a function that computes the alternating sum of all elements in an array

[Lwen8888]

I'm working on a homework require me to write a function that print out alternating sum of all elements enter in an array. Followin is what I got so far.

#include <iostream>

using namespace std;

double alternating_sum ( int numbers[] )

{

double sum;

for (int i = 0; i < 100; i++)

{
sum = sum + numbers[i-1] - numbers[i+1];

}


return sum;

}


int main()

{
double input_numbers[];

cout << " Enter a group of numbers to find the sum: " << endl;

cin >> input_numbers;

double result = alternating_sum(input_numbers[]);

system("pause");

return 0;

}

Question:

1. When I do double result = alternating_sum(input_numbers[]);
it shows error.

2. how do I use main program to calls this function using an array of your choice and prints out the resulting sum.

3. Did I mess up makin the function?

[Lwen8888]

example if alternatingSum is called with an array containing

1 4 9 16 9 7 4 9 11

then it computes

1 - 4 + 9 - 16 + 9 - 7 + 4 - 9 + 11 = - 2

[vlad from moscow]

You may not define an array with unknown size

double input_numbers[];

Ypu shall specify its size in the declaration.

Also you cna not enter elements of the array such a way as you are doing

cin >> input_numbers;

You shall eneter each element of the array separately in a loop.

Your function alternating_sum could look like

double alternating_sum ( double numbers[], int size )
 
{
 
   double sum = 0.0;

   for ( int i = 0; i < size; i++ )
   {
      sum += ( i % 2 == 0 ) ? numbers[i] : -numbers[i];
   }

 
   return sum;
}

[NerdTastic]

Vlad from moscow's point about the array with unknown size seems off, because you CAN declare dynamic arrays like that, but I didn't actually read the code too carefully. Also, his comment about

cin >> input_numbers;

is logical and possible, but an easier way to replace this code (which won't work) is to use cin.get, if you know how long you want the input to be. i.e.

cin.ignore(80, '\n') //basically a buffer flush
cin.get(input_numbers, NUM_OF_DIGITS);
cin.ignore(80, '\n'); //another flush 

where NUM_OF_DIGITS is replaced with the number of characters you want to get.

[NerdTastic]

Also, I do not think this:

for (int i = 0; i < 100; i++)

{
sum = sum + numbers[i-1] - numbers[i+1];

}

will accomplish your goal. Instead, try:

sum=numbers[0]; //start with initial value

for(int i=1; i<100; i++) //initialize i to 1 because the value at numbers[0] is already inside the sum
{

if(((i+2)%2)==0) //if POSITION IN ARRAY is even, then...
{sum=sum+numbers[i];}

else //if POSITION IN ARRAY is odd, then...
sum=sum-numbers[i];

}

that should work. The reason I added 2 is because I don't know what 0%2 would return, but it may not be necessary. It was just a precaution.

[vlad from moscow]

NerdTastic (39)

Mar 14, 2012 at 8:23pm

Vlad from moscow's point about the array with unknown size seems off, because you CAN declare dynamic arrays like that,

There is no such type as "dynamic arrays" in C++. So you are wrong and one more wrong. In C++ it is impossible to declare arrays of unknown size. When you are using, for example, code as the following

double * p = new double[x];

then you are declaring a pointer not an array.

[NerdTastic]

In c++ you can declare

char x[];

This is perfectly legal. It will take the size of the first use given it. For example:

char x[];

cin.get(x, 10);
cin.ignore(80, '\n');

after the input, x[] will be somewhere between 1 and 10, depending on how long the input is.

[Zephilinox]

I believe there are dynamic arrays in c++, not that I understand them, I've seen people talk about it here before, something like "int **x", that may be way off though.

[cire]

NerdTastic wrote:

]n c++ you can declare

char x[];

This is perfectly legal. It will take the size of the first use given it. For example:

This is completely false.

[NerdTastic]

This is completely false.

I literally just did it in a program, that runs fine.

[cire]

Either you're trolling, or you're using one screwed up compiler. Either way, that isn't valid C++.

[Moschops]

It's an ancient, pre-standard (and thus not C++) compiler (comes with iostream.h)

[naqib]

Im new in C++ i hope this work.




#include <cstdlib>
#include <iostream>

using namespace std;

int main(int argc, char *argv[])
{
int z;
int S_a = 0 ;
cout <<"Enter the number of array element you to sum" <<endl;
cin >> z;
cout <<"Enter the numbers " <<endl;
int x[z];


for (int i = 0; i<z; i++)
{
cin >> x[i];
};

for (int i=0; i<z; i++)
{
S_a = S_a + x[i];
}




cout << S_a <<endl;


system("PAUSE");
}

[cire]

int z;
int S_a = 0 ;
cout <<"Enter the number of array element you to sum" <<endl;
cin >> z;
cout <<"Enter the numbers " <<endl;
int x[z];

This is not valid C++. Variable length arrays are a part of C which was not included in C++. A compiler allowing it does not make it valid. It makes the compiler non-conforming. If you're using a gcc variant compile with -pedantic.

[vlad from moscow]

naqib (4)

Mar 16, 2012 at 9:09pm

Im new in C++ i hope this work.

int main(int argc, char *argv[])
{
int z;
int S_a = 0 ;
cout <<"Enter the number of array element you to sum" <<endl;
cin >> z;
cout <<"Enter the numbers " <<endl;
int x[z];

No, thsi code does not suttisfy C++ standard. This can be done only in a C-compiler that supports the C99 Standard. Nevertheless thsi is incorrect code according to the C++ Standard of any release.

[Zephilinox]

why is it not in the standard? seems like a pretty useful thing to be able to do...

then again, a lot of useful things tend to be non-standard.

[vlad from moscow]

There are much useful things that are present in C and are absent in C++ and vice versa there are much useful things in C++ that are absent in C.
In fact C and C++ are two different languages. .

[Moschops]

why is it not in the standard?

int x[z]; is forbidden in C++, unless z is a constant known at compile-time. Why? Here's a good chat about it: http://groups.google.com/group/comp.std.c++/browse_thread/thread/2bfe25800d4961e8/9545494bbb336dfa

[naqib]

I told u all im new in C++ but i did this According to the request
by the way i Just start C++ one week ago without any course just by self
i will try do thing standard

Im using Dev C++.

how do i know if this is a C# language;



tnx from all of u

[Zephilinox]

dev c++ is out of date, I have no idea why so many beginners seem to pick it, use Code::Blocks