, save to the original file

class Program
{
    static void Main(string[] args)
    {
        var peopleList = FileOperations.Read();

        var person = peopleList.FirstOrDefault(p => p.FirstName == "Roberto" && p.LastName == "Tamburello");
        if (person is not null)
        {
            person.Phone = "303-555-0187";
        }

        FileOperations.Write(peopleList);

    }
}

Answer 2

Hi @Thomas McIver-Chave (1000102302) ,

You can refer to the following code to search the txt and change a line in a text file based on your input.

I recommend that you use List to store information.

class Program  
{  
    static void Main(string[] args)  
    {  
        List<Person> list = new List<Person>();  
        string[] lines = File.ReadAllLines(@"test.txt");  
        for (int i = 0; i < lines.Count()/3; i++)  
        {  
            if(lines[i * 3]!=string.Empty|| lines[i * 3+1] != string.Empty|| lines[i * 3 + 1]!=string.Empty)  
            {  
                list.Add(new Person { FistName = lines[i * 3], LastName = lines[i * 3 + 1], Interests = lines[i * 3 + 2] });  
            }  
        }  
        Console.WriteLine("Please input FirstName");  
        string firstname = Console.ReadLine();  
        Console.WriteLine("Please input LastName");  
        string lastname = Console.ReadLine();  
        Person p = list.Where(i => i.FistName == firstname && i.LastName == lastname).FirstOrDefault();  
        if (p == null)  
        {  
            Console.WriteLine("Please input the correct firstname and lastname");  
        }  
        else  
        {  
            Console.WriteLine("Please input modified Intersts");  
            string interst = Console.ReadLine();  
            p.Interests = interst;  
            File.Delete(@"test.txt");  
            foreach (var item in list)  
            {  
                File.AppendAllLines(@"test.txt", new string[] { item.FistName, item.LastName, item.Interests });  
            }  
        }  
         
    }  
}  

public class Person  
{  
    public string FistName { get; set; }  

    public string LastName { get; set; }  

    public string Interests { get; set; }  


}