[C++] Tutorial 1: Discussion

Yersinia Pestis

First off, dduardo, thank you for what has been, so far, a most excellent tutorial.

Having said that, the pascals triangle task has seemed daunting to me, and I had to look up how to make my own functions to do it efficiently, and I came up with this:

Code:

#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

long double factorial (int num)
{
 if (num==1)
  return 1;
 return factorial(num-1)*num; // recursive call
}

int pscl (int n, int r)
{
    int out;
    long double nf=factorial(n), rf=factorial(r), nrf=factorial(n-r);

    long double btm=rf * nrf;

    long double tout=nf/btm;

    out = (int)tout;

    return out;
}

int main(int argc, char *argv[])
{
    int rin;
    int nin;
    int pout;
    
    cout << "input row: ";
    cin >> rin;

    cout << "input number: ";
    cin >> nin;

    pout=pscl(nin, rin);
    cout << pout << endl;
    
    system("PAUSE");
    return EXIT_SUCCESS;
}

the function 'factorial' is fairly obvious, and is a double float because factorials can be big. 'pascal' uses the binomial coefficient thing to calculate the number at a given place and row (n, r).

but this code crashes, and the debugger (when it decides to work) tells me it gets an 'access violation'. I tried breakpoints, but as I said, the debugger is a bit temperamental. Can you see anything wrong with my code?

KTC

No idea what the original question were, but I can tell you your program is crashing because / when the end user is entering a number for 'rin' that's larger than 'nin'. I.e. when 'n-r' < 0. (It might also crash when n-r == 0, but too lazy to try.)

Yersinia Pestis

Could you be kind enough to explain why it's crashing when n-r==0 ?

EDIT: I changed the condition for the if statement in 'factorial' to (num==1 || num==0) and now it works fine. I'm never nicking code off the internet again.

Thanks for bringing it to my attention. I can't believe I didn't spot that after going over the problem for 40 minutes.

Yersinia Pestis

Well, I've finished the tutorial. Again, great material dduardo.

But the open goal at the end seems a bit steep. A text adventure? With out object-orientation? But, I guess it's a challenge.

Except i ran into a major hurdle. Text inputs. This code:

Code:

#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main(int argc, char *argv[])
{
    char inp[256];
    cin >> inp ;
    cout << inp << endl;
    system("PAUSE");
    return EXIT_SUCCESS;
}

Only seems to grab input up to the first space. How can I get the whole string that is entered?

Also, did the other parts to the tutorial get thrown in the bin? If so, is there anywhere I can look for the higher levels of C++ programming?

KTC

Code:

#include <iostream>
#include <string>

int main()
{
    std::string s;
    std::getline(std::cin, s);
    std::cout << s << std::endl;
}

Yersinia Pestis

Thanks man! I'm gonna makes me a text adventure!

And look for more C++ stuff that's got stuck in the tubes.

i.mehrzad

I know nothing in programming and wanted to know that is this the right place to start as i am finding it a little difficult.

archermcr

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

int main(int argc, char *argv[])
{
int x,j;
double N,T,S,D,X;

cout<<"Enter Value of x:"<<endl;
cin>>x;
cout<<"Enter No. of Iterations:"<<endl;
cin>>j;
cout<<endl;
X=(x*3.1416)/180; //conversion from degrees to rad
N=X; //inital value
D=1; //inital value
T=1; //inital value
S=1; //inital value
cout<<"i"<<"\t\t"<<setw(6)<<"Term"<<"\t\t"<<"cos("<<x<<")"<<endl;
cout<<endl;
for(int i=1;i<=j;i++)
{
N=(-N)*X*X;
D=D*(2*i)*((2*i)+1);
T=N/D; //term
S=S+T; //summation
cout.setf(ios::fixed|ios::showpoint|ios::left);
cout<<i<<setprecision(3)<<"\t\t"<<T<<"\t\t"<<setprecision(5)<<S<<endl;
}
cout<<endl;
cout<<endl;
cout<<"sin("<<x<<")"<<"="<<S<<endl;
system("PAUSE");
return EXIT_SUCCESS;
}
i derived the formula from the expansion of the function of sin(x) which is
x-(x^3/3!)+(x^5/5!)-(x^7/7!)... pls help me..whats wrong w/ my program =(..it wont seem to display the cortrect value..

Dendi

Why don't u separate the code for searching the factorial of n (n!) in another function.....it will be much easier to debug.......

chaoseverlasting

If you write out the code for calculating factorial separately, and use cmath(for the pow() function), you could do it like this:

s=0;
for(int i=1;i<=j;i++)
{
s+=pow(-1,i+1)*pow(x,2*i+1)/fact(2*i+1);
};

cout<<"sinx= "<<s;

where pow(a,b)=a to power b, and fact(x) =x!

bleeker

Will C or C++ work better to program a pic?

soul

Quote by bleeker

Will C or C++ work better to program a pic?

Well, It can be wrong but I think C would be better, since it is an event driven programming language and it work faster, in more efficient way.

Ben Niehoff

Quote by Yersinia Pestis

First off, dduardo, thank you for what has been, so far, a most excellent tutorial.

Having said that, the pascals triangle task has seemed daunting to me, and I had to look up how to make my own functions to do it efficiently, and I came up with this:

Code:

#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

long double factorial (int num)
{
 if (num==1)
  return 1;
 return factorial(num-1)*num; // recursive call
}

int pscl (int n, int r)
{
    int out;
    long double nf=factorial(n), rf=factorial(r), nrf=factorial(n-r);

    long double btm=rf * nrf;

    long double tout=nf/btm;

    out = (int)tout;

    return out;
}

int main(int argc, char *argv[])
{
    int rin;
    int nin;
    int pout;
    
    cout << "input row: ";
    cin >> rin;

    cout << "input number: ";
    cin >> nin;

    pout=pscl(nin, rin);
    cout << pout << endl;
    
    system("PAUSE");
    return EXIT_SUCCESS;
}

the function 'factorial' is fairly obvious, and is a double float because factorials can be big. 'pascal' uses the binomial coefficient thing to calculate the number at a given place and row (n, r).

but this code crashes, and the debugger (when it decides to work) tells me it gets an 'access violation'. I tried breakpoints, but as I said, the debugger is a bit temperamental. Can you see anything wrong with my code?

Ack! There are FAR more efficient ways to calculate Pascal's triangle than to use a factorial function. I think the Mathworld page failed to explain the key property of Pascal's Triangle, which was Pascal's original motivation in the first place:

The numbers in each row are obtained by adding the two numbers above them in the previous row.

The Wikipedia page has a much better explanation: http://en.wikipedia.org/wiki/Pascal%27s_triangle

Maybe you should try re-writing your program not using any factorial function. Even a long double will not be big enough for you to get a triangle more than 20 rows or so. And it will also be inaccurate due to rounding errors (a double only holds 15 significant digits of accuracy; it represents higher numbers by using scientific notation).

If you use a more efficient method (simply summing terms from the previous row), then you can create huge triangles using only ints, and your program will run much, much faster.

Malvenuto

Assignment 3 requires use of the sqrt() function and the math.h library. The tutorial before that point doesn't mention them. Not sure whether or not you meant to leave them for the reader to find out on his own, but if not it might make sense to add them.

computerex

Quote by soul

Well, It can be wrong but I think C would be better, since it is an event driven programming language and it work faster, in more efficient way.

C is procedural, not event driven.

Can anyone please try to see what's wrong with this program? I wrote it for school, to help simply radicals. It seems to work fine most of the times. However there is a problem when trying to simply perfect cubes. Try and entering 64 with an index of 3, it'll give you an unexpected answer. Now try entering 8 with an index of 3, and it'll give you the right answer. I have tried debugging it, but it's not making any sense to me. I am sorry if this shouldn't be posted here, let me know and I can move it. :)

Code:

#include <iostream>
#include <math.h>

typedef struct 
{
	double x, y;
}pair;

bool breakcube(double radi, double inx, pair& pr)
{
	double cb = 0;
	for (int i = 2; i < radi; i++)
	{
		cb=i;
		for (int j = 1; j < inx; j++)
		     cb*=i;
		for (int j = 1; j < radi; j++)
		{
			if (cb*j==radi)
			{
				pr.x = cb;
				pr.y = j;
				return true; 
				
			}
		}
	}
	return false;
}
int main()
{
	double radi = 0;
	double cb   = 0;
	double inx  = 0;
	char   buff[500];
	while(true)
	{
		std::cout << "\nEnter radicand ('quit' to terminate): ";
		std::cin >> buff;
		if (!strcmp(buff, "quit"))
		    return 0;
		radi=atof(buff); 
		std::cout << "\nEnter index: ";
		std::cin >> inx;
		cb = pow(radi, 1.0/inx);
		if (cb == (int)cb)
		{
			std::cout << "\nPerfect " << inx << " : "; 
			for (int i = 0; i < inx; i++)
			{
				std::cout << cb;
				if (i!=inx-1)
				    std::cout << " * ";
		    }
		    std::cout << " = " << pow(cb, inx);
			continue;
		}
		pair pr;
		if (breakcube(radi, inx, pr))
		   std::cout << "\n" << radi << " can be broken by: " << pr.x << " * " << pr.y;
		else
		   std::cout << "\n" << radi << " cannot be broken";
	}
	return 0;
}

Malvenuto

In Exercise 5, the do-while example code prints out HELLO only twice instead of the 3 times shown in the example output.

robertkurti

guys can somebody help me with a java programming? It's a really short homework but I'm just stucked there. Here is the code if u can please teplay to me asap.

public class DaysPerWeek
{
public static void main(String[] args)
{

final int DAYS_PER_WEEK = 7;
int days;
int weeks;
int totalDays;
Scanner scan = new Scanner(System.in);

System.out.println("This program convert days to week");
System.out.print("Enter number of days; ");
days = scan.nextInt();
days = totlaDays % DAYS_PER_WEEK;
weeks = totalDays / DAYS_PER_WEEK;
System.out.println("Days Per Week" + totaldays);
}

}

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Yersinia Pestis	Could you be kind enough to explain why it's crashing when n-r==0 ? EDIT: I changed the condition for the if statement in 'factorial' to (num==1 \|\| num==0) and now it works fine. I'm never nicking code off the internet again. Thanks for bringing it to my attention. I can't believe I didn't spot that after going over the problem for 40 minutes.

KTC	Code: #include <iostream> #include <string> int main() { std::string s; std::getline(std::cin, s); std::cout << s << std::endl; }

i.mehrzad	I know nothing in programming and wanted to know that is this the right place to start as i am finding it a little difficult.

Dendi	Why don't u separate the code for searching the factorial of n (n!) in another function.....it will be much easier to debug.......

chaoseverlasting	If you write out the code for calculating factorial separately, and use cmath(for the pow() function), you could do it like this: s=0; for(int i=1;i<=j;i++) { s+=pow(-1,i+1)pow(x,2i+1)/fact(2*i+1); }; cout<<"sinx= "<<s; where pow(a,b)=a to power b, and fact(x) =x!

Malvenuto	Assignment 3 requires use of the sqrt() function and the math.h library. The tutorial before that point doesn't mention them. Not sure whether or not you meant to leave them for the reader to find out on his own, but if not it might make sense to add them.