Can I use std::find() with lambda function in vc++ [VS 2019]? - c++

Question

OS: Windows 10 Pro 64-bit(10.0, build 18362)
IDE: Visual Studio 2019(Version 16.4.3) [Before this I was using VS 2010]
Language: c++ and VC++
Here's my simple code in which i just want to find element 3 and print it if found
std::vector<int> intVect;
for (int counter = 0; counter < 5; counter++)
{
intVect.push_back(counter);
}
std::find(intVect.begin(), intVect.end(), [](int a)
{
if (a == 3)
{
std::cout << "Item Found.." << std::endl;
}
});
The probelm is when i compile this code it's giving me error like below:
Error C2678 binary '==' : no operator found which takes a left-hand
operand of type 'int' (or there is no acceptable conversion)
c:\Program Files (x86)\Microsoft Visual Studio
10.0\VC\include\algorithm 41

Answer 1

You can do this easily without a lambda:
auto val = std::find(intVect.begin(), intVect.end(), 3);
if (val != intVect.end()) {
std::cout << "Value found\n";
}
However, it's also possible to use a lambda, but you need to use std::find_if (available in the same header as std::find: <algorithm>):
auto val = std::find_if(intVect.begin(), intVect.end(), [](int i) { return i == 3; });
if (val != intVect.end()) {
std::cout << "Value found\n";
}
But there's really no point. You'd only use find_if in this case if you had something more complex, like a struct and you were searching for a particular member:
struct complex {
int id;
...
};
std::vector<complex> things = ...;
auto val = std::find_if(things.begin(), things.end(), [](const complex& c) { return c.id == 3; });
if (val != things.end()) {
std::cout << "Value found\n";
}

Answer 2

std::find expects a value to be compared to elements, so it should be
if (std::find(intVect.begin(), intVect.end(), 3) != intVect.end())
{
std::cout << "Item Found.." << std::endl;
}
If you want to use a lambda as predicate, you should use std::find_if; note that the lambda should return bool.
if (std::find_if(intVect.begin(), intVect.end(), [](int a) { return (a == 3); }) != intVect.end())
{
std::cout << "Item Found.." << std::endl;
}