recursion linked list ? I think

Question

I am very new and I have trouble understanding the macro steps I need to learn how to effectively code. These assignments feel extremely abstract and have to learn everything about recursion before I can even do it. Coding up a program is not easy, and I do really well when someone "helps me stay between the mayonnaise and mustard" so to speak. What am I doing wrong and what direction do I need to continue in?

I was thinking that I needed to sort the list first then have two seperate functions for merge sort and insertion sort per the assignment:

You are spending most of your time at home in this pandemic. It is of most importance for people to be aware of where other people, who are infected with COVID-19 are, and who they've been near. Keeping track of this information is known as "contact tracing." You've heard that there might be some very high paying jobs if you can show your contract tracing skills to the government, so you've decided to write a little program to highlight those skills.Your area can be modeled on the Cartesian plane. You are located at the point (x, y). In addition, you have the Cartesian coordinates of all people currently infected with COVID-19. What you would like to do is write a program that sorts these locations based on their distance from you, followed by handling queries. The queries are of the form of a point you are thinking of visiting. Your program should identify if someone who is infected is at that location, and if so, what their rank is on the sorted list of infected people. If no one is infected at that location, you should correctly identify this.

Note: There are many important implementation restrictions for this assignment, so to make sure everyone reads these, the section on implementation restrictions will be next, changing the order of the sections as compared to other assignments.

Implementation Restrictions

1. You must use a specified combination of Merge Sort and Insertion Sort to sort the point data. Specifically, for each input case, a threshold value, t, will be given. If the subsection of the array to sort has t or fewer values to sort, Insertion Sort should be used. Otherwise, Merge Sort should be used.Further details about the comparison used for the sorting are below.
2. You must store your coordinates in a struct that contains two integer fields.
3. You must write a function compareTo which takes in two pointers, ptrPt1 and ptrPt2, to coordinate structs and returns a negative integer if the point pointed to by ptrPt1 is closer to you than the point pointed to by ptrPt2, 0 if the two locations pointed to by both are identical locations, and a positive integer if the point pointed to by ptrPt1 is farther from you than the point pointed to by ptrPt2. Exceptions to this will be when the two pointers are pointing to points that are the same distance from you, but are distinct points. In these cases, if ptrPt1's x coordinate is lower than ptrPt2's x coordinate, a negative integer must be returned.Alternatively, if ptrPt1's x coordinate is greater than ptrPt2's x coordinate a positive integer must be returned. Finally, if the x coordinate of both points is the same, if ptrPt1's y coordinate is lower than ptrPt2's y coordinate, a negative integer must be returned. If ptrPt1's y coordinate is greater than ptrPt2's y coordinate, a positive integer must be returned.
4. Since your location must be used for sorting, please make the variable that stores your x and y coordinates global. Your program should have no other global variables.
5. A Binary Search function must be used when answering queries.
6. Your sort function should take in the array to be sorted,the length of the array as well as the threshold value, t, previously mentioned. This function should NOT be recursive. It should be a wrapper function.
7. The recursive sort function (you can call this mergeSort) should take in the array, a starting index into the array, an ending index into the array and the threshold value t. In this function, either recursive calls should be made OR a call to an insertion sort function should be made.

The Problem

Given your location, and the location of each person who has COVID-19, sort the list by distance from you from shortest to longest, breaking ties by x-coordinate (lower comes first), and then breaking those ties by y coordinate (lower comes first). After sorting, answer several queries about points in the coordinate plane. Specifically, determine if a query point contains someone who is infected or not. If so, determine that person's ranking on the sorted list in distance from you.

The Input(to be read from standard input)-Your Program Will Be Tested on Multiple Files

The first line of the input contains 5 integers separated by spaces. The first two of these values are x and y (|x|, |y| ≤ 10000), representing your location. The third integer is n (2 ≤ n ≤ 106), representing the number of infected people. The fourth integer is s (1 ≤ s ≤ 2x105), representing the number of points to search for. The last integer, t (1 ≤ t≤ 30), represents the threshold to be used for determining whether you run Merge Sort of Insertion Sort. The next n lines of the input contain x and y coordinate values, respectively, separated by spaces, representing the locations of infected people. Each of these values will be integers and the points will be distinct (and also different from your location) and the absolute value of x and y for all of these coordinates will not exceed 10,000.Then the next s lines of the file contains x and y coordinate values for searching. Both values on each line will be integers with an absolute valueless than or equal to 10,000.

The Output (to be printed to standard out)

The first n lines of output should contain the coordinates of the people infected, sorted as previously mentioned. These lines should have the x-coordinate, followed by a space, followed by the y-coordinate.The last s lines of output will contain the answers to each of the s queries in the input. The answer for a single query will be on a line by itself. If the point queried contains an infected person, output a line with the following format:

x y found at rank R

, where (x, y) is the query point, and R is the one-based rank of that infected person in the sorted list. (Thus, R will be 1 more than the array index in which (x, y) is located, after sorting.) If the point queried does NOT contain an infected person, output a line with the following format:

x y not found

Sample Input

(Note: Query points in blue for clarity. last five)

0 0 14 5 53
1 -6 
-2 -4 
3 4 
-4 2 
4 -1 
3 2 
2 0 
-5 -4 
-2 -6 
6 4 
4 -2 
4 0 
5 -4 
6 2 
-13 1  
0 -5

my code so far

#include <stdio.h>

int x = 0;//global coordinates
int y = 0;

typedef struct {
    int xInput, yInput;
}coordinates;

void scanPoints(coordinates[], int infectedPeople);
void scanSearchValues(coordinates[], int pointsToSearch);

void SortPoints(coordinates[], int);
int lessThan(coordinates[], int, int);
void printPoints(coordinates[], int);

void
scanPoints(coordinates pts[], int infectedPeople){
    for (int i = 0; i < infectedPeople; i++){
        scanf("%d %d", &pts[i].xInput, &pts[i].yInput);
    }
}

void
scanSearchValues(coordinates pts[], int pointsToSearch){
    for (int i = 0; i < pointsToSearch; i++){
    scanf("%d %d", &pts[i].xInput, &pts[i].yInput);
    }
}

void
sortPoints(coordinates pts[], int infectedPeople){
  
    int i, start, min_index, temp;

    for (start = 0; start < infectedPeople - 1; start++) {
    min_index = start;

    for (i = start + 1; i < infectedPeople; i++) {
    if (lessThan(pts, i, min_index)) {
    min_index = i;
    }
    }

    if (min_index != start) {
    coordinates temp = pts[start];
    pts[start] = pts[min_index];
    pts[min_index] = temp;
    }
    }
}

int
lessThan(coordinates pts[], int p, int q) {

if ((pts[p].xInput < pts[q].xInput) || ((pts[p].xInput == pts[q].xInput) && (pts[p].yInput < pts[q].yInput))) {
return 1;
    }
}




int
main(int argc, const char * argv[]) {
    
    int infectedPeople;
    int pointsToSearch;
    int threshold;
   
    scanf("%d%d", &x, &y);
    if(x > 10000 || y > 10000 )
        return 0;

    scanf("%d", &infectedPeople);
    if(infectedPeople < 2 || infectedPeople > 1000000)
        return 0;
   
    scanf("%d", &pointsToSearch);
    if(pointsToSearch < 1 || pointsToSearch > 200000)
        return 0;
    
    scanf("%d", &threshold);
    if(threshold < 1 || threshold > 30)
        return 0;
    
    
    return 0;
    
}

Answer 1

This is a challenging exercise for someone new to programming, but the first step is to read the problem description carefully. It might help to print it out on paper, so that you can easily mark it up with highlighter and / or pen. Additionally, you may be intimidated by all the details specified in the exercise. Don't be! Although some make work for you, most make decisions for you. The latter kind save you work, and do you exactly the service you asked of us: help you stay on track.

One of the keys to programming is learning to divide a problem into smaller pieces. Sometimes those pieces will also need to be divided into even smaller pieces. Many of these pieces will correspond naturally to functions, and accordingly, a second key to programming is recognizing how to choose the pieces so that they have well-defined inputs and outputs, and, to some extent, so that pieces can be re-used. In your case, the overall problem statement gives you a starting point for performing such an analysis:

Given your location, and the location of each person who has COVID-19, sort the list by distance from you from shortest to longest, breaking ties by x-coordinate (lower comes first), and then breaking those ties by y coordinate (lower comes first). After sorting, answer several queries about points in the coordinate plane. Specifically, determine if a query point contains someone who is infected or not. If so, determine that person's ranking on the sorted list in distance from you.

(Emphasis added.) The three main pieces I see there are

• read and store input data
• sort the data
• analyze the result and produce output

Reading the input

The implementation restrictions in the problem description have a lot to say about how you read and store the data. In particular,

2. You must store your coordinates in a struct that contains two integer fields.

You've prepared such a structure type.

4. Since your location must be used for sorting, please make the variable that stores your x and y coordinates global. Your program should have no other global variables.

Reading the restrictions carefully, I think the expectation is that you use the coordinates structure to represent all coordinates appearing in the program, including the (one) global variable representing your own coordinates.

6. Your sort function should take in the array to be sorted

You mentioned a linked list, but this indicates that you are expected to store the data in an array, not a linked list. From my more experienced vantage point, I have more reasons to believe that that is the expectation.

The detailed description of the input format gives you additional guidance on how to perform the reading, as of course the code needs to be suited to the data. So, read the first line of input to get the main program parameters, and store them appropriately. Among those is the number of infected person records to read; you'll need to store all those in memory in order to sort them and answer multiple queries about them, so allocate an array of structs large enough to hold them, then proceed to read those data.

You could similarly read and store the queries in advance, but I would suggest instead performing the required sorting first, and then processing each query immediately after reading it, without storing the whole list of queries.

Sorting the data

You write,

I was thinking that I needed to sort the list first then have two seperate functions for merge sort and insertion sort

Yes, I too read the problem description to be asking for separate merge sort and insertion sort functions -- and that's not what you seem presently to be providing. It also asks for a wrapper function that accepts the input and passes it on to the appropriate sort implementation, either (recursive) merge sort or insertion sort. Note that the wrapper function does not itself sort the list, except inasmuch as it passes the list to one of the other functions for sorting:

void sortCoordinates(coordinates coords[], int count, int threshold) {
    if (/* your condition here */) {
        insertionSortCoordinates(coords, count);
    } else {
        mergeSortCoordinates(coords, count);
    }
}

(The names and most of the details of these particular functions are at your discretion.)

Additionally, the restrictions help you out again here, though you need to read between the lines a bit. Both sorting and searching require that you have a way to compare the objects in the list, and look! The restrictions tell you exactly what form that should take:

3. You must write a function compareTo which takes in two pointers, ptrPt1 and ptrPt2, to coordinate structs [...]

In other words,

int compareTo(coordinates *ptrPt1, coordinates *ptrPt2) {
    /* your code here */
}

Your insertion and merge sort functions and also your binary search function (see below) will compare structures (when needed) by calling that function.

Do pay careful attention to the restrictions, though, as one of the decisions they make for you is the name for this function: compareTo, not lessThan. Deviating from the restrictions in this regard would likely cost you some marks.

Computing the output

Whether you read and store the query lines in advance or process them as you read them (having first sorted the input), the main computation to be performed is a binary search of the coordinates, per restriction 5. You'll wan't a function for that, maybe

int binarySearch(coordinates *target, coordinates coords[]) {
    /* your code here: returns 1-based rank if found, zero if not found */
}

Again, this function will use your compareTo function to compare coordinate structures. Note in particular that if implemented correctly according to the restrictions, compareTo() will return zero if and only if the two objects being compared are equal.

Answer 2

in

int
lessThan(coordinates pts[], int p, int q) {

if ((pts[p].xInput < pts[q].xInput) || ((pts[p].xInput == pts[q].xInput) && (pts[p].yInput < pts[q].yInput))) {
return 1;
    }
}

if ((pts[p].xInput < pts[q].xInput) || ((pts[p].xInput == pts[q].xInput) && (pts[p].yInput < pts[q].yInput))) is false the function does not return, introducing an undefined behavior in sortPoints

you wanted

int lessThan(coordinates pts[], int p, int q)
{
  return ((pts[p].xInput < pts[q].xInput) || ((pts[p].xInput == pts[q].xInput) && (pts[p].yInput < pts[q].yInput)));
}

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in sortPoints the variable temp in int i, start, min_index, temp; is useless, remove it

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In main you only read the 5 values, nothing more, so the other functions are useless, and you do not print nor compute something

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Not sure my answer is really usefull ...