# permutations and combinations

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 6 Exercise 10
Using std_lib_facilities.h by Bjarne Stroustrup.

//	Philipp Siedler
//	Bjarne Stroustrup's PP
//	Chapter 6 Exercise 10

/*
input:
12
3
c
*/

#include "std_lib_facilities.h"

int factorial(int _n) {
int fact = 1;
while (1 < _n){
fact *= _n;
--_n;
if (fact < 1) {
error("factorial overflow");
}
}
return fact;
}

int permutations(int _a, int _b) {
if (_a < _b || _a < 1 || _b < 1) {
}
return factorial(_a) / factorial(_a - _b);
}

int combinations(int _a, int _b)
{
return permutations(_a, _b) / factorial(_b);
}

int main()
try
{
int a;
int b;
string aCheck;
string bCheck;
bool permCheck = false;
bool combCheck = false;

cout << "Enter two integer values for permutation and combination: \n";
cout << "Enter value for a\n";

if (cin >> a) {}
else{
cin.clear();
cin >> aCheck;
error("Wrong value for a: ", aCheck);
}

cout << "Enter value for b\n";
if (cin >> b) {}
else {
cin.clear();
cin >> aCheck;
error("Wrong value for b: ", aCheck);
}
if (b > a) {
error("Value b must be smaller than a, b: ", b);
}

cout << "a = " << a << "\n";
cout << "b = " << b << "\n";

cout << "Enter 'p' for permutations, 'c' for combinations or 'b' for both:\n";
permCheck = true;
}
else if (answer == 'c') {
combCheck = true;
}
else if (answer == 'b') {
permCheck = true;
combCheck = true;
}
else {
}

if (permCheck) {
cout << "Permutation: " << permutations(a, b) << "\n";
}
if(combCheck){
cout << "Combination: " << combinations(a, b) << "\n";
}

keep_window_open(".");
}

catch (runtime_error e) {
cout << e.what() << "\n";
keep_window_open(".");
}
catch (...) {
cout << "Extiting" << "\n";
keep_window_open(".");
}

Output:

Enter two integer values for permutation and combination:
Enter value for a
12
Enter value for b
3
a = 12
b = 3
Enter 'p' for permutations, 'c' for combinations or 'b' for both:
c
Combination: 220