Moving through every character in a string array - c++

Question

I am trying to go through every string index and print the even indexes first and then print all the odd indexes. However I am getting a segmentation error and I don't know what it is caused by.
Here is my code:
int sum;
vector<string> array;
string temp;
for (int i = 0; i < num; i++) {
cin >> array[i];
}
for (int i = 0; i < num; i++) {
temp = array[i];
for (int j = 0; j < temp.size(); j++) {
sum = j;
if (sum % 2 == 0) {
cout << array[i][j];
}
}
}
cout << " ";
for (int i = 0; i < num; i++) {
temp = array[i];
for (int j = 0; j < temp.size(); j++) {
sum = j;
if (sum % 2 != 0) {
cout << array[i][j];
}
}
}
Does anyone see what I am doing wrong?

Answer 1

You are not populating the array correctly, so your loops are going out of bounds.
Try something more like this instead:
vector<string> array;
string temp;
array.resize(num);
for(int i = 0; i < num; ++i)
{
cin >> array[i];
}
/* Alternatively:
for(int i = 0; i < num; ++i)
{
cin >> temp;
array.push_back(temp);
}
*/
/* Alternatively:
copy_n(istream_iterator<string>(cin), num, back_inserter(array));
*/
for(int i = 0; i < num; ++i)
{
temp = array[i];
for(int j = 0; j < temp.size(); j += 2)
{
cout << temp[j];
}
}
cout << " ";
for(int i = 0; i < num; ++i)
{
temp = array[i];
for(int j = 1; j < temp.size(); j += 2)
{
cout << temp[j];
}
}

Answer 2

vector<string> array is an empty vector, but you're trying to access its ith value with cin >> array[i]; in the first loop, thus getting out of bounds (there's no ith index!) and a segfault.
You should append new values to the vector dynamically or pre-allocate the needed amount of space:
// in the first loop
string test;
cin >> test;
array.push_back(test);
Another option:
// change the declaration
vector<string> array(num); // a vector of size num
// or pre-allocate memory after initialisation
vector<string> array;
array.resize(num);
// then the loop...

Answer 3

You have not declared the size of the std::vector <std::string> array and you are trying to index it which gives segmentation fault. You can use array.push_back() to insert the values, or you have to define the size while declaring the vector.
Either:
std::vector <std::string> array (num); // given num is known beforehand
Or:
vector<string> array;
string temp;
for (int i = 0; i < num; i++) {
cin >> temp;
array.push_back(temp);
}