C programming - A array and a random number combined question?

Question

this is the part of my code I'm having trouble with. I can't understand why its doing it wrong. I have an array where it stores numbers 0 - 25 which are cases. The numbers are to be randomized and overwritten into the array. Only condition is is that no number can be doulbes, there can only be one of that number. I'm not asking you to do my code but do hint me or point me in the write directions. I am trying to learn :)

The problem lies within the second do loop. I can get the numbers to be randomized, but I get doubles. I have created a loop to check and fix this, but it's not working. The code does run, and doubles do still happen and I can't see why. It looks correct to me. Please look, thank you (:

This is what I have done originally (at the very end is where I am at now):

int check_double = 0;
int i = 0;
int counter = 0;
int array_adder = 0;
int random_number = 0;

int cases[] = {
    1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26
    };

float money[] = {
    0.01,1,5,10,25,50,75,100,200,300,400,500,750,1000,5000,10000,25000,50000,750000,100000,200000,300000,400000,500000,750000,1000000
    };

//Randomize all case number and realine them in the array
srand ( time(NULL) );
do
{
    cases[counter]= rand() % 26;
    counter += 1;
    printf("%d\n", cases[counter]);
}
while (counter <= 25);

//make sure there are no doubles in the array, just 0 - 25 and not a single number repeated twice
do
{
    check_double = 0;

    for (i = 0; i < counter; i++)
    {
        if (cases[counter] == cases[i])
        {
            cases[counter] = rand()% 26;
            check_double == 1;
        }
    }
}

while (check_double != 0);

Currently, what I had achived after that was combing both loops and check for doubles as the array goes. This is what I made, it still has doubles and im not sure why, I only posted the cose with both loops combined:

do
{

cases[counter]= rand() % 26;

if (cases[counter]>=1);

    for(i=0;i<=counter;i++)

    if (cases[counter]==cases[i])
        {
            cases[counter]=rand()% 26;
        }

printf("%d\n",cases[counter]);
counter+=1;
}

Answer 1

Robsta, you could try the following piece of code, I have run this in Dev-C++, any changes that you require can be made from your side. But, I assure you that this code generates what you intend.

int check_double = 0;
int i = 0;
int counter = 0;

int cases[] = {
    1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26
    };
//Randomize all case number and realine them in the array
srand ( time(NULL) );
do
{

cases[counter]= rand() % 26;

   for(i=0;i<counter;i++)
    if (cases[counter]==cases[i]){
      while (cases[counter]==cases[i])
        {
            cases[counter]=rand()% 26;
        }
      i=0;
    }

printf("%d\t%d\n",counter,cases[counter]);
counter+=1;
}while (counter <= 25);

If you have any clarifications required, I would love to discuss with you.

-Sandip

Answer 2

You're only ever writing over the last value in the array:

for(i=0;i<counter;i++)
    if (cases[counter]==cases[i])

You need to loop through as you are, then have an inner loop, where you compare all the other entries to the current one.

Even easier would be to do the loop where you set each random number, so when you set cases[3] for example, loop from 0 to 2 and check to see if your new value for 3 clashes, if so, wash - rinse - repeat!

Answer 3

You have this line of code:

check_double==1;

That doesn't change check_double because it's ==, not =. == compares; it doesn't assign. Change that line to this:

check_double=1;

A helpful compiler (clang in this example) will give you a warning about this:

test.c:5:14: warning: expression result unused [-Wunused-value]
        check_double==1;
        ~~~~~~~~~~~~^ ~

Answer 4

You can't check for duplicates with a single loop. You need to at least compare every possible pair of elements to be able to see if there's a duplicate. I'm guessing you forgot to loop over counter somewhere inside the second do...while?

Note that your method is not guaranteed to terminate. (Very, very likely but not certain.) Why don't you simply shuffle the cases array? Shuffling is simple but tricky; see Fisher-Yates (or Knuth) Shuffle for a simple algorithm.

Answer 5

If you are asking how to randomly sequence the number 1-25 then you could do something like this. This is a very brute-force way of generating the sequence, but it does work and might give you a starting point for something more optimized.

#include "stdafx.h"
#include <stdlib.h>
#include <time.h>
#include <conio.h>

const int LastNumber = 25;

bool HasEmpty(int available[LastNumber][2])
{
    bool result = false;
    for(int i = 0; i < LastNumber; i++)
    {
        if (available[i][1] == 0)
        {
            result = true;
            break;
        }
    }

    return result;
}

int _tmain(int argc, _TCHAR* argv[])
{
    int available[LastNumber][2];
    int newSequence[LastNumber];

    srand((unsigned int)time(NULL));

    for(int i = 0; i < LastNumber; i++)
    {
        available[i][0]=i;
        available[i][1]=0;
    }

    int usedIndex = 0;

    while (HasEmpty(available))
    {
         int temp = rand() % (LastNumber + 1);
         if (available[temp][1] == 0)
         {
             newSequence[usedIndex++] = available[temp][0];
             available[temp][1] = 1;
         }
    }   

    for(int i = 0; i < LastNumber; i++)
    {
        printf("%d\n",newSequence[i]);
    }

    getch();

    return 0;
}