how can I (if possible) accept the user pressing return to give a default reading to the prompt below


/************************************************** ******************


ERASES the existing information in a file,
then APPENDS the pressure to that file!!!
ios::trunc == delete information in file // say IF entries>12
ios::app == open file for appending


************************************************** ******************/

#include<iostream.h>
#include<fstream.h>
#include<iomanip.h>
#include <time.h>
#include <conio.h>

int index;//acts as counter for pressure array

float riser1;//used to count the number of consecutively rising readings
float riser2;//
int rcount;//riser counter also used

void main()
{
float voltage[12];
float pressure[12];


ofstream outfile;//setting up stream (called outfile)
outfile.open("C:\\pressure.txt",ios::trunc);//clear existing file BEFORE the loop starts
outfile<<"TEXT FILE WRITTEN BY **your name here**TO DISPLAY PRESSURE READINGS\n\n";
outfile.close();//close it!!!

riser1=0;//set riser1 to 0
riser2=0;//set riser2 to 0

for(index=0;index<12;index++)//setting counter for loop (12 months)
{//begin for loop1

cout<<"\nEnter the voltage for reading number: "<<index+1<<" >> ";//prompt for input
cin>>voltage[index];//input voltage

pressure[index]=((voltage[index]*20)+500);//convert to pressure

if(pressure[index]>=700)//if equal to or greater than print this to screen
{
cout<<endl<<endl<<endl<<endl<<endl<<endl<<"\n\..........* ************ WARNING THE PRESSURE IS ABOVE RECOMENDED LEVELS!****************"<<endl;
}

//target stream (outfile) to a text file (pressure.txt)
//then open it ready to APPEND data

outfile.open("C:\\pressure.txt",ios::app);//open to append

//send data
if(pressure[index]>=700)//if pressure is >= 700 print asterix's as warning in file
{
outfile<<"*** "<<pressure[index]<<" *** THIS IS ABOVE MAXIMUM ALLOWED TEMPERATURE"<<endl;
}//end if

else//otherwise just print the pressure
{
outfile<<pressure[index]<<endl;//remembering to change line
}//end if

outfile.close();//close file
//
riser2=pressure[index];//set riser2 to pressure,

if(riser2>riser1)//now if riser2 is greater than riser1
{
rcount++;//add one to the counter (keeping count of consecutively rising readings)
}//end if
else
{
rcount=0;//else reset counter
}

if(rcount>=4)//if counter is greater than or equal to 4 (consecutively risen readings)
{
cout<<"\n\..........*********** WARNING! THE LAST "<<rcount<<" ENTRIES HAVE RISEN CONSECUTIVELY ************\n";//alert
cout<<"\n**** TRY RELEASING SOME OF THE PRESSURE FROM THE MAIN TANK USING THE VALVE ****\n";
//the user to how many consecutive rises there has been
}//end if


cout<<endl<<"file written\n\n\n";//inform the user the file has been written


riser1=riser2;//set riser1 to riser2
}//end loop1

cout<<setw(5)<<"readings"<<setw(12)<<"pressure"<<e ndl;//setting column width
for(index=0;index<12;index++)//12 times
{
cout<<setw(4)<<index+1<<setw(12)<<pressure[index]<<endl;//print the readings and pressures
}//end for

time_t hold_time;
hold_time=time(NULL);
outfile.open("C:\\pressure.txt",ios::app);//open to append
outfile<<"\nThis file was modified on: "<<ctime(&hold_time);
outfile.close();

}//end program



why does the text come out all mixed up??? when i copy and paste the code below???