how to check string start in C++

Question

Is there any way in C++ to check whether a string starts with a certain string (smaller than the original) ? Just like we can do in Java

bigString.startswi         


        

Answer 1

I'm surprised no one has posted this method yet:

#include <string>    
using namespace std;

bool starts_with(const string& smaller_string, const string& bigger_string) 
{
    return (smaller_string == bigger_string.substr(0,smaller_string.length()));
}

Answer 2

To optimize a little bit:

if ( smallString.size() <= bigString.size() &&
     strncmp( smallString.c_str(), bigString.c_str(), smallString.length() ) == 0 )

Don't forget to #include <cstring> or #include <string.h>

Answer 3

Either create a substring that is the length of your smallString variable, and compare the two. Or do a search for the substring smallString and see if it returns index 0

Answer 4

strstr() returns a pointer to the first occurrence of a string within a string.

Answer 5

I thought it makes sense to post a raw solution that doesn't use any library functions...

// Checks whether `str' starts with `start'
bool startsWith(const std::string& str, const std::string& start) {
    if (&start == &str) return true; // str and start are the same string
    if (start.length() > str.length()) return false;
    for (size_t i = 0; i < start.length(); ++i) {
        if (start[i] != str[i]) return false;
    }
    return true;
}

Adding a simple std::tolower we can make this case insensitive

// Checks whether `str' starts with `start' ignoring case
bool startsWithIgnoreCase(const std::string& str, const std::string& start) {
    if (&start == &str) return true; // str and start are the same string
    if (start.length() > str.length()) return false;
    for (size_t i = 0; i < start.length(); ++i) {
        if (std::tolower(start[i]) != std::tolower(str[i])) return false;
    }
    return true;
}

Answer 6

The simplest approach would be:

if ( smallString.size() <= bigString.size()
    && std::equals( smallString.begin(), smallString.end(), bigString.end() )

(This will also work if one of the two, or both, is a vector. Or any other standard container type.)