C++ Program Functions - c++

Question

I am having trouble with my functions. When I use a function to manipulate an array, and print it and move on to the next manipulation function, it uses the array that was previously manipulated instead of the original array. For example, when my function converts every negative number to a positive, I call the next function which zeros out all even numbers, and my array prints out all zeros, instead of using the array from the original.
#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
#define NUMS_PER_LINE 10 // maximum numbers to be printed on each line.
int numbers[100]; // array to hold upto 100 integer numbers.
int numcnt; // actual count (<=100) of numbers in above array.
// reads file content into array
void read_array_from_file (const char filename[])
{
ifstream inpfile(filename);
if (!inpfile.is_open())
{
cout << "Can't open file : " << filename << endl;
exit(1);
}
numcnt=0; // Initialise count of read numbers
// Read numbers from the file into array.
inpfile >> numbers[numcnt];
while (!inpfile.eof()) // Read until EOF is reached.
{
numcnt++; // Got one more number from input file.
inpfile >> numbers[numcnt];
}
inpfile.close();
return;
}
// Print out all the values in the array
void print_array_content (int numsinaline)
{
int i;
for (i=0; i<numcnt+1; i++)
{
if ((i % numsinaline) == 0)
cout << endl;
cout << numbers[i] << " ";
}
cout << endl << endl;
return;
}
// calculate average
double calculate_average ()
{
int i;
float sum=0;
for (i=0; i<(numcnt-1); i++)
{
sum += numbers[i];
}
return (sum/(numcnt-1));
}
// Find numbers larger and smaller than the average.
void find_numbers_smaller_and_larger_than_average (int &larger, int &smaller, int average)
{
int i;
for (i=0; i<(numcnt-1); i++)
{
if (numbers[i] < average)
smaller++;
else if (numbers[i] > average)
larger++;
}
return;
}
// Convert negative numbers to positive in the array 'numbers'.
void convert_negative_to_positive ()
{
int i;
for (i=0; i<(numcnt-1); i++)
{
if (numbers[i] < 0)
numbers[i] *= -1;
}
return;
}
// Convert all even numbers into zero.
void zero ()
{
int i;
for (i=0; i<numcnt; i++)
{
if (numbers[i] > 0)
numbers[i] *= 0;
}
return;
}

Answer 1

First of all, you are using a global variable for your array, so you are never passing it to your function. When you change a global variable in the function, it changes the data in the array. You should be passing that data into the function and NOT using global variables.
Second, while(!inpFile.eof()) is bad! Don't do it.
For file streams:
std::vector<int> numbers;
std::ifstream fin("myfile");
std::copy(std::istream_iterator<int>(fin), std::istream_iterator(), std::back_inserter<vector<int> >(numbers));
Those 3 lines will read the entire file into the vector "numbers".
Third, when declaring your functions, pass the array:
void myFunction(const std::vector<int>& vec); // if you aren't going to change the vector
or
void myFunction(std::vector& vec); // if you are going to change it
and you would call it by simply:
myFunction(numbers);

Answer 2

" it uses the array that was previously manipulated instead of the original array."
Obviously because you have your array declared globally
int numbers[100];
Outside all functions.
When you perform one operation on this array, the element get modified and the new values will be used for next functions.
Instead of this, save of copy of your original array and then use this copy whenever you wish to work on original array

Answer 3

All your operations act on a single global variable, numbers. If you modify it in any of your functions its values will also change in every other occurrence.
Instead, provide a way to tell your functions which array you want to use, how many elements it contains and use several arrays. This also enables you to get rid of the global variable.
Example:
#include <iostream>
using namespace std;
typedef unsigned int array_size_t;
void print_array(int array[], array_size_t size){
for(array_size_t i = 0; i < size; ++i){
cout << array[i] << endl;
}
}
int main(){
int a1[] = {1,2,3,4};
int a2[] = {1,3,3,7,0,0,0,0};
print_array(a1,4);
print_array(a2,8);
}
Remark
If you're allowed use standard containers such as std::vector instead. The solution above is more C than C++-like.

Answer 4

You are using global variable. All your operation on numbers whatever index will change your certain position's value.
Another potential risk is if your input file contains more than 100 integers, you will do
inpfile >> numbers[100];
or some index number greater than 100.
which will cause a segmentation fault.
You should be very careful when using global variables

Answer 5

You are directly manipulating your array inside of your functions since it is defined globally, instead of passing in a copy as a parameter.
void modify(int[] array) {
//Modify copied array here
}
int main() {
int numbers[100];
int copyNumbers[100];
//Copy numbers
memcpy(copyNumbers, numbers, sizeof(numbers));
modify(copyNumbers);
//Use modified array
memcpy(copyNumbers, numbers, sizeof(numbers)); //Set back to original
modify(copyNumbers); //Modify copy again as original
}