Filling in a vector with random numbers

Question

I'm trying to print random numbers using srand. This is my code.

I want each subsequent number larger than the previous number in the list.

This is my code.

#include <iostream>
#include <ctime>
#include <cmath>
#include <random>
#include <vector>
#include <algorithm>
#include <climits> // for INT_MAX;


int main(){
srand((int)time(0));

std::vector<int> myVec;


int number = 5;
for(size_t index = 0; index <100; index++){
  int  result = (rand() % number) +1;
  if((result % 5) == 0 or (result % 5 == 3 && index == 50)){
    number += 2;
    myVec.push_back(number);
}
}

for(int vec : myVec){
    std::cout << vec << "  ";
}

return 0;
}

The output I'm looking for is

0 2 4 6 9 12 so on

Can someone please guide me on how can I achieve it?

Answer 1

You should be able to write this a lot simpler. Also, the reason your code seems to be not getting any random values is because your pushing back your bound with myVec.push_back(number). You should instead be doing myVec.push_back(result).

As for your actual question, it doesn't seem like your code is written as to what you're trying to achieve. If you want to get values 0-4 greater than the subsequent value, it would be better to write it something like this:

#include <iostream>
#include <ctime>
#include <random>
#include <vector>

int main(){
    srand(time(0));

    std::vector<int> nums;

    int lower = 0;
    for(int i=0; i<100; i++) {
        // generate random number from lower bound to upper bound
        // starting lower bound is 0
        nums.push_back((rand() % 5) + lower);
        lower = nums[nums.size()-1];
    }

    for(int num : nums){
        std::cout << num << "  ";
    }
    std::cout << "\n";
    return 0;
}

So, generally, if a random function is returning any value x where 0 <= x < 1 (which I believe C/C++ rand() does), then to get a value within a given range you want to have it in this format:

(rand() % (upper_bound - lower_bound + 1)) + lower_bound

However, you said that you want all values to be 0-4 greater than the lower bound. This means we essentially want to start from our lower bound, and add 0-4 to it. So really, we just need to generate a random number from 0-4. The code above does rand() % 5 because rand() % N will return any number x where 0 <= x < N. So, we want to do rand() % 5 which will get any number from 0 to 4.

Also, this is just semantics, but in my personal experience it's always good to try and maintain some level of inference with your variable naming. In your for-each loop you wrote for(int vec : myVec), but to the glancing eye this may appear that myVec is actually a vector of vectors, and every element of myVec is a vector itself. Although the type int is clearly declared, it's still easy to get confused as to why the developer named it "vec" if its type is not a vector. Once again, some of this comes down to preference and if this is for a personal project, assignment, or actual production code. But I'd start making a good habit of making sure things are named intentionally and purposefully.