How to generate 256 random numbers without repeat?

Question

I need to generate 256 random numbers in range 0-255 without repeating.
I came up with many options and the numbers are constantly repeated.
I usually compare in cycles.
If the generated number matches the one already existing in the array, I again generate a new value in the same array element.
And so on until you get 256 unique numbers.

int newNums[256];
for(int i=0;i<256;i++){
newNums[i] = -1;
}
for(int el = 0; el<256;el++){
newNums[el] = rand() % 255; //earlier i use srand(time(0))
for(int pos=0, matches = 0; pos<=el; pos++){
if(newNums[el] == newNums[pos]){
matches++;
if(matches > 1){
el--;
break;
}
}
}
}

Answer 1

Doing it this way is very slow, because as the array fills up, you will get lots of duplicate numbers and waste a lot of loop cycles.
A better approach would be to fill the array with the numbers and just shuffle it randomly afterwards. Here is an example making full use of STL features:

https://code.sololearn.com/cO7vK5K9S5ya/?ref=app

The sample() algorithm is actually very easy to implement, and you could do it yourself using rand() if you wish. As a reference, here is how the STL implements it:

https://en.cppreference.com/w/cpp/algorithm/random_shuffle

If the library implementation feels not understandable, feel free to ask again.

Answer 2

Влад Цислевский , what about the code you presented? Is it creating an error. Please give a clear description. Thanks!

Answer 3

Влад Цислевский Unfortunately, my messenger is not working correctly. Regarding your DM:

std::iota is just a convenience function to fill the array. You might have to include <numeric> where it is actually defined:

https://en.cppreference.com/w/cpp/algorithm/iota

A simple for loop would do the job too.