Abs () not working ....

#include <iostream>

#include<cmath>
#include<limits.h>
#include<stdio.h>
#include<algorithm>
#include<stdlib.h>


using namespace std;

int main()
{

    long int n=0;
    long int a[n];
    long int value;
    cin>>n;
    for(int i=0; i<n;>    {
        cin>>a[i];
    }
    cin>>value;
    cout<<endl;
    int temp=0,temp2=0;
    float diff=0.0,diff2=0.0;
    for(int i=0; i<n;>    {
        temp=a[i];

        for(int j=i+1; j<n;>        {
            if(i==0)
            {
                if(temp+a[j]==value)
                {
                    temp2=a[j];
                    diff=temp2-temp;
                         diff=abs(diff);
                    cout<<"temp="<<temp<<"temp2="<<temp2<<"diff="<<diff<<endl;
                }
            }


            else
            {
                if(temp+a[j]==value)
                {
                    temp2=a[j];
                    diff2=temp2-temp;
                    diff2=abs(diff2);


                    if(diff2<diff)>
                        diff=diff2;


                    cout<<"temp="<<temp<<"temp2="<<temp2<<"diff="<<diff<<endl;
                }
            }

        }
    }
    cout<<"diff="<<diff<<" value="<<value<<endl;
    int aa=(value+diff)/2;
    int b=(value-diff)/2;
    cout<<"Peter should buy books whose prices are "<<b<<" and "<<aa<<endl;</stdlib.h></algorithm></stdio.h></limits.h></cmath></iostream>