Case-insensitive string comparison in C++ [closed]

Answer 1

The trouble with boost is that you have to link with and depend on boost. Not easy in some cases (e.g. android).

And using char_traits means all your comparisons are case insensitive, which isn't usually what you want.

This should suffice. It should be reasonably efficient. Doesn't handle unicode or anything though.

bool iequals(const string& a, const string& b)
{
    unsigned int sz = a.size();
    if (b.size() != sz)
        return false;
    for (unsigned int i = 0; i < sz; ++i)
        if (tolower(a[i]) != tolower(b[i]))
            return false;
    return true;
}

Update: Bonus C++14 version (#include <algorithm>):

bool iequals(const string& a, const string& b)
{
    return std::equal(a.begin(), a.end(), b.begin(),
                      [](char a, char b) {
                          return tolower(a) == tolower(b);
                      });
}

Answer 2

Doing this without using Boost can be done by getting the C string pointer with c_str() and using strcasecmp:

std::string str1 ="aBcD";
std::string str2 = "AbCd";;
if (strcasecmp(str1.c_str(), str2.c_str()) == 0)
{
    //case insensitive equal 
}

Answer 3

Looks like above solutions aren't using compare method and implementing total again so here is my solution and hope it works for you (It's working fine).

#include<iostream>
#include<cstring>
#include<cmath>
using namespace std;
string tolow(string a)
{
    for(unsigned int i=0;i<a.length();i++)
    {
        a[i]=tolower(a[i]);
    }
    return a;
}
int main()
{
    string str1,str2;
    cin>>str1>>str2;
    int temp=tolow(str1).compare(tolow(str2));
    if(temp>0)
        cout<<1;
    else if(temp==0)
        cout<<0;
    else
        cout<<-1;
}

Answer 4

str1.size() == str2.size() && std::equal(str1.begin(), str1.end(), str2.begin(), [](auto a, auto b){return std::tolower(a)==std::tolower(b);})

You can use the above code in C++14 if you are not in a position to use boost. You have to use std::towlower for wide chars.

Answer 5

As of early 2013, the ICU project, maintained by IBM, is a pretty good answer to this.

http://site.icu-project.org/

ICU is a "complete, portable Unicode library that closely tracks industry standards." For the specific problem of string comparison, the Collation object does what you want.

The Mozilla Project adopted ICU for internationalization in Firefox in mid-2012; you can track the engineering discussion, including issues of build systems and data file size, here:

• https://groups.google.com/forum/#!topic/mozilla.dev.platform/sVVpS2sKODw
• https://bugzilla.mozilla.org/show_bug.cgi?id=724529 (tracker)
• https://bugzilla.mozilla.org/show_bug.cgi?id=724531 (build system)

Answer 6

For my basic case insensitive string comparison needs I prefer not to have to use an external library, nor do I want a separate string class with case insensitive traits that is incompatible with all my other strings.

So what I've come up with is this:

bool icasecmp(const string& l, const string& r)
{
    return l.size() == r.size()
        && equal(l.cbegin(), l.cend(), r.cbegin(),
            [](string::value_type l1, string::value_type r1)
                { return toupper(l1) == toupper(r1); });
}

bool icasecmp(const wstring& l, const wstring& r)
{
    return l.size() == r.size()
        && equal(l.cbegin(), l.cend(), r.cbegin(),
            [](wstring::value_type l1, wstring::value_type r1)
                { return towupper(l1) == towupper(r1); });
}

A simple function with one overload for char and another for whar_t. Doesn't use anything non-standard so should be fine on any platform.

The equality comparison won't consider issues like variable length encoding and Unicode normalization, but basic_string has no support for that that I'm aware of anyway and it isn't normally an issue.

In cases where more sophisticated lexicographical manipulation of text is required, then you simply have to use a third party library like Boost, which is to be expected.