How to extract specific elements from a string?

Question

I am trying to extract the first numbers from each block of numbers from the next string.

string s = "f 1079//2059 1165//2417 1164//2414 1068//1980";

In this example I need to extract 1079, 1165, 1164 and 1068

I have tried with getline and substr but I have not been able to.

Show us what you tried (substr attempt in particular). Also describe us how your solution didn't work. We may be able to help you to fix it. — eerorika, Mar 03 '17 at 10:10
Cut the line in substring using spacing as separator, then extract the first number from each of these substrings. — Jean-Baptiste Yunès, Mar 03 '17 at 10:11

score 2 · Answer 1 · answered Mar 03 '17 at 10:51

You can utilize the <regex>(C++ regular expression library) with pattern (\\d+)//. Locate the numbers before double slashes. Also using the parentheses to extract the numbers only by submatch.

Here is usage.

string s = "f 1079//2059 1165//2417 1164//2414 1068//1980";

std::regex pattern("(\\d+)//");
auto match_iter = std::sregex_iterator(s.begin(), s.end(), pattern);
auto match_end = std::sregex_iterator();

for (;match_iter != match_end; match_iter++) 
{
    const std::smatch& m = *match_iter;
    std::cout << m[1].str() << std::endl;   // sub-match for token in parentheses, the 1079, 1165, ...
                                            // m[0]: whole match, "1079//"
                                            // m[1]: first submatch, "1070"
}

score 1 · Answer 2 · answered Mar 03 '17 at 10:34

I usually reach for istringstream for this kind of thing:

std::string input = "f 1079//2059 1165//2417 1164//2414 1068//1980";
std::istringstream is(input);
char f;
if (is >> f)
{
    int number, othernumber;
    char slash1, slash2;
    while (is >> number >> slash1 >> slash2 >> othernumber)
    {
        // Process 'number'...
    }
}

score 0 · Answer 3 · answered Mar 03 '17 at 10:37

here is an attempt with getline and substring which works.

auto extractValues(const std::string& source)
-> std::vector<std::string>
{
    auto target = std::vector<std::string>{};
    auto stream = std::stringstream{ source };
    auto currentPartOfSource = std::string{};
    while (std::getline(stream, currentPartOfSource, ' '))
    {
        auto partBeforeTheSlashes = std::string{};
        auto positionOfSlashes = currentPartOfSource.find("//");
        if (positionOfSlashes != std::string::npos)
        {
            target.push_back(currentPartOfSource.substr(0, positionOfSlashes));
        }
    }
    return target;
}

score 0 · Answer 4 · edited May 23 '17 at 11:53

Or there is another split way to extract tokens, but it may involve some string copy.

Consider a split_by function like

std::vector<std::string> split_by(const std::string& str, const std::string& delem);

Possible implementations in Split a string in C++?

Make string be splitted by first, then splitted by // and extract first item.

std::vector<std::string> tokens = split_by(s, " ");
std::vector<std::string> words;
std::transform(tokens.begin() + 1, tokens.end(),  // drop first "f"              
               std::back_inserter(words), 
               [](const std::string& s){ return split_by(s, "//")[0]; });

How to extract specific elements from a string?

4 Answers4

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