Generating random characters from strings

Homework Statement

Write a program that takes a positive integer N and a string as command line arguments (N is assumed to be smaller than the length of the string). The program should pick N random characters from the string and construct and print a new string composed of these random characters.

Homework Equations

for loops. random(?)

The Attempt at a Solution

We so far learned how to generate random double variables using Math.random(). Is there a way (method) to generate random characters in strings?

Code:
<method signature><method name><parameters>

for i = 0; i < some integer; i++
string.random.charAt(i); // not really a command but something as this exists?

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Borek
Mentor
It depends on the language, but you have to use random number as an index to the string. Something like - for example - string[rand()] - it won't work, but should push you in the right direction.

NascentOxygen
Staff Emeritus
A more descriptive title would be, "Choosing N characters at random from a string of k characters". I imagine that you are not allowed to choose the ith character more than once, for any i?

Random numbers are fascinating. Choosing things at random is fun to program.

A foolproof method is to set up an array A[1]...A[k] storing the numerals 1..k in that order. Then shuffle (i.e., randomly mix up or swap around) the elements of array A. (I'll leave you to think about how to shuffle.)

Then simply print out the characters in your string that correspond in position to the numbers found in the first N elements of array A (because they have now been well shuffled).

This way, you aren't making any great demands on your random number generator, and you can monitor everything along the way to make sure it is working out exactly as you planned. Yes, you can plan random events!

Unfortunately, we haven't covered arrays in the course so using them will result in a failing grade at the moment. But thank you for the answer; this will sure come in handy when we take arrays. I actually thought of it and I think it worked:

Code:
public class RandomString
{
public static void main(String[] args)
{
int n = Integer.parseInt(args[0]); // n should be less than the length of the string
String str = args[1];

for (int i = 1; i <= n; i++)
{
int position = randomGenerator(str.length());
char c = str.charAt(position);
System.out.print(c);
}
}

public static int randomGenerator(int n)
{
double rand = Math.random();
double num = 1 + rand * (n - 1); // the string's length starts at 1 hence we add 1
int position = (int) num;
return position;
}
}
The output blabbered a string of random characters, so I'm guessing it worked.

NascentOxygen
Staff Emeritus