how to compare a vector of pointers and string ? - c++

Question

struct compare{
string k;
compare(string a) : k(a){}
bool operator()(const product* t)const{
return (t->productdetails.getName()<k);
}
};
void cashier::calculate(string s,vector<product*> &a){
//this is lambda (tried to use it ,but doesn't work)
auto comp=[](const product* t, const string b)
{
return (b < t->productdetail.getName());
};
if (std::binary_search ( a.begin(), a.end(),s, compare(s)))
cout << "found!\n";
else
std::cout << "not found.\n";
}
I'm stuck in this part for a long time.
the vector holds product(class) pointer. product pointer points to productdetail which has two variables (name and price)
I need to look up (string s) in the vector and if the string s is in vector (product->productdetail.getName()), I need to return the price..
how to compare proudct* and string s ?
my teacher gave me advice that i need to make anther string conversion function since the comparing type need to be the same..
(I tried to use lambda and it didn't work and changed to compare function)

Answer 1

I concur with #Fureeish, using bare pointers in modern C++ is not a good choice, unless you absolutely have to.
That said, writing predicates for the standard library requires a good knowledge of the library methods you are using. Have a look at std::binary_search documentation. A simple application is to search for the same type as you are searching through. In other words your needle is the same type as what is in your haystack. Here is an example:
#include <iostream>
#include <vector>
#include <algorithm>
class product {
public:
class product_details {
public:
std::string name;
double price;
};
product_details productDetails{};
product(std::string const &name, double price) : productDetails{ name, price} {}
};
struct product_compare {
bool operator()(product const &lhs, product const &rhs) {
return lhs.productDetails.name < rhs.productDetails.name;
}
};
int main(int argc, char *argv[])
{
std::vector<product> producList{
{ "toaster", 74.99 },
{ "blender", 103.99 },
{ "slow cooker", 142.99 }
};
product_compare productCompare{};
std::sort(producList.begin(), producList.end(), productCompare);
product searchProduct{"slow cooker", 142.99};
if (std::binary_search(producList.begin(), producList.end(), searchProduct, productCompare))
std::cout << "Found!\n";
else
std::cout << "Not found!\n";
}
But that lacks elegance. Your needle can be a different type, the answer is actually in this SO question. Here is a rewrite that takes advantage of this. This idiom is a bit more complicated to write but is better related to the actual problem and therefore easier to understand. Ultimately you never know which side of the compare is going to be needle and which side is going to be hay. So you have to write your compare predicate to accept both comparisons.
#include <iostream>
#include <vector>
#include <algorithm>
class product {
public:
class product_details {
public:
std::string name;
double price;
};
product_details productDetails{};
product(std::string const &name, double price) : productDetails{ name, price} {}
};
struct product_compare {
bool operator()(std::string const &lhs, product const &rhs) {
return lhs < rhs.productDetails.name;
}
bool operator()(product const &lhs, std::string const &rhs ) {
return lhs.productDetails.name < rhs;
}
};
// Used by std::sort
bool operator<(product const &lhs, product const &rhs) {
return lhs.productDetails.name < rhs.productDetails.name;
}
int main(int argc, char *argv[])
{
std::vector<product> producList{
{ "toaster", 74.99 },
{ "blender", 103.99 },
{ "slow cooker", 142.99 }
};
product_compare productCompare{};
std::sort(producList.begin(), producList.end());
if (std::binary_search(producList.begin(), producList.end(), std::string("slow cooker"), productCompare))
std::cout << "Found!\n";
else
std::cout << "Not found!\n";
}