How to find reverse relations between jobs?

I've been trying to find reverse relations between jobs. To be more specific I will tell it with an example.

Suppose I have n jobs i.e {0,1,2,3,4,...n}. I also have relations between jobs. I know only successor jobs, i.e., 2 is followed by 4,5. 5 is followed by 7,8 etc. I have it in a text file. I want to obtaing precedence relations between jobs (what is the predecessor job of 5?).

Having text output would be great. I have some code but it does not work.

#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
using namespace std;

#define TOTAL_ACTIVITY 123

void readFile();
void change ();
void writeFile ();

struct Activity {
    int precedessor [3];
    int successor [3];
    int id;
};

Activity activityList[TOTAL_ACTIVITY];

void main() {
    readFile();
    change();
    writeFile();
}

void  readFile() {
    ifstream myReadFile;
    myReadFile.open("pre.txt");
    if (!myReadFile) { //check whether the file can be opened
        cerr << "Unable to open file"; // terminate with error
    }

    while (!myReadFile.eof()) {
        int Id,suc1, suc2, suc3;
        int t = 0;
        while (myReadFile >> Id >>  suc1 >> suc2 >> suc3) //data should be in this order
        {
            activityList[t].id = Id;
            activityList[t].successor [0] = suc1;
            activityList[t].successor [1] = suc2;
            activityList[t].successor [2] = suc3;
            t++;
        }
    }
    return;
}

void change() {
    int act;
    for (int i=1;i<TOTAL_ACTIVITY;i++){
        for (int j=0;j<TOTAL_ACTIVITY;j++){
            for (int k=0;k<3;k++) {
                if (activityList[j].successor[k]==i;)
            }
        }
    }
}

void writeFile() {
    ofstream out("out.txt");
    out << "id\t" << "Pre1\t" << "Pre2\t" << "Pre3\t"<<"\n";
    for (int j = 0; j < TOTAL_ACTIVITY; j++) {
        out << activityList[j].id << "\t";
        out << activityList[j].precedessor[0]<< "\t";
        out << activityList[j].precedessor[1] << "\t";
        out << activityList[j].precedessor[2] << "\t";
        out << "\n"; 
    }
    out.close();
}

Here is a sample input:

ID  Successor1  Successor2  Successor3
1   2   3   4
2   6   11  15
3   7   8   13
4   5   9   10
5   20      
6   30      
7   27      
8   12  19  27
9   14      
10  16  25  
11  20  26  
12  14      
13  17  18  
14  17      
15  25      
16  21  22  
17  22      
18  20  22  
19  24  29  
20  23  25  
21  28      
22  23      
23  24      
24  30      
25  30      
26  31      
27  28      
28  31      
29  32      
30  32      
31  32      

Output should be something like this:

Id开发者_如何学JAVA Predecesor1  Predecesor2  Predecesor3
........................................
...........................................
...........................................

You are given the successors of a job, but you do not need to retain this information.

For example, if I say: 5 -> 6, 7 meaning that 5 is followed by 6 and 7, then it is equivalent to saying that 6 and 7 are preceded by 5, right.

You can then either:

• directly output the precedence when reading the successors
• store in an associative container, using the job id as the key and the predecessor as value

Specifics details... are for you to work out to conclude your homework.