SJF Non-preemptive scheduling algorithm

Question

I'm fresh on these scheduling algorithms. I've become comfortable with SJF non-preemptive and I understand it from a pen and paper Gantt chart perspective but not quite so from a programming perspective. My code is below and though it runs successfully my math is incorrect. How can I fix it?

TEST CASE Process Burst Arrival P1 20 0 P2 3 3 P3 2 5

Expected Outputs Avg wait of 11.3 Avg turnaround time of 19.6 Avg response time of 10.6

Actual outputs avg wait of 8.3 avg turnaround time of 6.6 avg response time of 14

    #include <iostream>
    using namespace std;

    void SJF_NP(int n, int burst[], int arrival[], int throughput)
    {
      cout << "Output for SJF_Non_Preemptive scheduling algorithm" << endl;

      int i, j, temp, temp2;
      double tot, avgwait, avgturnaround, avgresponse, tp;

      //array instantiations                                                                                          
      int start[n], end[n], wait[n];

      //calculations                                                                                                  
      for(i=1;i<=n;i++)
        {
          for(j=i+1;j<=n;j++)
            {
              if (i>=2 && burst[i-1]>burst[j-1])
                {
                  temp = burst[i-1];
                  temp2 = arrival[i-1];
                  burst[i-1]=burst[j-1];
                  arrival[i-1]=arrival[j-1];
                  burst[j-1]=temp;
                  arrival[j-1]=temp2;
                }
            }
          if(i==1)
            {
              start[0]=0;
              end[0]=burst[0];
              wait[0]=0;
            }
          else
            {
              start[i-1]=end[i-2];
              end[i-1]=start[i-1]+burst[i-1];
              wait[i-1]=start[i-1]+arrival[i-1];
            }
          //throughput                                                                                                
          if (start[i+1] <= throughput)
            tp = i+1;
        }

      //output                                                                                                        
      cout << "\n\nPROCESS \t BURST TIME\tARRIVAL TIME\tWAIT TIME\tSTART TIME\tEND TIME\n";
      for (i=0;i<n;i++){
        cout << "\nP[" << i + 1 << "]" << "\t\t" << burst[i] << "\t\t" << arrival[i] << "\t\t" << wait[i] << "\t\t" << start[i] << "\t\t" << end[i];
      }
      //avg wait time                                                                                                 
      for(i=1,tot=0;i<n;i++){
        tot+=wait[i-1];
        avgwait=tot/n;
      }
      //avg turnaround time                                                                                           
      for(i=1,tot=0;i<n;i++){
        tot+=end[i-1];
        avgturnaround=tot/n;
      }
      //avg response time                                                                                             
      for(i=1,tot=0;i<n;i++){
        tot+=start[i-1];
        avgresponse=tot/n;
      }
      cout << "\n\nAverage Wait Time: " << avgwait;
      cout << "\nAverage Response Time: " << avgturnaround;
      cout << "\nAverage Turnaround Time: " << avgresponse;
      cout << "\nThroughput for (" << throughput << "): " << tp << endl;
    }

Answer 1

#include<iostream>
using namespace std;
struct Process
{
    int pid,at,bt,wt,tt,ct; //at=arrival time;bt=burst time
                                 //wt=waiting time; tt=turnaround time
                                //ct=completion time
};

//to swap processes
void swap(struct Process *t ,struct Process *w )
{
struct Process v;
v=*t;
*t=*w;
*w=v;
}
 //sort according to arrival time
 void sort1(struct Process *t,int p)
{
int i,j;
struct Process *q,s;

for(i=0;i<p;i++,t++)
{
for(j=i+1,q=t+1;j<p;j++,q++)
{
 if((t->at)>(q->at))
 {
 s=*t;
 *t=*q;
 *q=s;
 }
 }
}
}

//sort according to burst time
void sort2(struct Process *t,int p)
{
int i,j;
struct Process *q,s;
if(t->pid==p)
{
    return;
}
for(i=1;i<p;i++,t++)
{
for(j=i+1,q=t+1;j<p;j++,q++)
{
 if((t->bt)>(q->bt))
 {
 s=*t;
 *t=*q;
 *q=s;
 }
 }
}
}

int main()
{
    int n;
    struct Process p[80];
    int at,bt;
    cout<<"Enter number of processes";
    cin>>n;
    for (int i=0;i<n;i++)
      {
        cout<<"for process"<<i+1<<"\n";
        cout<<"pid";
        cin>>p[i].pid;
        cout<<"at";
        cin>>p[i].at;
        cout<<"bt";
        cin>>p[i].bt;
      } 


    sort1(p,n);
    p[0].wt=0;
    p[0].ct=p[0].bt;
    p[0].tt=p[0].ct;



    cout<<"ProcessId\t"<<"Arrival time\t"<<"Burst time\t";
    for(int i=0;i<n;i++)
    {
        cout<<p[i].pid<<"\t\t"<<p[i].at<<"\t\t"<<p[i].bt<<"\n";
    }

    int t=1;
    do{

    int diff=p[t].at-p[t-1].ct;
    if(diff<=0)
    {
      sort2(&p[t],n);   
      for( int i=t+1;i<n;i++)
      {
        if(p[t].bt==p[i].bt&&p[i].at<p[t].at)
        {
            swap(&p[t],&p[i]);
            break;
        }

      }

      p[t].ct=p[t-1].ct+p[t].bt;
      p[t].tt=p[t].ct-p[t].at;
      p[t].wt=p[t].tt-p[t].bt;
    }

    else
    {
        p[t].ct=p[t].at+p[t].bt;
        p[t].tt=p[t].ct-p[t].at;
        p[t].wt=0;
        sort2(&p[t+1],n);
    }
    t++;

    }   
    while(t < n);
    //Output
    cout<<"ProcessId\t"<<"Arrival time\t"<<"Burst time\t"<<"Waiting Time\t"      
    <<"Turnaround Time\n";
    for(int i=0;i<n;i++)
    {
        cout<<p[i].pid<<"\t\t"<<p[i].at<<"\t\t"<<p[i].bt<<"\t\t"
        <<p[i].wt<<"\t\t"<<p[i].tt<<"\n";
    }

    return 0;
}