Generate random keys

I was given task to generate random 80 byte keys and i have decided following strateges

in my computer sizeof(char)=1 so i have created array of english alphabetical letters

char *p=" ";
char a[0..26] and in cycle 


for (int  i=0;i<=80;i++){
   *(p+i)+= a[(rand()+100) % 26];
}

but it does not work it stops execution please help sorry if my code is stupid but i can't think at this time otherwise thanks code

#include <iostream>
#include <string.开发者_StackOverflowh>
#include <cstdlib>
using   namespace std;
int main(){

    char *p=" ";
    char  a[]= { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
 for (int i=0;i<=80;i++){
        *(p+i)+=(a[(rand()+100)%26]);
    }

     cout<<p<<endl;


     return 0;

}

Well, normally I'd say that you need to provide more information than "it stops executing," but a few things jump out at me:

    char *p=" ";
    char  a[]= { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    for (int i=0;i<=80;i++){
         *(p+i)+=(a[(rand()+100)%26]);
    }

Your loop is writing values into *p from index 0 to 80 (81 total elements).

The first iteration iteration:

*(p+0) = a[...]; 

will work, but the second one

*(p+1) = a[...];

should fail since there's no reserved memory at address *(p+1). This may be off by one if you can write to the space reserved for the null \0 that's appended to the string literal.

When you declare *p as

char *p=" ";

You're only allocating 1 byte. So, when your loop writes to p[1], p[2]... you're attempting to write into unallocated memory. Change your declaration to something like

char pArr[81];
char *p=pArr;

and go from there.

Try this out:

#include <iostream>
#include <string.h>
#include <cstdlib>
using   namespace std;
int main(){
    // ensure the target has enough memory for the key and a null terminator
    char p[81];

    // this string will do as nicely as the character array
    char a[] = "abcdefghijklmnopqrstuvwxyz";

    // no += here. I assign the random character directly to the target buffer
    for (int i=0;i<=80;i++)
        p[i] = a[rand()%26];

    // alternately, you can calculate a random English character with:
    // p[i] = rand()%26 + 'a';
    // which removes the need for the a[] buffer at all

    // don't forget to null-terminate
    p[80] = '\0'

    // output results
    cout<<p<<endl;
    return 0;
}

You are assigning characters in your p variable but you have not allocated memory to assign those characters. You probably want something like this:

char p[81];

and then go from there.

I have modified your code...try it out.

#include <iostream>
#include <string.h>
#include <cstdlib>
using namespace std;
int main(){

    char *p= new char[81];
    char  a[]= { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
 for (int i=0;i<=80;i++){
        *(p+i)=(a[(rand()+100)%26]);
    }

     cout<<p<<endl;


     return 0;

}

So what is the output?

A question about this code:

Why generate an array of alphabetical characters? Why not get a random, modulate by 26, and add the offset to ASCII "a"? This saves a memory allocation, and is, in my mind, more clear.

You need to allocate space for the full size of your char array p before writing into it in the loop.

Update By the way, rand() isn't very random if you don't seed it with a unique value first using srand.

For completeness:

std::vector<char> BuildKey(std::size_t keyLength)
{
    const char elements[] = "abcdefg0123!@_";
    const std::size_t numElements = sizeof(elements) / sizeof(char);
        //division technically superfluous, as sizeof(char) == 1, but in general its needed.

    std::vector<char> key;
    key.reserve(keyLength);
    for(std::size_t i = 0; i < keyLength; ++i) 
        key.push_back(elements[rand()%numElements]);
    //there are a handful of ways using standard algorithms to achieve this,
    //  but I'm largely unconvinced the added complexity is justified for simple loops

    return key;
}

There are a few more changes one could make. I would probably paramaterize out rand so I can choose my PRNG later, and possibly provide the elements array as an input as well. And string might be more appropriate than vector depending on how you plan on using the key. Or just accept a ForwardIterator, and return void. Lots of choices.