Implementing a Case Insensitive string::find in C++

In this article we will discuss how to find a Case Insensitive Sub String in a given string in C++ using,

• STL
• Boost Library

std::string provides a method std::string::find to search for the sub string inside a given string, but this function is case sensitive i.e.

std::string data = "Hi this is a sample string";
size_t pos = data.find("SAMPLE");

In above code, value of pos will be std::string::npos i.e.

pos == std::string::npos

Because, std::string::find is case sensitive and can not find ‘SAMPLE’ in the given string.  Now let’s implement a case sensitive version of std::string::find

Finding A Case Insensitive Sub String in C++ using STL

Logic:

Create a new find function that will convert both the given string and sub string to lower case and then use the std::string::find to search sub string in the string i.e.

/*
 * Find Case Insensitive Sub String in a given substring
 */
size_t findCaseInsensitive(std::string data, std::string toSearch, size_t pos = 0)
{
	// Convert complete given String to lower case
	std::transform(data.begin(), data.end(), data.begin(), ::tolower);
	// Convert complete given Sub String to lower case
	std::transform(toSearch.begin(), toSearch.end(), toSearch.begin(), ::tolower);
	// Find sub string in given string
	return data.find(toSearch, pos);
}

Let’s see how to use this function,

std::string data = "Hi this is a sample string";
size_t pos = findCaseInsensitive(data, "SAMPLE");
if( pos != std::string::npos)
	std::cout<<"'SAMPLE' Found at "<<pos<<std::endl;
else
	std::cout<<"'SAMPLE' Not Found"<<std::endl;

Unlike std::string::find, here we are able to find “SAMPLE” in “Hi this is a sample string”

Complete code example is as follows,

#include <iostream>
#include <string>
#include <algorithm>
/*
 * Find Case Insensitive Sub String in a given substring
 */
size_t findCaseInsensitive(std::string data, std::string toSearch, size_t pos = 0)
{
	// Convert complete given String to lower case
	std::transform(data.begin(), data.end(), data.begin(), ::tolower);
	// Convert complete given Sub String to lower case
	std::transform(toSearch.begin(), toSearch.end(), toSearch.begin(), ::tolower);
	// Find sub string in given string
	return data.find(toSearch, pos);
}

int main()
{
	std::string data = "Hi this is a sample string";

	// Using std::string::find to find a Sub string in given string
	size_t pos = data.find("SAMPLE");

	if( pos != std::string::npos)
		std::cout<<"'SAMPLE' Found at "<<pos<<std::endl;
	else
		std::cout<<"'SAMPLE' Not Found"<<std::endl;

	// Using Customized find function to find a Sub string in given string
	pos = findCaseInsensitive(data, "SAMPLE");

	if( pos != std::string::npos)
		std::cout<<"'SAMPLE' Found at "<<pos<<std::endl;
	else
		std::cout<<"'SAMPLE' Not Found"<<std::endl;

	return 0;
}

Output

'SAMPLE' Not Found
'SAMPLE' Found at 13

Finding A Case Insensitive Sub String using Boost::icontains

With boost::icontains we can check if a sub string exists in a given string i.e.

std::string data = "Hi this is a sample string";

// Case Insensitive Sub String Search
bool result = boost::icontains(data, "SAMPLE");

But we can not find out the position of substring with it 🙁

Complete code example is as follows,

#include <iostream>
#include <string>
#include <boost/algorithm/string.hpp>



int main()
{
	std::string data = "Hi this is a sample string";

	// Case Insensitive Sub String Search
	bool result = boost::icontains(data, "SAMPLE");

	if(result)
		std::cout<<"'SAMPLE' is Present in ---"<<data<<std::endl;
	else
		std::cout<<"'SAMPLE' is Not Present in ---"<<data<<std::endl;


}

Output

'SAMPLE' is Present in ---Hi this is a sample string