The std::find_if Algorithm C++

1. Use the std::find_if Function to Search for Elements Satisfying the Given Criteria
2. Use the std::find_first_of Function to Search for Element Matches in Two Ranges
3. Use the find_end Function to Split the String With Delimiters

This article will demonstrate how to utilize the std::find_if algorithm from C++ Standard Template Library.

Use the std::find_if Function to Search for Elements Satisfying the Given Criteria

The std::find_if function is part of STL algorithms, and it provides a searching method for elements in the range satisfying the given condition. Namely, the condition is specified as a callable object that returns a bool value.

The basic overload of the std::find_if function accepts two iterators denoting the range that needs to be searched. The third parameter represents the callable object used to evaluate the elements in the range. Note that range iterators should at least satisfy LegacyInputIterator requirements.

std::find_if returns the iterator to the first element satisfying the given criteria or the second argument iterator if no such element is found.

In the following code snippet, we employ the std::find_if algorithm on std::string objects and use the isupper function as the callable object. Consequently, the function should help us determine if the string contains only lowercase letters or not.

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

int main() {
  string str1 = "this is random String";
  string str2 = "this is";
  string str3 = "wqz";

  std::find_if(begin(str1), end(str1), isupper) != end(str1)
      ? cout << "str1 contains uppercase letters" << endl
      : cout << "str1 contains only lowercase letters" << endl;

  std::find_if(begin(str2), end(str2), isupper) != end(str2)
      ? cout << "str2 contains uppercase letters" << endl
      : cout << "str2 contains only lowercase letters" << endl;

  return EXIT_SUCCESS;
}

Output:

str1 contains uppercase letters
str2 contains only lowercase letters

Use the std::find_first_of Function to Search for Element Matches in Two Ranges

std::find_first_of is another powerful algorithm from STL that can be utilized to search for the same elements in two given ranges. The functions accept four iterators as parameters, the first two of which denote the range that needs to be searched for elements passed as the last two iterator arguments.

The latter two iterators must meet the requirements of LegacyForwardIterator. In this case, we use the std::find_first_of algorithm to check if the characters match in two strings. The function’s return value is the iterator to the first element in the searched range that matches an element from the second range. If no such element is found, the second iterator of the searched ranges is returned.

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

int main() {
  string str1 = "this is random String";
  string str2 = "this is";
  string str3 = "wqz";

  std::find_first_of(str1.begin(), str1.end(), str2.begin(), str2.end()) ==
          str1.end()
      ? cout << "no letters match in str1 and str2" << endl
      : cout << "some letters match in str1 and str2" << endl;

  std::find_first_of(str2.begin(), str2.end(), str3.begin(), str3.end()) ==
          str2.end()
      ? cout << "no letters match in str2 and str3" << endl
      : cout << "some letters match in str2 and str3" << endl;

  return EXIT_SUCCESS;
}

Output:

some letters match in str1 and str2
no letters match in str2 and str3

Use the find_end Function to Split the String With Delimiters

On the other hand, we have the find_end algorithm to search for the last occurrence of the given range sequence in another range. Like the std::find_first_of, this algorithm accepts four iterators denoting both ranges, but it tries to find the exact sequence match in the first range.

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

int main() {
  string str1 = "this is random String";
  string str2 = "this is";
  string str3 = "wqz";

  auto ret = std::find_end(str1.begin(), str1.end(), str2.begin(), str2.end());
  ret == str1.end() ? cout << "no such sequence found" << endl
                    : cout << "last occurrence found at "
                           << std::distance(str1.begin(), ret) << endl;

  ret = std::find_end(str2.begin(), str2.end(), str3.begin(), str3.end());
  ret == str2.end() ? cout << "no such sequence found" << endl
                    : cout << "last occurrence found at "
                           << std::distance(str2.begin(), ret) << endl;

  return EXIT_SUCCESS;
}

Output:

last occurrence found at 0
no such sequence found