How to generate a set of unique hash strings in C++? - c++

Question

This code produce 26*26*26*26 hasname (using combinations of a-z)or you can say random names which i want to assign to a structure member.Now when i am assigning that by first allocating that structure member sufficient memory and then using strcpy, only last hashname generated by this code is being passed to the structure i.e zzzz(it is the last hashname).What can i do so that hashname is assigned from the starting.
vcd_xyz[4] = '\0';
int count = 0;
for(int i=0;i<26;i++)
{
vcd_xyz[0] = 'a'+i;
for(int j=0;j<26;j++)
{
vcd_xyz[1] = 'a'+j;
for(int k = 0;k<26;k++)
{
vcd_xyz[2] = 'a' + k;
for(int l=0;l<26;l++)
{
vcd_xyz[3] = 'a' +l;
count ++;
printf("%s\n",vcd_xyz);
}
}
}
}
So i am using
sss->Variables[0].hashname = (char*)calloc(strlen((char*)vcd_xyz)+1,sizeof(char));
strcpy(sss->Variables[0].hashname,(char*)vcd_xyz);
to copy the hasname produced but it is copying the last hashname produced, so where ever i am using tr0->Variables[0].hashname = (char*)calloc(strlen((char*)vcd_xyz)+1,sizeof(char));
strcpy(tr0->Variables[0].hashname,(char*)vcd_xyz); only zzzz get printed.what i am doing wrong or what should i do so that hashname are assigned in a sequential manner.

Answer 1

At first you need to realize that char vcd_xyz[4] is an array of 4 characters, meaning that you can put there 3 characters + terminating character '\0'. If you treat this array as it is null-terminated when it is not, it results in undefined behavior.
What your code actually does is that it iterates through all possible combinations of 4 letters long strings, from "aaaa" to "zzzz", leaving the vcd_xyz array filled with the last combination (i.e. "zzzz").
If you want to generate random 4-letters long string, here's the C-style function you might use:
int irand(int min, int max) {
return ((double)rand() / ((double)RAND_MAX + 1.0)) * (max - min + 1) + min;
}
it generates random number from <min;max> interval and it can be used like this:
std::string generateHashTag() {
char str[5];
for (int i = 0; i < 4; ++i)
str[i] = irand('a', 'z');
str[4] = '\0';
return std::string(str);
}
But in case you want to generate a set of unique 4-letters long hash tags, you will need more complex solution. In C++ you might easily generate these strings in a loop that will put them into an std::set container till you have enough of them or you might generate more unique combinations of this string, put all of these into an std::vector, shuffle it and pick first N, e.g.:
const size_t N = 5;
std::set<std::string> myHashTags;
srand(time(0));
while (myHashTags.size() < N)
myHashTags.insert(generateHashTag());
for (std::set<std::string>::iterator i = myHashTags.begin();
i != myHashTags.end(); ++i)
std::cout << *i << ' ';
outputs kemy snwv vnmi wfmm wqeg. Full example here.