Case-Insensitive Search Using std::ranges::find_if()

To make std::ranges::find_if() case-insensitive when searching through a container of strings, you need to provide a predicate that performs a case-insensitive comparison.

Case-Insensitive Comparison Function

First, define a case-insensitive comparison function. You can use the std::tolower() function to convert characters to lowercase:

1#include <algorithm>
2#include <iostream>
3#include <string>
4#include <vector>
5#include <cctype>
6
7bool caseInsensitiveCompare(char a, char b) {
8  return std::tolower(a) == std::tolower(b);
9}

Using std::ranges::find_if() with the Predicate

Next, use this function within a predicate for std::ranges::find_if():

1#include <algorithm>
2#include <iostream>
3#include <vector>
4#include <string>
5#include <cctype>
6
7bool caseInsensitiveCompare(char a, char b) {
8  return std::tolower(a) == std::tolower(b);
9}
10
11bool caseInsensitiveStringCompare(
12  const std::string& str,
13  const std::string& target
14) {
15  return std::equal(
16    str.begin(),
17    str.end(),
18    target.begin(),
19    target.end(),
20    caseInsensitiveCompare
21  );
22}
23
24int main() {
25  std::vector<std::string> words{
26    "Apple", "banana", "Cherry" };
27
28  std::string target = "BANANA";
29  auto result = std::ranges::find_if(words,
30    [&](const std::string& word) {
31      return caseInsensitiveStringCompare(
32        word, target
33      ); 
34  });
35
36  if (result != words.end()) {
37    std::cout << "Found: " << *result << "\n";
38  } else {
39    std::cout << "Not found\n";
40  }
41}

1Found: banana

Explanation

In this example:

• caseInsensitiveCompare() converts each character to lowercase before comparing.
• caseInsensitiveStringCompare() uses std::equal() to compare two strings case-insensitively.
• std::ranges::find_if() calls caseInsensitiveStringCompare() within a lambda function to perform the case-insensitive search.

Handling Mixed Case and Special Characters

This method works well for ASCII characters. For more complex scenarios involving Unicode characters, consider using libraries like ICU (International Components for Unicode) to handle case conversion and comparison.

By using a custom comparison function, you can effectively perform case-insensitive searches in a container of strings with std::ranges::find_if().