How can I use a string read from `std::cin` to look up an existing variable by name?

Question

I'm currently trying to make a sort of a shopping cart. When the program asks for items, i type them in; but it needs to remember the values so it can use them later in the code.

I have this code so far:

#include <iostream>

int main() {
    int item{};
    int apple = 5;
    std::cout << "what item do you want to buy?";
    std::cin >> item;
    std::cout << item;
    return 0;
}

Can I make it so that when i type apple as input, item actually gets the value 5, from the variable named apple? With this code, I get 0 as the result.

No, in C++ the name of a variable *only exists* while the code is being compiled (except perhaps for some debug-mode hacks designed to help find problems in the code). The program cannot look for the name `apple` because it is not actually stored anywhere that it could be found. Instead, use a data structure that explicitly does that mapping, such as `std::map`. — Karl Knechtel, Jun 30 '22 at 06:35
Hello, you can also check this post about hashmap in cpp https://stackoverflow.com/questions/3578083/what-is-the-best-way-to-use-a-hashmap-in-c — Débart Rédouane, Jun 30 '22 at 06:41

score 3 · Accepted Answer · edited Jun 30 '22 at 06:45

3

You could create a map with string keys and int values, store the necessary data in that map (instead of separate variables), and use the value read from std::cin as an index.

The code looks like:

std::map<std::string, int> fruits;
fruits["apple"] = 5;
std::string choice;
std::cin >> choice;
std::cout << fruits[choice] << std::endl;

edited Jun 30 '22 at 06:45

Karl Knechtel

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answered Jun 30 '22 at 06:35

MFischer

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9

yep thats it thnaks also if i wanted to add more values to fruits how would i do that? – Maravic Karlo Jun 30 '22 at 06:42
The same way that the `apple` value is added. – Karl Knechtel Jun 30 '22 at 06:43
would this work? fruits["apple", "grape"] = 5, 10; or something like this – Maravic Karlo Jun 30 '22 at 06:44
No; use two separate statements. – Karl Knechtel Jun 30 '22 at 06:45
Alternately, use this initialization syntax: https://stackoverflow.com/questions/138600/initializing-a-static-stdmapint-int-in-c – Karl Knechtel Jun 30 '22 at 06:46
@MaravicKarlo `fruits["apple"] = 5; fruits["grape"] = 10;` – Aykhan Hagverdili Jun 30 '22 at 06:46
After reading `choice`, it will be necessary to check that the map contains an entry for that string before printing it. Unless you want to create an entry for the string entered, and (depending on user input) have `fruits["apple"]` and `fruits["Apple"]` map to different values. – Peter Jun 30 '22 at 06:47
my bad im brain dead how would i make it so when i enter more than one fruit it makes it sum up the price? – Maravic Karlo Jun 30 '22 at 06:50
2

You should probably use `.at()` for reading from the map, which will protect you from using a wrong key. – HolyBlackCat Jun 30 '22 at 07:19

score 0 · Answer 2 · answered Jun 30 '22 at 06:43

You can use a map, as shown in the other answer. Another choice would be to show the user a numbered list:

1: apple - $10
2: banana - $15
3: ...

And then ask them to enter a number. Then use the number to access the element in the array of products. This might be a more robust interface for the user.

How can I use a string read from `std::cin` to look up an existing variable by name?

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