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Join Date: Mar 2003
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OK I need some help... so far ive been able to get everything on my own but...im making a program that selects questions from an array numbered 1-40. i put in a random fuctions so the questions are chosen randomly. but i want to have it so that theres no duplication in the questions. like if the number is already chosen... skip and pick the next. any help would be much appreciated.
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#2
Mar 6th, 2003
It seems easy enough to set up another array to store a reference of string pointers to the questions chosen. And each time a random question is selected search the other array for a matching pointer. Make sure you set each of the other array cells to NULL so you don't compare some unknown something. 
Of course I don't know C++ but that's how I'd do it in C. I'd probably set the array only to 10 pointers and run it like a queue once it's filled up. A repeat after 10 questions wouldn't be bad.
Of course I don't know C++ but that's how I'd do it in C. I'd probably set the array only to 10 pointers and run it like a queue once it's filled up. A repeat after 10 questions wouldn't be bad.
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Join Date: Feb 2002
Location: Lawn Guylen, NY
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#3
Mar 6th, 2003
Here's an extremely simplistic/mediocre way of doing it in C++, based on Aelfinn's posted.
Lets say you have array of strings A[40] with 40 cells, that looks sorta like the following?
A[0] = Question 1?
A[1] = Question 2?
A[2] = Question 3?
and, well, you get the idea. Now, basically create a second array, int B[40]. Also create a variable int so_far which will be an accumulator each time a question is asked.
So, lets say a random number 5 is chosen. First, look up A[5] and ask the question. Then, set B[0] equal to 5, and increment so_far by 1. Now, lets say a random number 8 is chosen. Look up A[8], ask the question, and set B[1] equal to 8, and increment so_far by 2. Each time a question is asked, do a linear search on B[0] through B[so_far - 1] to see if any of the values match up to 8. Make sense?
Lets say you have array of strings A[40] with 40 cells, that looks sorta like the following?
A[0] = Question 1?
A[1] = Question 2?
A[2] = Question 3?
and, well, you get the idea. Now, basically create a second array, int B[40]. Also create a variable int so_far which will be an accumulator each time a question is asked.
So, lets say a random number 5 is chosen. First, look up A[5] and ask the question. Then, set B[0] equal to 5, and increment so_far by 1. Now, lets say a random number 8 is chosen. Look up A[8], ask the question, and set B[1] equal to 8, and increment so_far by 2. Each time a question is asked, do a linear search on B[0] through B[so_far - 1] to see if any of the values match up to 8. Make sense?
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#4
Mar 7th, 2003
One easy way to achieve this would be as follows:
- Start with your array of 40 questions (strings).
- Initialise a counter to the number of strings (40)
- Select a random string in the range 1 to 40 (e.g. string 10)
- Swap the selected string (10) with the last unused string (40)
- Output string 40
- decrement the number of strings (to 39)
The string you used is now outside the range (as string 40)
- Select a random string in the range 1 to 39 (e.g. string 26)
- Swap the selected string (26) with the last unused string (39)
- Output string 39
- decrement the number of string to 38
The used strings are at positions 39 and 40, outside of the range
- repeat as required, handle the condition whereby zero strings are left
It sounds more complicated than it really is, here's a function that achieves it (using a vector of strings):
You populate your vector with strings, then set an integer variable equal to the number of strings, and off you go - make calls to the getQuestion() function.
Here's a demo program:
- Start with your array of 40 questions (strings).
- Initialise a counter to the number of strings (40)
- Select a random string in the range 1 to 40 (e.g. string 10)
- Swap the selected string (10) with the last unused string (40)
- Output string 40
- decrement the number of strings (to 39)
The string you used is now outside the range (as string 40)
- Select a random string in the range 1 to 39 (e.g. string 26)
- Swap the selected string (26) with the last unused string (39)
- Output string 39
- decrement the number of string to 38
The used strings are at positions 39 and 40, outside of the range
- repeat as required, handle the condition whereby zero strings are left
It sounds more complicated than it really is, here's a function that achieves it (using a vector of strings):
string getQuestion(vector<string>& v, int& numberOfStrings)
{
// handle errors
if(numberOfStrings <= 0) return "error";
if(numberOfStrings > v.size()) return "error";
// select a random number in the range 0 - numberOfStrings-1
int next = rand() % numberOfStrings;
// decrement the number of strings
numberOfStrings--;
// swap it with the last unused string
swap(v[next], v[numberOfStrings]);
// retrieve the string
return v[numberOfStrings];
}You populate your vector with strings, then set an integer variable equal to the number of strings, and off you go - make calls to the getQuestion() function.
Here's a demo program:
#include <iostream>
#include <vector>
#include <string>
#include <cstdlib>
#include <algorithm>
using namespace std;
string getQuestion(vector<string>& v, int& numberOfStrings)
{
if(numberOfStrings <= 0) return "Error";
if(numberOfStrings > v.size()) return "Error";
int next = rand() % numberOfStrings;
numberOfStrings--;
swap(v[next], v[numberOfStrings]);
return v[numberOfStrings];
}
int main()
{
vector<string> q;
q.push_back("One");
q.push_back("Two");
q.push_back("Three");
q.push_back("Four");
q.push_back("Five");
q.push_back("Six");
q.push_back("Seven");
q.push_back("Eight");
q.push_back("Nine");
q.push_back("Ten");
int num=q.size();
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
cout << getQuestion(q,num) << endl;
} •
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#5
Mar 7th, 2003
I like that...much faster than my idea and with no guaranteed work to prove the string hasn't been used.
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