Thread: C program to generate array[5] of random characters composed of A-Z and 0-9

There is no space for the string terminator.
Try:

Code:

…
    char string[6];
…
    string[5] = '\0';
    printf("String: '%s'\n",string);

The headerfile 'conio.h' is not standard, avoid it.
Also, you should not use getch().

You could do something like this:

Code:

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

#define MAX_ARR 5

int main(void)
{
  char arr[MAX_ARR + 1];
  int i, n, rnd;
  srand(time(NULL));
  for (i = 0; i < MAX_ARR; ++i)
  {
    rnd = rand();
    n = (rnd >> 4) & 0xF;
    *(arr + i) = (rnd & 0xF) & 1
                 ? (n % 10) + '0'
                 : (n % 26) + 'A';
  }
  arr[MAX_ARR] = 0;
  puts(arr);
  return 0;
}

You could also do 'rnd = rand() % 0xFF;' if you wanted... Technically with the size of the integer generated by rand(), you could use that number to generate multiple characters and digits at a time using the method I posted. Instead of a single 32-bit integer going to waste, you could split that up into 4 8-bit integers to generate data for 4 elements, and you'd only have to fill the last slot after a single call to rand() -- the only problem with this idea is that it depends on the value of the number.. Even though technically if rand() returned 5, it would still be random, but the majority of the values derived from the bits wouldn't appear that way.

Originally Posted by algorism

I can't understand why cstryx's code is so complicated. Why not just:

Code:

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

#define NUM_LETTERS 26
#define NUM_DIGITS  10
#define NUM_CHARS   (NUM_LETTERS + NUM_DIGITS)
#define SIZE        5

int main() {
    srand(time(NULL));
    char a[SIZE+1] = {0};
    for (int i = 0; i < SIZE; ++i) {
        int r = rand() % NUM_CHARS;
        a[i] = r < NUM_LETTERS
               ? 'A' + r
               : '0' + r - NUM_LETTERS;
    }
    puts(a);
    return 0;
}

My code at least gives a theoretical 50/50 that the generated char is a digit or an alpha (depending on whether or not you consider rand() to return a theoretical equal amount of odds and evens in terms of probability, given an even # of calls to that function). With yours it's 26:10 odds. If it's not important, then sure you can do this. I was partially trying to get him to think about a few unique alternatives rather than just calling rand() and returning a single char element to an array placeholder. The idea behind my code was that individual bits would represent parts of the condition, and offset from the first value in either set. My code doesn't do it, but with this logic, you could theoretically, equally determine 4 elements at a time with every single call to rand().

Originally Posted by WoodSTokk

There is no space for the string terminator.
Try:

Code:

…
    char string[6];
…
    string[5] = '\0';
    printf("String: '%s'\n",string);

The headerfile 'conio.h' is not standard, avoid it.
Also, you should not use getch().

String's are not required to be null-terminated. If his purpose for this data doesn't require that it should be null-terminated, then his code would be fine. The only problem is that his printf() call requires it to be. If he modified it, it would be fine as:

Code:

printf("%.5s\n", string);

Originally Posted by algorism

Your code "at least" gives something that was neither asked for nor implied in the OP's question. It's a bizarre interpretation.

And you have a somewhat idealized picture of rand if you think you could use it to generate 4 elements at a time. (Hint: What's the guaranteed (by the standard) value of RAND_MAX? What is it on your system?)

If he didn't ask that the chance of generating a alpha vs. a digit be the same, then what makes you think that it's correct to assume that he isn't going for it, if the reverse was neither "asked for nor implied" otherwise? That's "bizarre" logic.

I also never said that unused bits had to be used, I was simply pointing out bit manipulation which allows you to take advantage of letting a single number do multiple things for you, and allowed my code to hint at a strategy. You could let them overlap and use 5 bits instead if you wanted.. There is an issue with my previous code but it was an example aside from the naïve way of doing things. If you'd rather teach originality and avoid creativity--which is undoubtedly beneficial with problem solving, I'll let you do that... There's nothing advantageous about discouraging it though. Aside from that I don't see how you couldn't use the previous value and 4-bits each from rand() itself to generate 4 elements at a time, but I'd be glad to show you that it's possible if you're doubtful. It seems you just thought it was appropriate to make assumptions dictate your judgement before second thought, and I don't respect that.

In case the question means that at least a Latin letter and a digit is always present in the result (that's how I've interpreted anyway) the logic changes.

A quick & dirty approach could be something like the following:

Code:

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

#define AB      "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
#define ABLEN   ( sizeof(AB) - 1 )
#define ARRSIZ  5

int main( void )
{
    srand( time(NULL) );

    char arr[ARRSIZ] = {'\0'};    // not a cstring
    int  nletters = 1 + (rand() % (ARRSIZ-1));
    int  ndigits  = ARRSIZ - nletters;
    printf( "nletters: %d, ndigits: %d\n", nletters, ndigits );

    int i = 0;
    while ( ndigits || nletters ) {
        int c = AB[ rand() % ABLEN ];
        if ( nletters && isalpha(c) ) {
            nletters--;
        }
        else if ( ndigits && isdigit(c) ) {
            ndigits--;
        }
        else {
            continue;
        }
        arr[i++] = c;
    }
    printf( "%.5s\n", arr );

    return 0;
}

PS. I chose to NOT treat arr as a cstring, since the question seems not to imply such a thing. However, if further manipulation of arr can benefit of cstring-like operations, it should be more useful to define/treat it as a cstring from the beginning.

Jeez.
A-Z + 0-9 = 36 possibilities

Code:

string[i]=(rand()%36)+'0';        // get 0 to 35, add '0' to convert to number
if (string[i] > '9') string[i] +=7;  // if above '9', convert to letter