Trouble using find function in C++ - c++

Question

I am trying to find the difference in my code when I use std::find.
For my test code. I made a Vector called Test
std::vector<const char*> Test;
To test the find function, I filled the Test vector with dummy data by using push_back function
Test.push_back("F_S");
Test.push_back("FC");
Test.push_back("ID");
Test.push_back("CD");
Test.push_back("CT");
Test.push_back("DS");
Test.push_back("CR");
Test.push_back("5K_2");
Test.push_back("10K_5");
Test.push_back("10K_1");
Test.push_back("10K_2");
Test.push_back("10K_3");
Test.push_back("10K_4");
Test.push_back("10K_5");
What I want to do with the find function is to go through the Test and see if there are any repeated data. The first time a encounter the data, I will save it to a vector called Unique_Data.
std::vector<const char*> Unique_Data;
So for the 14 data points above, only 13 will be saved because 10K_5 repeated.
The Code I am using looks like this
for(int i = 0; i < Test.size(); i++)
{
if( Unique_Data.empty())
{
Unique_Data.push_back(Test[i]);
}
else if (std::find(Unique_Data.begin(), Unique_Data.end(), Test[i]) != Unique_Data.end())
{
// Move on to next index
}
else
{
Unique_Data.push_back(Test[i]);
}
}
The problem I am having is when I am using the dummy data. I am getting a correct answer for Unique_Data.
However, if I save the actual data into the Test vector which are saved in linked list. I get that they are all unique.
The code looks like this
p_curr = List.p_root;
while(p_curr != NULL)
{
// id starts from 0
if(atoi(p_curr->id) == 14) break;
Test.push_back(p_curr->Descriptor);
p_curr = p_curr->p_next;
}
I tested with the same 14 data. They are all const char* types. However, when I used the linked list data. The find function thinks all the data is unique.
Can anyone tell me what is wrong with this?

Answer 1

Using C-style strings is a bit tricky, they are just a pointer, and pointers are compared by identity. Two C strings with the same sequence of characters, but different addresses will compare different.
const char first[] = "Hi";
const char second[] = "Hi";
assert(first == second); // will fail!
There are two solutions to this problem. The simple one is using std::string in your container, as std::string will provide value comparisons. The alternative is to pass a comparison functor to std::find as a last argument. But this will still leave the problem of managing the lifetime of the const char*-s stored in the vector.

Answer 2

This is a pointers problem. You're not storing strings in your array, you're storing the memory address of the data in the string.
This strange behaviour is probably because in your example case you have literal strings that cannot be changed, so the compiler is optimising the storage, and when two strings are the same then it stores the same address for all strings that have the same text.
In your real data example, you have a bunch of strings that hold the same data, but each of these strings lives at a different memory address, so the find function is saying that all strings have a different address.
In summary, your find function is looking at the memory address of the string, not the data (text) in the string. If you use std::strings then this problem will disappear.
I would highly recommend using strings, as performance is going to be more than good enough and they eliminate a vast number of problems.

Answer 3

As David Rodriguez mentions in his answer, you're only comparing pointers, and not the contents of the strings themselves. Your solution will work as is if you were storing std::strings instead of char const *. With the latter, you need to resort to std::find_if and a predicate that calls strcmp to determine whether the strings are identical.
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
int main()
{
std::vector<const char*> Test;
Test.push_back("F_S");
Test.push_back("FC");
Test.push_back("ID");
Test.push_back("CD");
Test.push_back("CT");
Test.push_back("DS");
Test.push_back("CR");
Test.push_back("5K_2");
Test.push_back("10K_5");
Test.push_back("10K_1");
Test.push_back("10K_2");
Test.push_back("10K_3");
Test.push_back("10K_4");
Test.push_back("10K_5");
std::vector<const char*> Unique_Data;
for(auto const& s1 : Test) {
if(std::find_i(Unique_Data.cbegin(), Unique_Data.cend(),
[&](const char *s2) { return std::strcmp(s1, s2) == 0; })
== Unique_Data.cend()) {
Unique_Data.push_back(s1);
}
}
for(auto const& s : Unique_Data) {
std::cout << s << '\n';
}
}
Here's a live example