Question

Every time this code runs in UNIX, it hangs and displays a new line without a prompt. It compiles without error. Anyone know what’s up here?

/*
This program uses a Monte Carlo simulation to maximize widget profits.
 */

#include <stdio.h>
#include <stdlib.h>

// constant values used in the simulation
#define MIN_WIDGETS 1000 // minimum widgets per month
#define MAX_WIDGETS 1500 // maximum widgets per month

#define MIN_COST 0.25 // minimum cost per widget
#define MAX_COST 0.82 // maximum cost per widget

#define MIN_CONVERSION 0.01 // minimum converstion rate
#define MAX_CONVERSION 0.05 // maximum converstion rate

#define MIN_PROFIT 38 // minimum profit per sale
#define MAX_PROFIT 43 // maximum profit per sale

#define FIXED_OVERHEAD 400 // fixed overhead cost

#define NUM_SIMULATIONS 100 // number of simulations performed

void main()
{

  // to keep track of inputs in highest-profit simulation
  int bestW = 0; // widgets per month
  float bestC = 0; // cost per widget
  float bestR = 0; // rate of converstion
  float bestP = 0; // profit per sale
  float bestProfit = 0; // highest profit

  srand(time(0)); // initialize the random number generator
  float h = FIXED_OVERHEAD;

  int i; // loop index 
  for(i = 0; i < NUM_SIMULATIONS; i++)
    {
        // initialize inputs for this individual simulation
        int w = 0; // quantity of widgets bought
        float c = 0; // cost of a widget
        float r = 0; // conversion rate
        float p = 0; // profit per sale
        float profit = 0; // profit

        // simulate quantity of widgets bought per month, between MIN_WIDGETS and MAX_WIDGETS
        w = random() % (MAX_WIDGETS + 1); // to set the maximum value of w at MAX_WIDGETS
        while (w < MIN_WIDGETS) {
            w = random() % (MAX_WIDGETS + 1); // fetch new random number that may fit parameters
        }

        // simulate cost per widget, between MIN_COST and MAX_COST
        c = random() % 100; // to convert random number into an integer between 0 and 99
        while (c < (MIN_COST*100) || c > (MAX_COST*100)) {
            c = random() % 100; // fetch new random number that may fit parameters
        }
        c = c / 100.0; // convert cost back from cents into dollars

        // simulate conversion rate, between MIN_CONVERSION and MAX_CONVERSION
        r = random() % 100; // to convert random number into an integer between 0 and 99
        while (r < (MIN_CONVERSION*100) || r > (MAX_CONVERSION*100)) {
            r = random() % 100; // fetch new random number that may fit parameters
        }
        r = r / 10.0; // convert back into fraction

        // simulate profit per sale, between MIN_PROFIT and MAX_PROFIT
        p = random() % ((MAX_PROFIT + 1)*100); // to convert random number into an integer between 0 and 4300
        while (p < MIN_PROFIT*100) {
            p = random() % (MAX_PROFIT + 1); // fetch new random number that may fit parameters
        }
        p = p / 100.0; // convert back into floating point with two decimal places after period

        profit = (w * r * p) - (h + c * w);

        printf("Current profit is $%.2f, with %d widgets at a %.2f cost per widget with a %.1f conversion rate and %.2f profit/sale.\n",
            profit, w, c, r, p);

        if (profit > bestProfit) {
            bestW = w;
            bestC = c;
            bestR = r;
            bestP = p;
            bestProfit = profit;
        }

    }

  printf("Maximum profit is $%.2f, with %d widgets at a %.2f cost per widget with a %.1f conversion rate and %.2f profit/sale.\n",
    bestProfit, bestW, bestC, bestR, bestP);

}

Answer 1

I think the error is here:

while (p < MIN_PROFIT*100) {
    p = random() % (MAX_PROFIT + 1); // fetch new random number that may fit parameters
}

If the loop body is entered p will be set to a number between 0 and MAX_PROFIT (= 43). It will never be greater than or equal to MIN_PROFIT*100 (= 3800), so it will go into an infinite loop.

As a side-note, you might want to consider using do { } while loops here instead of using a while loop and writing the loop body twice. Writing code twice is an excellent way to make errors, because when you change one of the implementations you always have to remember to change the other. If you forget to update both you will introduce a bug. And that seems to be what happened here.