Thread: Computing a ratio with an array

What is wrong with this code cause im not getting the correct ratio of total people to cumulative length?

Code:

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


//define constant variables
#define FIRST_CAR_LENGTH 10
#define NORMAL_CAR_LENGTH 8
#define CAR_CAPACITY 4


int main(void){
    
    //Set up some variable with values that come from input or calculations
    double t_track;
    double train_length;
    double useable_track;
    int t_cars, maxcar, n;
    int t_people, maxpeep;
    int t_trains;
    int i; 
    float ratio_array[100];
    float sum;
    
    //initialize array values to zero
    for(i=0;i<99;i++){
        ratio_array[i]=0;
    }
    
    maxpeep = 0;
    maxcar = 0;
    
    
    //Prompting the user for data and assigning it to variables
    printf("What is the total length of the track, in feet?\n");
    scanf("%lf", &t_track);
    printf("what is the maximum length of a train, in feet?\n");
    scanf("%lf", &train_length);
    
    useable_track = (t_track * .25);
   
   //for loop to calculate all different train lengths
   for(i=10;i<=train_length;i+=8){ 
    
    t_cars = ((i-FIRST_CAR_LENGTH)/(NORMAL_CAR_LENGTH))+1;
    t_trains = (useable_track/i);
    t_people = (t_cars * CAR_CAPACITY * t_trains);
    
    if(maxpeep < t_people)
        maxpeep = t_people;
    if(maxcar < t_cars)
        maxcar = t_cars;
    
    ratio_array[t_cars-1] = (t_people/(i*t_cars)); 
    n++;
     }
    
    for(i=0;i<99;i++){
        sum += ratio_array[i];
    }
    
    printf("your ride can have at most %d people on it at one time\n", maxpeep);
    printf("This can be achieved with trains of %d cars\n", maxcar-1);
    printf("AVG ratio: %.3f\n", (sum/n));
    
    printf("\n");
    
    system("Pause");
    return 0;
}

Let's say you do 25/4. (I'm looking at line 53 as posted.) The result of that division is 6 (there is a remainder of 1, but you didn't ask for it so you don't get it). Note that this implies that if i*t_cars > t_people, then ratio_array will be zero (since the remainder is ignored). If you want to do division with decimals, you need to have decimals in the problem (or introduce one, by doing something like "t_people*1.0/(i*t_cars)").

No that's not working, but I appreciate the help! here is an example out put:What is the total length of the track, in feet?
4025
What is the maximum length of a train, in feet?
89
Your ride can have at most 480 people on it at one time.
This can be achieved with trains of 6 cars.
AVG Ratio: 0.467

and my AVG Ratio is out putting 0.251

As I'm looking over that loop again: (1) The whole point of putting the macros at the top is so that you wouldn't type things like "i=10" and "i+=8"; (2) What is the significance of the divisor i*t_cars? i is the length of a train, t_cars is the number of cars; I don't see why those should be multiplied.

IMO, you shouldn't be doing your math calculations, inside a for loop. A switch statement or a sequence of if statements, is needed.

To debug what you have, simply add a printf() statement right after your calculation line of code, and a getchar(). (Or use your debugger, and watch the result of the calculations, as they occur, step by step).

In general, loops with calculations in them, are used to refine the answer (think of a loop calculating the digits of Pi) - where the previous calculation is needed to reach the next answer. That shouldn't be the case with this problem.

Originally Posted by Adak

IMO, you shouldn't be doing your math calculations, inside a for loop. A switch statement or a sequence of if statements, is needed.

To debug what you have, simply add a printf() statement right after your calculation line of code, and a getchar(). (Or use your debugger, and watch the result of the calculations, as they occur, step by step).

In general, loops with calculations in them, are used to refine the answer (think of a loop calculating the digits of Pi) - where the previous calculation is needed to reach the next answer. That shouldn't be the case with this problem.

I think the idea (so far as I can tell) is to do the problem for each possible train-length (one car, two cars, three cars, etc) and storing the results for later work.

And a step in the right direction would be getting the math correct for one ratio (that is now incorrect), imo.

You have 100 elements and are only initialising 99 of them to 0... although you're only summing 99 of them as well. But then dividing by fubar. This is probably the issue.