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How do I check if a C++ std::string starts with a certain string, and convert a substring to an int?


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).

2018/11/16
1
257
11/16/2018 4:11:03 PM

Accepted Answer

Use an overload of rfind which has the pos parameter:

std::string s = "tititoto";
if (s.rfind("titi", 0) == 0) {
  // s starts with prefix
}

Who needs anything else? Pure STL!

Many have misread this to mean "search backwards through the whole string looking for the prefix". That would give the wrong result (e.g. string("tititito").rfind("titi") returns 2 so when compared against == 0 would return false) and it would be inefficient (looking through the whole string instead of just the start). But it does not do that because it passes the pos parameter as 0, which limits the search to only match at that position or earlier. For example:

std::string test = "0123123";
size_t match1 = test.rfind("123");    // returns 4 (rightmost match)
size_t match2 = test.rfind("123", 2); // returns 1 (skipped over later match)
size_t match3 = test.rfind("123", 0); // returns std::string::npos (i.e. not found)
2020/06/01
489
6/1/2020 4:48:33 PM


Just for completeness, I will mention the C way to do it:

If str is your original string, substr is the substring you want to check, then

strncmp(str, substr, strlen(substr))

will return 0 if str starts with substr. The functions strncmp and strlen are in the C header file <string.h>

(originally posted by Yaseen Rauf here, markup added)

For a case-insensitive comparison, use strnicmp instead of strncmp.

This is the C way to do it, for C++ strings you can use the same function like this:

strncmp(str.c_str(), substr.c_str(), substr.size())
2018/04/27

If you're already using Boost, you can do it with boost string algorithms + boost lexical cast:

#include <boost/algorithm/string/predicate.hpp>
#include <boost/lexical_cast.hpp>

try {    
    if (boost::starts_with(argv[1], "--foo="))
        foo_value = boost::lexical_cast<int>(argv[1]+6);
} catch (boost::bad_lexical_cast) {
    // bad parameter
}

This kind of approach, like many of the other answers provided here is ok for very simple tasks, but in the long run you are usually better off using a command line parsing library. Boost has one (Boost.Program_options), which may make sense if you happen to be using Boost already.

Otherwise a search for "c++ command line parser" will yield a number of options.

2019/06/18

Code I use myself:

std::string prefix = "-param=";
std::string argument = argv[1];
if(argument.substr(0, prefix.size()) == prefix) {
    std::string argumentValue = argument.substr(prefix.size());
}
2012/08/17

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;
  }
}
2015/05/21

Given that both strings — argv[1] and "--foo" — are C strings, @FelixDombek's answer is hands-down the best solution.

Seeing the other answers, however, I thought it worth noting that, if your text is already available as a std::string, then a simple, zero-copy, maximally efficient solution exists that hasn't been mentioned so far:

const char * foo = "--foo";
if (text.rfind(foo, 0) == 0)
    foo_value = text.substr(strlen(foo));

And if foo is already a string:

std::string foo("--foo");
if (text.rfind(foo, 0) == 0)
    foo_value = text.substr(foo.length());
2017/05/23

Source: https://stackoverflow.com/questions/1878001
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