Dynamic Arrays and user input c++ - c++

Question

I'm writing a program for my C++ class that takes the user input for the size of a int and char array, fills the arrays with random values (numbers 0-100, letters A-Z) then sorts, reverses, and displays both arrays.
For the most part the program works and I understand the logic I used here, but...
After running and debugging the code multiple times. I noticed that when the arrays were being filled with values, the first element, even though it was actually being given a value, it would not print the assigned value it was given in ascending order, but would in a descending order? I don't understand this at all.
NOTE: I have to use template functions for the sorting, reversing and display of the arrays.
template <class T>
void sort(T *arr, int a) {
T temp;
for (int i = 0; i < a; i++) {
for (int j = a; j > 0; j--) {
if (arr[i] > arr[j]) {
temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}
}
}
template <class T>
void reverse(T *arr, int a) {
T temp;
for (int i = 0; i < a / 2; i++) {
temp = arr[i];
arr[i] = arr[a - i];
arr[a - i] = temp;
}
}
template <class T>
void display(T *arr, int a) {
for (int i = 0; i < a; i++) {
cout << arr[i] << ", ";
}
cout << endl;
}
template<class T>
void save(T *arr, int a) {
sort(arr, a);
display(arr, a);
reverse(arr, a);
display(arr, a);
}
int main() {
int x, y;
cout << "Please enter a number for an array of data type \"int\"" << endl;
cin >> x;
cout << "Please enter a number for an array of data type \"char\"" << endl;
cin >> y;
int *arr1 = new int[x];
char *arr2 = new char[y];
for (int i = 0; i < x; i++)
cout << (arr1[i] = rand() % 100 + 1);
srand(time(nullptr));
for (int i = 0; i < y; i++)
cout << (arr2[i] = rand() % 26 + 65);
system("cls");
save(arr1, x);
save(arr2, y);
delete[]arr1;
delete[]arr2;
system("pause");
return 0;
}

Answer 1

You are using the complete length here:
save(arr1, x);
save(arr2, y);
So in reverse
arr[i] = arr[a - i];
arr[a - i] = temp;
you need to -1 on the length or you'll get an invalid index when i == 0
arr[i] = arr[a - 1 - i];
arr[a - 1 - i] = temp;
Like R Sahu says, in sort
for (int j = a; j > 0; j--) {
you need to -1 because a is the length which will be an invalid index.
for (int j = a-1; j > 0; j--) {
As a side note, you can declare Temp t inside of the for loop in reverse and inside of the if in sort because it is only used in those scopes.
EDIT:
Also I overlooked, in sort you need to change
j>0
to
j >= 0
that way you access the first element of the array as well.

Answer 2

You have a off-be-one error in couple of places.
for (int j = a; j > 0; j--) {
is incorrect. a is an invalid index for the array. Change that line to use j = a-1:
for (int j = a-1; j > 0; j--) {
You have a similar, off-by-one, error in reverse. Instead of
arr[i] = arr[a - i];
arr[a - i] = temp;
you need to use:
arr[i] = arr[a - i - 1];
arr[a - i - 1] = temp;
Your implementation of sort is not correct. I don't want to get into the algorithmic details here but changing the order of the values used for j seems to fix the problem.
for (int i = 0; i < a; i++) {
for (int j = i+1 ; j < a ; j++) {
// The swapping code.
}
}

Answer 3

You are using bubble sort that is O(n^2) time complexity. Consider using faster algorithm. If you don't want to implement it on your own, use sort() function. It's complexity is about O(n log n), which is very good.
http://www.cplusplus.com/reference/algorithm/sort/
#include <iostream>
#include <algorithm>
using namespace std;
bool comp(int i1, int i2) { // comp function is to compare two integers
return i1 < i2;
}
int main() {
int x[30];
int n;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> x[i];
}
sort(x, x + n, comp); // if you don't provide comp function ( sort(x, x+n) ), sort() function will use '<' operator
for (int i = 0; i < n; i++) {
cout << x[i] << " ";
}
return 0;
}