Chapter 4 Exercise // 2 - Principles & Practice Using C++

In all these exercises I am using Visual Studio Community 2015 and the header file "std_lib_facilities.h" which can be found here:


http://www.stroustrup.com/Programming/PPP2code/std_lib_facilities.h


My version is spelt differently so adjust the code accordingly if copying and pasting.

Chapter 4 Exercise // 4.2

If we define the median of a sequence as a "number so that exactly as many elements come before it in the sequence as come after it," fix the program in 4.6.3 so that it always prints out a median. Hint: A median need not be an element of the sequence.

#include "stdafx.h"
#include "std_lib_facilities_new_version.h"
using namespace std;


int main()
{
vector<double> temps;

cout << "Please enter integers to find mean & median, press enter, ctrl+z, then enter again to finish: \n";

for (double temp; cin >> temp;)
temps.push_back(temp);

//compute mean temp
double sum = 0;
for (double x : temps) sum += x;
cout << "Average temperature: " << sum / temps.size() << '\n';

//compute median temp
sort(temps);

if (temps.size() % 2 == 0)
cout << "Median temperature: " << (temps[temps.size() / 2 - 1] + temps[temps.size() / 2]) / 2 << endl;
else
cout << "Median Temperature: " << temps[temps.size() / 2] << endl;

keep_window_open();

return 0;

}

Ok this one took me a while, not because I don't know how to calculate the median but I just completely forgot that values inputted into a vector are not labelled 1,2,3,4 etc...but 0,1,2,3,4 etc....

The median on an even amount of numbers is calculated as so:
Lets say we put the numbers 1, 2, 3,4 in. In a vector these are labelled as 1[0], 2[1], 3[2], 4[3]. Therefore when we divide the size by 2 it will return 2 which is the number 3 (it's confusing right?) So temps[temps.size() / 2 - 1] will now return 2 and then we add it to temps[temps.size() /2] to give us 5. We then divide all that by 2 to get 2.5 which is the median.

I made the mistake of having +1 in my calculations and got very annoyed as didn't work, especially since in normal maths terms it should've worked. I've seen the above code a lot whilst I was searching for answers and not one person replied with "vectors start at 0, just -1 instead of +1". Just the standard "Use this instead"....