Remove string from array by index

Question

I'm trying to build a simple country capitals quiz program using c#. Currently, I am giving the user a random country from the countries list and asking for input on what the capital is. Then there is a conditional that checks if the correct capital is equal to the input. After this completes, I need to be able to remove the country and capital asked from each array so that the loop does not repeat them. Here is the code I have so far:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace program {
  class MainClass {
    public static void Main (string[] args) {
      string[] countries = new string[] {"Germany", "Poland", "France", 
                                        "United States", "Russia", "Japan"};
      string[] capitals = new string[] {"berlin", "warsaw", "paris", 
                                       "washington dc", "moscow", "tokyo"};

      int score = 0;
      int length = countries.Length;

      string current_country = countries[random_number];
      string current_capital = capitals[random_number];

      for (int i = 0; i <= length;) {
        Random rnd = new Random();
        int random_number = rnd.Next(0, length);
        Console.WriteLine("What is the capital of {0}?", current_country);
        string capital_input = Console.ReadLine();

        if (capital_input == current_capital) {
          Console.WriteLine("Correct!");
          countries.Remove[random_number];
          capitals.Remove[random_number];
          score += 1;
          continue;
        } else {
          Console.WriteLine("Incorrect!");
          countries.Remove[random_number];
          capitals.Remove[random_number];
          continue;
      }

      if (length == 0) {
        break;
      }
    }

    int score_percent = score * 20;
    Console.WriteLine("Congratulations! You scored {0}% of questions 
                      correct.", score_percent);
  }
}

}

The program fails to compile with these errors:

exit status 1
main.cs(27,34): error CS0201: Only assignment, call, increment, decrement, 
await, and new object expressions can be used as a statement
main.cs(28,28): error CS0201: Only assignment, call, increment, decrement, 
await, and new object expressions can be used as a statement
main.cs(33,29): error CS0201: Only assignment, call, increment, decrement, 
await, and new object expressions can be used as a statement
main.cs(34,28): error CS0201: Only assignment, call, increment, decrement, 
await, and new object expressions can be used as a statement
Compilation failed: 4 error(s), 0 warnings

Any ideas?

The first problem is that you can't remove elements from arrays, or add to them. I suspect you want List<string> instead. Next, Remove is a method, not an indexer - so you want Remove(...) rather than Remove[...]. — Jon Skeet
– Jon Skeet, Commented Feb 8, 2018 at 19:48
Third, you're using random_number before you've declared it. So put current_country and current_capital inside the loop, below random_number — Balah
– Balah, Commented Feb 8, 2018 at 19:52
Dont declare the Random in the loop though, make it a static member of the class and call the Next() function inside the loop. — maccettura
– maccettura, Commented Feb 8, 2018 at 19:57

cowboydan · Accepted Answer · 2018-02-08 19:51:22Z

1

Consider using the following:

List<string> remainingCountries = new List<string>(){ "US", "Germany", "Russia"};
void RemoveIncorrectGuess(string countryToRemove)
{
   remainingCountries.Remove(countryToRemove);
}

answered Feb 8, 2018 at 19:51

cowboydan

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1 Comment

Wyck Over a year ago

Mostly a good answer, but the OP asked to remove "by index", so you should have offered a solution using RemoveAt

Roman Marusyk · Accepted Answer · 2018-02-08 20:27:17Z

There are many issues in your code. If you want working code then use this:

 public static void Main(string[] args) {
    List<string> countries = new List<string> { "Germany", "Poland", "France", "United States", "Russia", "Japan" };
    List<string> capitals = new List<string> { "berlin", "warsaw", "paris", "washington dc", "moscow", "tokyo" };

    int score = 0;
    Random rnd = new Random();

    while(countries.Count > 0) {
        int random_number = rnd.Next(0, countries.Count);

        string current_country = countries[random_number];
        string current_capital = capitals[random_number];

        Console.WriteLine("What is the capital of {0}?", current_country);

        string capital_input = Console.ReadLine();

        if (!string.IsNullOrWhiteSpace(capital_input) && capital_input.Equals(current_capital, StringComparison.InvariantCultureIgnoreCase)) {
            Console.WriteLine("Correct!");
            score++;
        } else {
            Console.WriteLine("Incorrect!");
        }
        countries.RemoveAt(random_number);
        capitals.RemoveAt(random_number);
    }

    int score_percent = score * 20;
    Console.WriteLine("Congratulations! You scored {0}% of questions correct.", score_percent);
    Console.ReadKey();
}

Now I try to explain some cases (also using comments):

You can't remove elements from arrays using Remove[random_number]. As said J.Skeet: Remove is a method, not an indexer
It would be better to use List<string> instead. Then you can remove elements like RemoveAt(random_number)
You're using random_number before you've declared it.
Don't declare the Random in the loop.
In this case using while instead of for is more preferable
Check of input capital with ignore case

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