Multiple string comparisions

Question

I'm trying to implement a very basic clone of redis in C++. So when I get the queries, I need to parse those. Right now I am doing this:

void Query::buildQuery(){
        std::string query_type = lower(args[0]);
        if(query_type == "get"){ //do something }
        else if(query_type == "set"){ //do something }
        else if(query_type == "getbit"){ //do something }
        else if(query_type == "setbit"){ //do something }
        else if(query_type == "zadd"){ //do something }
        else if(query_type == "zcard"){ //do something }
        else if(query_type == "zcount"){ //do something }
        else if(query_type == "zrange"){ //do something }
        else if(query_type == "save"){ //do something }
        else { throw(QueryException("Invalid query type")); }
}

Is there any other, shorter way to do this? I don't want to include any other library than the STL.

this answer is the way to use a switch to replace the `if()else if()...` [Evaluate a string with a switch in C++ [duplicate]](http://stackoverflow.com/a/16388594/5852567) — DIEGO CARRASCAL, Mar 07 '16 at 18:37
@DIEGOCARRASCAL Actually, no. I wasn't looking for a way to change if else to switch. I'm looking for a method other than these 2(if any) that could somehow do the required. — Ayush Gupta, Mar 07 '16 at 18:55

score 3 · Answer 1 · answered Mar 07 '16 at 18:45

3

If those do_somethings can be extracted into separate methods, then you could create a pre-initialized hash map (unordered_map) from string to pointer to member function and do something like

(this->*queryHandlers[query_type])();

You'll have to choose between lots of functions and one large function, though.

answered Mar 07 '16 at 18:45

Sergei Tachenov

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This is actually a great idea! Thanks! – Ayush Gupta Mar 07 '16 at 18:59

score 0 · Answer 2 · edited May 23 '17 at 11:45

0

Without if...else if, you can do this by switch statement. Like:

void Query::buildQuery(){
        std::string query_type = lower(args[0]);
        switch(str2int(query_type) ){
            case str2int("set"):// do something 
                break;
            case str2int("getbit"):// do something 
                break;
            ................
            ..........
            default:
                throw(QueryException("Invalid query type")); 
        }
}

According to Serhiy where str2int is like:

constexpr unsigned int str2int(const char* str, int h = 0)
{
    return !str[h] ? 5381 : (str2int(str, h+1)*33) ^ str[h];
}

edited May 23 '17 at 11:45

Community

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answered Mar 07 '16 at 18:32

Sakib Ahammed

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Since when C++ supports switch over strings? – Sergei Tachenov Mar 07 '16 at 18:34
1

You **can't** use `switch` with a `std::string` – NathanOliver Mar 07 '16 at 18:35
switch can be used only with integer values. So, you just need to convert your "case" values to integer. You can achieve it by using constexpr from c++11, thus some calls of constexpr functions can be calculated in compile time. – Sakib Ahammed Mar 07 '16 at 18:37
for more details you can see this post http://stackoverflow.com/questions/16388510/evaluate-a-string-with-a-switch-in-c – Sakib Ahammed Mar 07 '16 at 18:39
2

You should include the code for `str2int` with attribution into your answer. Any answer should be stand alone. I should not have to go somewhere else to make your answer complete. – NathanOliver Mar 07 '16 at 18:45
@NathanOliver - thanks for your comment. I edited my answer. – Sakib Ahammed Mar 07 '16 at 21:21

score 0 · Answer 3 · answered Mar 07 '16 at 18:50

If you're running on an Intel/AMD processor and feeling 'brave' you might like to take a look at these implementations of strcmp, strlen, etc that use SSE instructions. That'd be quicker.

As for the general structure, you could turn "set" & "do something" into an class which has a test method and a do-something method. Have an array of those, and iterate across it calling the test method passing query_type. The test method for the object would automatically call the do-something method if it matches the string.

Multiple string comparisions

3 Answers3