Pointer/Template Exercise - Not Out Putting Anything

[pcannons]

Hi, I have this piece of code,

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

template <typename T>
T sum(T a[], int size){
	T result = 0;
	for(int i = 0; i<size; i++)
		result += a[i];
	return result;
}

int main(int argc, char* argv[]) {
	int intarray[argc-1];
	int *ia = intarray;
	int result=0;
	char **argt = argv;
	while(*argt){
		*ia=atoi(*argt);
		(*argt)++;
		ia++;
	}
	result = sum(intarray, argc);
	cout << result << endl;
	cout << "Yes" << endl;
	return 0;
}

However, when I run it, it outputs nothing for some reason, any idea what the issue may be?

Thank you!

[Robertlzw]

Line 14 does not compile. Array subscript has to be a constent expression.

[pcannons]

It compiles...

[krishnendu]

I have made some modification. Try the following code:

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

template <typename T>
T sum(T a[], int size)
{
	T result = 0;
	for(int i = 0; i < size; i++)
		result += a[i];

	return result;
}

int main(int argc, char *argv[])
{
	int totalNumbers = (argc == 1) ? 0 : (argc - 1);
	int intarray [totalNumbers];
	int i = 0;
	int result = 0;

	for (i = 0; i < totalNumbers; i++)
		intarray[i] = atoi (argv[i + 1]);

	result = sum (intarray, totalNumbers);
	cout << result << endl;
	cout << "Yes" << endl;
	return 0;
}

Note: argc contains the total number of words in the command line. Thus it will always be greater than 0 and the value of argv[0] is the command itself.

Robertlzw is partially correct. Some compiler does not support variable length array.

Which compiler(s) are you people using? I have tested it with g++.

[Disch]

Robertlzw is partially correct. Some compiler does not support variable length array.

FWIW the C++ standard doesn't support it, so in a sense it's technically not valid C++.

Although it is legal in C, which is probably why some compilers (namely gcc) support it.

[pcannons]

Hi,

Thank you everyone for the help.

Robertlzw, Disch, you are right, I was using gcc and should of been using g++, hence why it compiled.

I see from this website's tutorial section:

NOTE: The elements field within brackets [] which represents the number of elements the array is going to hold, must be a constant value, since arrays are blocks of non-dynamic memory whose size must be determined before execution. In order to create arrays with a variable length dynamic memory is needed, which is explained later in these tutorials.

krishnendu, thank you for your time, however I was doing this as an exercise on pointers

Turns out, I had number of elements for my array wrong, it should not be argc - 1, I mistakenly thought it was # of elements starting at 0 while I should of just passed agrc.

As for why my code didn't work, I had (*argt)++ which increased the value of the first array elemnt instead of increasing the pointer position of the argv array.

Here is my working code:

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

template <typename T>
T sum(T a[], int size){
	T result = 0;
	for(int i = 0; i < size; i++)
		result += a[i];
	return result;
}

int main(int argc, char *argv[]) {
	int intarray[argc];
	int *ia = intarray;
	int result=0;
	char **argt = argv;
	while(*argt){
		*ia=atoi(*argt);
		ia++;
		*argt++;
	}
	result = sum(intarray, argc);
	cout << result << endl;
	cout << "Yes" << endl;
	return 0;
}

Thank you everyone!