read string into array - c++

Question

I want to read a string with integers and whitespaces into an array. For example I have a string looks like 1 2 3 4 5, and I want to convert it into an integer array arr[5]={1, 2, 3, 4, 5}. How should I do that?
I tried to delete the whitespaces, but that just assign the whole 12345 into every array element. If I don't everything element will all assigned 1.
for (int i = 0; i < str.length(); i++){
if (str[i] == ' ')
str.erase(i, 1);
}
for (int j = 0; j < size; j++){ // size is given
arr[j] = atoi(str.c_str());
}

Answer 1

A couple of notes:
Use a std::vector. You will most likely never know the size of an input at compile time. If you do, use a std::array.
If you have C++11 available to you, maybe think about stoi or stol, as they will throw upon failed conversion
You could accomplish your task with a std::stringstream which will allow you to treat a std::string as a std::istream like std::cin. I recommend this way
alternatively, you could go the hard route and attempt to tokenize your std::string based on ' ' as a delimiter, which is what it appears you are trying to do.
Finally, why reinvent the wheel if you go the tokenization route? Use Boost's split function.
Stringstream approach
std::vector<int> ReadInputFromStream(const std::string& _input, int _num_vals)
{
std::vector<int> toReturn;
toReturn.reserve(_num_vals);
std::istringstream fin(_input);
for(int i=0, nextInt=0; i < _num_vals && fin >> nextInt; ++i)
{
toReturn.emplace_back(nextInt);
}
// assert (toReturn.size() == _num_vals, "Error, stream did not contain enough input")
return toReturn;
}
Tokenization approach
std::vector<int> ReadInputFromTokenizedString(const std::string& _input, int _num_vals)
{
std::vector<int> toReturn;
toReturn.reserve(_num_vals);
char tok = ' '; // whitespace delimiter
size_t beg = 0;
size_t end = 0;
for(beg = _input.find_first_not_of(tok, end); toReturn.size() < static_cast<size_t>(_num_vals) &&
beg != std::string::npos; beg = _input.find_first_not_of(tok, end))
{
end = beg+1;
while(_input[end] == tok && end < _input.size())
++end;
toReturn.push_back(std::stoi(_input.substr(beg, end-beg)));
}
// assert (toReturn.size() == _num_vals, "Error, string did not contain enough input")
return toReturn;
}
Live Demo

Answer 2

Your code arr[j] = atoi(str.c_str()); is fault. The str is a string, not a char. When you used atoi(const char *), you should give the &char param. So the correct code is arr[j] = atoi(&str[j]). By the way, if you want to change the string to int, you could use the function arr[j] = std::stoul(str). I hope this can help you.

Answer 3

You have modified/parsing the string in one loop, but copying to integer array in another loop. without setting any marks, where all the embedded integers in strings start/end. So we have to do both the actions in single loop.
This code is not perfect, but to give you some idea; followed the same process you followed, but used vectors.
string str = "12 13 14";
vector<int> integers;
int start=0,i = 0;
for (; i < str.length(); i++){
if (str[i] == ' ')
{
integers.push_back(atoi(str.substr(start,i).c_str()));
start = i;
}
}
integers.push_back(atoi(str.substr(start,i).c_str()));