C++ Test if map element exists

Question

I have an "array" of strings defined as such:

typedef map<int, string> strArr;

Whenever I do this:

strArr args;
if(!args[1]) { /*do stuff*/ }

The compiler tells me that there's no match for 'operator!' Why is this so, and how can I fix this?

EDIT: Is there a way of making this work with bool operator! ()!

It's so because you failed to read any documentation, instead choosing to randomly guess! — Lightness Races in Orbit, Mar 21 '15 at 03:11

Emil Laine · Answer 1 · 2015-03-21T03:12:58.773

5

With !args[1], you're trying to call operator! on a std::string, and indeed, the error message is right: std::string has no operator!.

To check whether an element exists in a std::map, use find. It will return std::map::end if the specified key is not in the map:

if (args.find(1) == args.end()) { ... }

edited Mar 21 '15 at 03:12

answered Mar 21 '15 at 03:07

Emil Laine

Then how do I fix it...? – Lee Yi Mar 21 '15 at 03:07
1

@LeeYi: What is preventing you from simply reading the documentation? – Lightness Races in Orbit Mar 21 '15 at 03:12
I didn't know why that `!args[1]` didn't work. I simply assumed you could do the C++ equivalent of the lua `not args[1]` – Lee Yi Mar 21 '15 at 03:14
2

It may be a good time to realize that C++ is not Lua. – Emil Laine Mar 21 '15 at 03:15
I know that, obviously, but it seems like there are some quirks with C++ that I am unaware of – Lee Yi Mar 21 '15 at 03:16
1

@LeeYi Documentation is a great resource, you should use it! http://en.cppreference.com/w/cpp/container/map/find – Julian Mar 21 '15 at 03:22
2

@LeeYi: Now's a great time to discover that "simply assuming" things _does not work_. – Lightness Races in Orbit Mar 21 '15 at 03:26

1 Answers1