Programa 3 (corregido), C++ (gcc)

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//g++  5.4.0
#include<math.h>
#include <iostream>
#include <iomanip>
#include<stdlib.h>
#include<stdio.h>
#include<vector>

using namespace std;

int programa3();
int diferencias(vector<double>,vector<double>,int);
int integral(vector<double>,vector<double>,int);
int llenar_dat(vector<double>,vector<double>,int);
int main()
{
    cout<<fixed<<setprecision(6);
    programa3();
    return 0;
}

int programa3(){
    int t;
    vector<double> x;
    vector<double> fx;
    double d1, d2;
    
    //Lectura de los datos
    x.clear();
    fx.clear();
    
    cin>>t;
    x.resize(t);
    fx.resize(t);
    
    for(int i=0; i<t; i++){
        cin>>d1;
        cin>>d2;
        x[i]=d1;
        fx[i]=d2;
    }
    
    integral(x,fx,t);
    diferencias(x,fx,t);
    
    return 0;
}

int diferencias(vector<double> x,vector<double> fx ,int t){
    
    vector<double> dif,dif2;
    double h, a, b;
    
    cout<<"\nDiferenciacion numerica\n\n";
    
    cin>>a;//Primer dato del intervalo
    cin>>b;//Segundo dato del intervalo
    
    dif.clear();
    dif2.clear();
        
    dif.resize(t);
    dif2.resize(t);
    
    h=x[1]-x[0];    
    
    for(int i=1; i<t-1; i++){
        dif[i]=(fx[i+1]-fx[i-1])/(2*h);
        dif2[i]=(fx[i+1]-2*fx[i]+fx[i-1])/(h*h);
    }
    cout<<"-----------------------------------------\n";
    cout<<"|x        |"<<"f(x)     |"<<"f'(x)     |"<<"f''(x)  |"<<"\n";
    cout<<"-----------------------------------------\n";
    for(int i=0; i<t; i++){
        cout<<"|"<<x[i]<<" |"<<fx[i]<<" |";
        if(x[i] >= a and x[i] <= b) cout<<dif[i]<<" |"<<dif2[i]<<"|\n";
        else cout<<"          |        |\n";
    }
    cout<<"-----------------------------------------\n";
    
    return 0;
}

int integral(vector<double> x,vector<double> fx ,int t){
    
    double suma=0,a,b,h,suma2=0;
    int lim_sup=0, lim_inf=0;
    bool normal=true;
    
    cout<<"\nIntegracion numerica\n\n";
    
    cin>>a;//Primer dato del intervalo
    cin>>b;//Segundo dato del intervalo
    
    h=x[1]-x[0];    
    
    for(int i=0; i<t; i++){
        if(a==x[i]) lim_inf=i;
        if(b==x[i]) lim_sup=i;
    }
    
    if(!((lim_inf-lim_sup)%2==0)) normal=false;
    
    if(normal){
        suma=suma+fx[lim_inf];
        suma=suma+fx[lim_sup-1];
        for(int i=lim_inf+1; i<lim_sup-1; i++){
            if(i%2!=0) suma=suma+4*fx[i];
            if(i%2==0) suma=suma+2*fx[i];
        }
        suma=suma*(h/3);
    }
    else{
        suma=suma+fx[lim_inf];
        suma=suma+fx[lim_sup-3];
        for(int i=lim_inf+1; i<lim_sup-3; i++){
            if(i%2!=0) suma=suma+4*fx[i];
            if(i%2==0) suma=suma+2*fx[i];
        }
        suma=suma*(h/3);
        cout<<fx[lim_sup-3];
        suma2=suma2+fx[lim_sup-3];
        suma2=suma2+fx[lim_sup];
        suma2=suma2+3*(fx[lim_sup-2]+fx[lim_sup-1]);
        suma2=suma2*((3*h)/8);
        suma=suma+suma2;        
    }cout<<"------------------------\n";
    cout<<"|x        |"<<"f(x)     |\n";
    cout<<"-------------------------\n";
    for(int i=lim_inf; i<lim_sup; i++){
        cout<<"|"<<x[i]<<" |"<<fx[i]<<" |\n";
    }
    cout<<"--------------------------\n";
    cout<<""<<suma<<"\n";
    return 0;
}

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Compilation time: 0.63 sec, absolute running time: 0.19 sec, cpu time: 0.13 sec, memory peak: 3 Mb, absolute service time: 0,83 sec