Thread: Playing with random numbers

Hi everyone I am hoping someone can point out the error in the following code fragment (please excuse the sloppy coding I am just starting to learn c).
First rotation through the for loop correctly returns the hex value however second time around it does not?

First myRand = 4D should result in FFFFFFFF and does
Second myRand = 15FC should result in FFFFFFFA doesn't work?

Code:

 srand(length);  // seed rand generator

    for (i = 0; i < length; ++i) {
        myRand =  rand();   // get random number
        myDivMod = myRand % length;
        // myRand = length / 2;
        myRand = myRand / length;  
        myRand = myRand * -1;               
        myRand = myRand + myDivMod;

//I would like to change string1 directly here instead of using a string2 can I do that?

        j = string1[i]; 
        j += myRand;
        string2[i] = j;
    }

Thanks for taking the time to read this and any suggestions are MOST welcome.

cheers

Originally Posted by Salem

Post a complete code we can copy/paste/run to see what you're seeing.

Object is to change an inputted string characters as per the massaged values of the random number
I want to use the ist byte of hex value eg. FA in hex value FFFFFFFA
and add that to the value of the character in the input string

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>




int main()
{
    char string1[30], string2[30], myHexBuf[50],j;
    int i, length, bufsize, myBuf, myDivMod, myDivAdd, myDivStrlen;
    unsigned int myRand; //myHexBuf;  //, myRand;




    printf("Enter String\n");


    scanf("%s",string1);


    length = strlen(string1);
    
    bufsize = length * 4;


    myBuf = (malloc(bufsize));  // allocate buffer size of strlen chars
    string * 4 for the hex values


    srand(length);  // seed rand generator


    for (i = 0; i < length; ++i) {
        myRand =  rand();   // get random number
        myDivMod = myRand % length; // - length / 2;  // get mod of random number
         myRand = myRand / length;  //  should equal 6 with 4D random
        myRand = myRand * -1;    // negate mRand by * -1
         myRand = myRand + myDivMod;
        
    //j = string1[i];
        //j += myRand;
        //string2[i] = j;
    }




    return 0;
}

Thanks for the reply but I am now definitely lost I am using Code Blocks and the length of my string is 12 characters

1st return from rand() returns 77 or 0x4D
2nd return from rand() returns 5628 or 0x15FC
3rd return from rand() returns 6232 or 0x1858
4th return from rand() returns 29052 or 0x717C

is the rand function machine specific ?

These are the same values which are returned under IDA

cheers

So should the rand function seeded with the same value return the same value on any machine it is used on ?

Thanks for your time and reply I must admit I don't follow half of what you are saying :-(
On my machine using "First Second" as an input string which has 12 characters the 12 is then used to seed the rand call.
The first rand call returns 0x4D.
That value is divided by the 12 characters entered the result is 6 into the EAX register and the modulo which is 5 is saved EDX.
The strlen value in this case 12 which is in EAX is divided by 2 by sar 1 so it is now 6.
The value is EAX is then negated by multiplying it by -1 which results in FFFFFFFA in the EAX register.
The mod value contained in the EDX register is added to EAX which now becomes FFFFFFFF.
The last FF byte is then added to the first character of the string entered which just deducts 1 from its value so the "F"
character 0x46 or decimal 70 becomes ox45 or decimal 69 so is changed to an "E".
The next rand call returns 0x15FC and the process above eventually ends in FFFFFFFA in the EAX register which
when added to the second character of the string entered an "i" subtracts 6 from it's value
"i" becomes "c".
The third call to rand returns 0x1858 and the loop continues changing all of the entered characters in the input string
by the values after the calculations on the rand number returned.

BUT if the rand value returned is only correct for my machine I can't do what I am trying to do.

rand() is a pseudo random number generator. It isn't really random, just appears to be. It depends on the initial value seeded to the algorithm. If this value is 12, rand() will always give you the same sequence. Run this program multiple times to see:

Code:

#include <stdio.h>
#include <stdlib.h>

int main( void )
{
  int i;

  srand( 12 );
  for ( i = 0; i < 10; i++ )
    printf( "%d ", rand() );
  putchar( '\n' );
}

Code:

$ cat test.c
#include <stdio.h>
#include <stdlib.h>

int main( void )
{
  int i;

  srand( 12 );
  for ( i = 0; i < 10; i++ )
    printf( "%d ", rand() );
  putchar( '\n' );
}
$ cc -o test test.c
$ ./test
1687063760 945274514 247215794 1768547008 1478315323 279012855 668221829 1109214873 1378701697 1054489464 
$ ./test
1687063760 945274514 247215794 1768547008 1478315323 279012855 668221829 1109214873 1378701697 1054489464 
$ ./test
1687063760 945274514 247215794 1768547008 1478315323 279012855 668221829 1109214873 1378701697 1054489464