# I rea......lly need your help ( C++ )

Discussion in 'C++' started by ashmangat@yahoo.com, Nov 12, 2005.

1. ### Guest

Hey I am new, but I don't have time to intruduce myself yet.
I am intro to C++
and this is a programme I have to write.
all the direction are here, It will be very nice of someone to figure
this out.
note: I only in intro C++ which is about to be finished by the next two
weeks.
The topic which I am on in my book is "POINTERS"

Text from the book:

Write a program that can be used to gather statistical data about the
number
of movies college students see in a month. The program should perform
the
following steps:
Ask the user how many students were surveyed. An array of
integers with this many elements should be dynamically allocated.
Allow the user to enter the number of movies each student saw into the
array.
Calculate and display the average, median, and mode of the values
entered.
(Use the functions you wrote in problem 6 and 7 to calculate the median
and mode.)

Input Validation: Do not accept negative numbers for input.

Use the following skeleton for the main program:

void main(void)
{
int *dyn_array;
int mode, students;
float avrg;

do
{
cout << "How many students will you enter? ";
cin >> students;
}while ( students <= 0 );

dyn_array = create_array( students );
//this funtion creates a dynamic array
//and is found in the book
enter_data( dyn_array, students );
//use 'pointer' notation in this function to
//access array elements
mode = get_mode( dyn_array, students );
//check out pc6 for an explanation of 'mode'
//median = median( dyn_array, students );
//don't do median in this exercise
avrg = average( dyn_array, students );
//should be trivial

cout << "The array is: ";
showArray( dyn_array, students);
//you should already have this function
cout << endl;
cout << "The Mode is " << mode << ".\n";
cout << "The average is " << avrg << ".\n";

delete [] dyn_array;
return 0;
}

Some help (and requirements) for the mode function:

'mode' as explained in pc6: "In Statistics, the 'mode' of a set of
value is the
value, which occurs most often or with the greatest frequency".

Note: Very important to completely understand the explanation below.
Get
on the discussion board if there is any confusion.

The call:
mode = get_mode(dyn_array, student);

int get_mode(int *student_data, int num_students)

get_mode should do the following:

create an array 'frequency' of 1000 integers. 1000 is arbitrarily
chosen as
the absolute maximum of movies a college student could see in a week.
This array
will keep track of frequencies of weekly movie watching values.

Intialize array 'frequency' elements to zeros (use a loop)

Populate the 'frequency' array with the data (how many students see 1,
2, 3,
etc. movies a week)

The following algorithm is a useful method to easily increment random
values:

Run a loop on 'student_data' from 0 to 'num_students'.

The content of 'student_data' array becomes the
index of the 'frequency' array and the content of that 'frequency'
array
element gets incremented. Very important to understand this process.

Here is a little animation help for
the sentence content of 'student_data' array becomes the index of the
'frequency' array'.

In subscript notation, it would look like this:

frequency [ student_data[index] ] ++

However, a requirement for the code above is to implement
student_data[index]' in pointer notation and leave frequency[] in
subscript
notation.

Example of processing the mode with 'frequency' and 'student_array':

If, at the beginning, the frequency array contains:

0 0 0 0 0 0 ...
and 'student_data' contains:

2 4 1 2 ...

In the loop the first element '2' points to the third element (index 2)
in the
'frequency' array and increments it. So now 'frequency' looks like
this:

0 0 1 0 0 0 ...

By the time the loop runs through all 4 elements of 'student_data',
'frequency'
will look like this:

0 1 2 0 1 0 ...

Notice that the 'frequency' array above now contains the mode: 2 movies
a week
(the third element, but index 2) contains the biggest number. Makes
sense? If
you aggree with the process described above, then you can write the
program to
accomplish it.

After the 'frequency' array is updated with frequencies of all data,
make a
new loop with the 'frequency' array to pick out the largest value. Use
the same
function used in earlier assignment. It should look something like:

for(i = 0; i < 1000; i++)
{
if(max < frequency)
{ max = frequency;
maxi = i;
}
}
return maxi -- remember that the index corresponds to the movies seen
per week!

Note: Do not worry about making the program handle bimodal values (mode
having
two values).
Don't forget to block-comment every function definition. Example:

//*******************************************
// enter_data()
// This function inputs data for all students
// into dynimacally created array
//*******************************************

, Nov 12, 2005

2. ### Jonathan McdougallGuest

wrote:
> Hey I am new, but I don't have time to intruduce myself yet.
> I am intro to C++
> and this is a programme I have to write.

And what is your standard C++ question?

> all the direction are here, It will be very nice of someone to figure
> this out.
> note: I only in intro C++ which is about to be finished by the next two
> weeks.
> The topic which I am on in my book is "POINTERS"
>
> Text from the book:
>
> Write a program that can be used to gather statistical data about the
> number
> of movies college students see in a month. The program should perform
> the
> following steps:
> Ask the user how many students were surveyed. An array of
> integers with this many elements should be dynamically allocated.

That is very unfortunate. This is not an application of dynamic memory
in C++. Usually, you'll want to allocate objects on the heap when you
deal with polymorphism (class hierarchies). You *don't* want to use
dynamic memory for collections. C++ already has classes that do the
job.

Here's a modification to this exercise:

Create a program which

1) has a list of predefined movies
2) asks X students to pick their favorite
3) display statistical data about the results (whatever you want,
that's not the point)

Use this skeleton:

# include <string>
# include <vector>

int main() // main() returns an int. Always.
{
typedef std::vector<string> Movies;

Movies movie_list;
movie_list.push_back("Star wars");
movie_list.push_back("Harry potter");
movie_list.push_back("Pulp Fiction");

Movies picked;

while (true)
{
// display the list of predefined movies as a menu
// display a prompt for a student to select their favorite movie

// get input from user
// validate input

// if input is "0" or "", break the loop

// add the selected movie name to the vector "picked"
}

// get statistics about the results
// display statistics
}

Hints:

1) use iterators

for (Movie::iterator itor=movie_list.begin(); itor!=movie_list.end();
++itor)
{
std::cout << *itor;
}

2) use std::string

std::string input;
std::cin >> input;

std::cout << "Your choice is: " << input;

3) don't use "new"

<snip>

4) do not hardcode any numbers else than 0.

5) change the book

Jonathan

Jonathan Mcdougall, Nov 12, 2005

3. ### John HarrisonGuest

wrote:
> Hey I am new, but I don't have time to intruduce myself yet.
> I am intro to C++
> and this is a programme I have to write.

Tell us what part you don't understand and we'll help. No-one is going
to figure it out for you, or do your homework for you.

Try postings some of the code you've written already and we'll help.

The most important piece of advice is to take things slowly.

Start with the skeleton code, and do *one* function at a time. Do *not*
try it all at once, you *will* fail if you do it that way. Start with
the create_array function and don't go any firther until you have got
that tested and working. The try the enter_data function and don't got
any further until you have got that tested and working. etc. This is the
most important piece of advice I can give, and one I've seen newbie and
newbie like yourself ignore.

john

John Harrison, Nov 12, 2005
4. ### benGuest

> That is very unfortunate. This is not an application of dynamic memory
> in C++. Usually, you'll want to allocate objects on the heap when you
> deal with polymorphism (class hierarchies). You *don't* want to use
> dynamic memory for collections. C++ already has classes that do the
> job.

Please note that you don't need dynamic memory to facilitate
polymorphism. In fact, you shouldn't use dynamic memory unless the logic
of your program requires your object to live beyond the scope where it
is created.

Ben

ben, Nov 12, 2005
5. ### Jonathan McdougallGuest

ben wrote:
> > That is very unfortunate. This is not an application of dynamic memory
> > in C++. Usually, you'll want to allocate objects on the heap when you
> > deal with polymorphism (class hierarchies). You *don't* want to use
> > dynamic memory for collections. C++ already has classes that do the
> > job.

>
> Please note that you don't need dynamic memory to facilitate
> polymorphism. In fact, you shouldn't use dynamic memory unless the logic
> of your program requires your object to live beyond the scope where it
> is created.

Many of the techniques often used in conjunction with polymorphism
imply dynamic memory, such as object factories and heterogeneous
(pointer-to-base) collections. Actually, except for the most trivial
uses (such as an argument to a function), polymorphism usually requires
dynamic memory.

Jonathan

Jonathan Mcdougall, Nov 12, 2005