How do I check if a C++ std::string starts with a

Question

How do I implement the following (Python pseudocode) in C++?

if argv[1].startswith('--foo='):
    foo_value = int(argv[1][len('--foo='):])

(For example, if argv[1] is --foo=98, then foo_value is 98.)

Update: I'm hesitant to look into Boost, since I'm just looking at making a very small change to a simple little command-line tool (I'd rather not have to learn how to link in and use Boost for a minor change).

Answer 1

You can also use strstr:

if (strstr(str, substr) == substr) {
    // 'str' starts with 'substr'
}

but I think it's good only for short strings because it has to loop through the whole string when the string doesn't actually start with 'substr'.

Answer 2

You would do it like this:

   std::string prefix("--foo=");
   if (!arg.compare(0, prefix.size(), prefix))
      foo_value = atoi(arg.substr(prefix.size()).c_str());

Looking for a lib such as Boost.ProgramOptions that does this for you is also a good idea.

Answer 3

Nobody used the STL algorithm/mismatch function yet. If this returns true, prefix is a prefix of 'toCheck':

std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()

Full example prog:

#include <algorithm>
#include <string>
#include <iostream>

int main(int argc, char** argv) {
    if (argc != 3) {
        std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
                  << "Will print true if 'prefix' is a prefix of string" << std::endl;
        return -1;
    }
    std::string prefix(argv[1]);
    std::string toCheck(argv[2]);
    if (prefix.length() > toCheck.length()) {
        std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
                  << "'prefix' is longer than 'string'" <<  std::endl;
        return 2;
    }
    if (std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()) {
        std::cout << '"' << prefix << '"' << " is a prefix of " << '"' << toCheck << '"' << std::endl;
        return 0;
    } else {
        std::cout << '"' << prefix << '"' << " is NOT a prefix of " << '"' << toCheck << '"' << std::endl;
        return 1;
    }
}

---

Edit:

As @James T. Huggett suggests, std::equal is a better fit for the question: Is A a prefix of B? and is slight shorter code:

std::equal(prefix.begin(), prefix.end(), toCheck.begin())

Full example prog:

#include <algorithm>
#include <string>
#include <iostream>

int main(int argc, char **argv) {
  if (argc != 3) {
    std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
              << "Will print true if 'prefix' is a prefix of string"
              << std::endl;
    return -1;
  }
  std::string prefix(argv[1]);
  std::string toCheck(argv[2]);
  if (prefix.length() > toCheck.length()) {
    std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
              << "'prefix' is longer than 'string'" << std::endl;
    return 2;
  }
  if (std::equal(prefix.begin(), prefix.end(), toCheck.begin())) {
    std::cout << '"' << prefix << '"' << " is a prefix of " << '"' << toCheck
              << '"' << std::endl;
    return 0;
  } else {
    std::cout << '"' << prefix << '"' << " is NOT a prefix of " << '"'
              << toCheck << '"' << std::endl;
    return 1;
  }
}

Answer 4

if(boost::starts_with(string_to_search, string_to_look_for))
    intval = boost::lexical_cast<int>(string_to_search.substr(string_to_look_for.length()));

This is completely untested. The principle is the same as the Python one. Requires Boost.StringAlgo and Boost.LexicalCast.

Check if the string starts with the other string, and then get the substring ('slice') of the first string and convert it using lexical cast.

Answer 5

text.substr(0, start.length()) == start

Answer 6

std::string text = "--foo=98";
std::string start = "--foo=";

if (text.find(start) == 0)
{
    int n = stoi(text.substr(start.length()));
    std::cout << n << std::endl;
}