how to make random score generator using array? - c++

Question

I am trying to make a program that calculates the average score of class and counting the number of A, B, C, D students. Now I am trying another idea: If the test scores are randomly given to students(between 60 to 100), how can I input those random scores into an array? Mine requires users to input all scores one by one. I would like to know how to fill in the array automatically.
#include <iostream>
#include <cstdlib>
using namespace std;
double Average(double *scores, int N)
{
int i;
double total = 0;
for(i = 0; i < N; i++)
{
total = total+scores[i];
}
return total / N;
}
int Agrade(double *scores, int N)
{
int i;
int count = 0;
for(i = 0; i < N; i++)
{
if(scores[i] >= 90 && scores[i] <= 100) count++;
}
return count;
}
int Bgrade(double *scores, int N)
{
int i;
int count = 0;
for(i=0; i < N; i++)
{
if(scores[i] >=80 && scores[i] < 90) count ++;
}
return count;
}
int Cgrade(double *scores, int N)
{
int i;
int count = 0;
for(i=0; i < N; i++)
{
if(scores[i] >=70 && scores[i] < 80) count ++;
}
return count;
}
int Dgrade(double *scores, int N)
{
int i;
int count = 0;
for(i=0; i < N; i++)
{
if(scores[i] >=60 && scores[i] < 70) count ++;
}
return count;
}
int Fgrade(double *scores, int N)
{
int i;
int count = 0;
for(i=0; i < N; i++)
{
if(scores[i] < 60) count ++;
}
return count;
}
int main(){
int i;
int N;
double *scores;
std::cout<<"How many test scores? "<<endl;
cin>>N;
if(N<1){
std::cout<<"Invalid input. try again"<<endl;
}
else if(N>25)
{
std::cout<<"1-25 only."<<endl;
}
else if(N>0 && N<25){
std::cout<<"Total number of test is: "<< N << endl;
}
scores = new double[N];
for(i = 0; i < N; i++)
{
cout << "Randomly generating score of students" << i + 1 << ": ";
cout << (rand() % 40 + 60) << endl; //Trying to give random scores between 60-100
if(!(cin >> scores[i]) || scores[i] < 0 || scores[i] > 100)
{
if(!cin)
{
cin.clear();
cin.ignore(1000, '\n');
}
cout << "Score must be between 0 to 100.\n\n"; i--; continue;
}
}
double averagescore = Average(scores, N);
int scoreAcount = Agrade(scores, N);
int scoreBcount = Bgrade(scores, N);
int scoreCcount = Cgrade(scores, N);
int scoreDcount = Dgrade(scores, N);
int scoreFcount = Fgrade(scores, N);
cout << "The average test score : " << averagescore << endl;
cout << "The number of A grades : " << scoreAcount << endl;
cout << "The number of B grades : " << scoreBcount << endl;
cout << "The number of C grades : " << scoreCcount << endl;
cout << "The number of D grades : " << scoreDcount << endl;
cout << "The number of F grades : " << scoreFcount <<endl;
cout << endl;
return 0;
}
Can I just modify this part to make it work?
for(i = 0; i < N; i++)
{
cout << "Randomly generating score of students" << i + 1 << ": ";
cout << (rand() % 40 + 60) << endl; //Trying to give random scores between 60-100
if(!(cin >> scores[i]) || scores[i] < 0 || scores[i] > 100)
{
if(!cin)
{
cin.clear();
cin.ignore(1000, '\n');
}
cout << "Score must be between 0 to 100.\n\n"; i--; continue;
}
}
or do I need to modify all functions?

Answer 1

If what you want is to assign result of std::rand() function to all elements in array, you should take a look at std::generate function from <algorithm> header, which does exactly what you want. Take a look at this simple example:
#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
int main() {
std::srand(0);
const int N = 5;
std::vector<int> scores(N);
std::generate(scores.begin(), scores.end(), [](){return std::rand() % 40 + 60; });
for(auto s : scores) {
std::cout << s << ' ';
}
}
This will fill vector scores with randomly generated numbers from specified interval using std::generate function and lambda function given as its third argument that returns random number.
In your case, if you're not allowed to use std::vector, you can still do it with plain C-arrays, like this:
int scores[5];
std::generate(std::begin(scores), std::end(scores), [](){return std::rand() % 40 + 60; });
Unfortunately, std::begin and std::end functions cannot be applied straight-forward to dynamically allocated arrays, but this still should work in your case:
const int N = 5;
double* scores = new double[N];
std::generate(scores, scores+N, [](){return std::rand() % 40 + 60; });
for(int i=0; i<N; ++i) {
std::cout << scores[i] << ' ';
}
// remember to release memory allocated using new

Answer 2

You need to seed the random number generator to make sure the random numbers have a reasonably random seed. So before the for loop you mentioned, you need to call something like srand(time(NULL)) and include time.h in your code. That should be enough.
And just as an aside, in your code, I only see allocation of the scores array. Remember to delete! Even better, use vectors throughout.