Reading integer input from file into a dynamic array and printing it

Question

I am attempting to write a program that reads in integers from an input file and outputs a dynamic array. This array would be the size of the input. For verification, I'd also like to print the array. The output will be passed to a function to create a sorted linked list.

I tried the following, but it did not work:

istringstream input(R"inp(
23 43 12 67 
18 15 22
12 xxx 23 12 xx 34556 11 11 www
)inp");

int get, count = 0;

while (input >> get)
    count++;

cout << "Number of integers: " << count << endl;

int *array = new int [count];


for (int i = 0; i < count; i++)
{
    input >> array[i];
}

for (int i = 0; i < count; i++)
{
    cout << *(array+i) << endl;
}

delete[] array;

Here's an online example of my code.

The problem is that the output shows some weird numbers, completely unrelated to the input:

Number of integers: 8
-1217944384
-1217944384
-1
538976266
540226080
824193844

Where did I go wrong?

Answer 1

Your code has several misconceptions and flaws:

(1) After applying this loop to count your inputs

 while (input >> get)
    count++;

the input stream's state is left over the result of last extraction operation (input >> get) that has failed. Thus no further input can be read, without completely resetting the stream.

(2) The second loop you're showing

for (int i = 0; i < count; i++) {
    input >> array[i];
}

uses the input stream in invalid state (the whole stream was already read to input.eof()), and thus reading from it results in 'weird values' (in other words: It's undefined behavior at this point).

---

I would write the following proven code to solve this

// Your literal input file formatting goes here
istringstream input(R"inp(
23 43 12 67 
18 15 22
12 xxx 23 12 xx 34556 11 11 www
)inp");

int current;
vector<int> allIntInputs;

while (input >> current || !input.eof()) {
    if(input.fail()) {
        input.clear();
        string crap;
        input >> crap; // read anything up to the next 
                       // whitespace delimiter (the default deleimiters)
        continue; // with the next item
    }
    // Everything's fine we'll add another number
    allIntInputs.push_back(current);
}

// Print all integer values extracted
cout << "Integer values read from input:" << endl;
for(vector<int>::iterator it = allIntInputs.begin();
    it != allIntInputs.end();
    ++it) {
    if(it != allIntInputs.begin()) {
        cout << ' ';
    }
    cout << *it;    
}
cout << endl;

Output

Integer values read from input:
23 43 12 67 18 15 22 12 23 12 34556 11 11

Answer 2

As πάντα ῥεῖ pointed out, the solutions that I did provide are not totally safe, that's why I will provide a third example, using boost::spirit.

See the points fast fix and good solution, as well as πάντα ῥεῖ's answer to get it working without using boost.

my personal favourite solution: note that this example does require to have read the text file into a string.

#include <boost/spirit/include/phoenix_stl.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>

template<typename Iterator>
bool
parse_numbers(Iterator first, Iterator last, std::vector<int>& v)
{
    bool r =
    boost::spirit::qi::parse(first, last,
    //  Begin grammar
        (boost::spirit::qi::int_[boost::phoenix::push_back(
            boost::phoenix::ref(v), _1)]
            % *(boost::spirit::qi::char_("a-zA-Z")
                | boost::spirit::ascii::space)));

    if (first != last) // fail if we did not get a full match
       return false;
    return r;
}

const std::string s = "23 43 12 67 \n18 15 22\n12 xxx 23 12 xx 34556 11 11 www";
std::string::const_iterator start = s.begin(), stop = s.end();
std::vector<int> results;
parse_numbers(start, stop, results)));
for(int i : results)
    std::cout << value << ' '; 

the result would be, as expected:

23 43 12 67 18 15 22 12 23 12 34556 11 11

The above example is partially built on the example given in the boost::spirit documentation.

input >> get moves the current curser position, so after your while loop you have nothing left to read.

fast fix:

ifstream input;
input.open("file.txt");

int get, count = 0;

while (input >> get)
    count++;
input.close();
input.open("file.txt");    
int *array = new int [count];    

for (int i = 0; i < count; i++)
{
    input >> array[i];
}
for (int i = 0; i < count; i++)
{
    cout << *(array+i) << endl;
}

input.close();
delete[] array;

To close and reopen the stream should work, but there are more efficient solutions out there...

good solution:

One could be to read and insert into a dynamically growing vector for example. See the documentation for further reference.

std::vector<int> dataArray;
while (input >> get)
{
    dataArray.insert(dataArray.end(), get);
}
for(auto&& value : dataArray)
{
    std::cout << value << std::endl; 
}

That would have multiple advantages:

1. allocating the vector on the stack prevents you from being forced to call delete. An alternative would be a standard smart pointer.
2. the for each loop works even without counting the elements. If you need the number of elements you could just ask the vector about his size.