Streams – Lazy evaluation in C++14

Post by jzwinck » Mon, Jun 23, 2014, 1:33 AM UTC

ptfoobar wrote: post #10

srean wrote: post #5

@wwwwwwwwww you have been hell-banned and I dont see anything particularly offensive in your immediate comment history. > is there any reason to add lazy evaluation to c++ [...] I think you are suffering from a misapprehension. Nothing is being added to C++ the language. This is just an implementation of lazy streams in C++14. Think of it as an optional 3rd party library. > lazy evaluation is one of those features wh…

Second example say you want to compute (x + y) * z - w where each of them are long vectors but want to keep the look of the code readable and explicit. Naive way to do it will create lots of temporaries and unnecessary loops. Every binary operation is going to create a temporary (and entail a loop). But if you have laziness you just need one loop. Can you elaborate on this? Why can laziness provide better code here?

Post by srean » Mon, Jun 23, 2014, 3:17 AM UTC

pixelglow wrote: post #13

ris wrote: post #2

Ah, python's itertools available in c++, at last.

There's a previous implementation of itertools in C++: https://github.com/ryanhaining/cppitertools .

Post by wyager » Mon, Jun 23, 2014, 4:47 AM UTC

wwwwwwwwww wrote: post #4

honest question: is there any reason to add lazy evaluation to c++ other than to just add more features to the language? lazy evaluation is one of those features where i have absolutely no idea what benefit it actually provides

    inputs = [raw_read() for i in range(1000)] # Read numbers from command line
    numbers = map(int, inputs) # Turn them into integers
    for num in numbers: # Check for 42
      if num == 42:
        return true
    return false # It's not there

Post by ori_b » Mon, Jun 23, 2014, 5:25 AM UTC

wyager wrote: post #16

wwwwwwwwww wrote: post #4

honest question: is there any reason to add lazy evaluation to c++ other than to just add more features to the language? lazy evaluation is one of those features where i have absolutely no idea what benefit it actually provides

>lazy evaluation is one of those features where i have absolutely no idea what benefit it actually provides Good question. Let's say you have a program like this little python example: inputs = [raw_read() for i in range(1000)] # Read numbers from command line numbers = map(int, inputs) # Turn them into integers for num in numbers: # Check for 42 if num == 42: return true return false # It's not there If you use lazy…

    for (auto& num = file.line_iterator(); num != file.end(); num++) {
        if (atoi(num) == 42)
            return true;
    return false;

Post by hrjet » Mon, Jun 23, 2014, 6:06 AM UTC

ori_b wrote: post #18

wyager wrote: post #16

Earlier quoted context omitted.

>lazy evaluation is one of those features where i have absolutely no idea what benefit it actually provides Good question. Let's say you have a program like this little python example: inputs = [raw_read() for i in range(1000)] # Read numbers from command line numbers = map(int, inputs) # Turn them into integers for num in numbers: # Check for 42 if num == 42: return true return false # It's not there If you use lazy…

But in a language with mutating side effects, and iterators, I'm not sure what this buys you over: for (auto& num = file.line_iterator(); num != file.end(); num++) { if (atoi(num) == 42) return true; return false; Note that 'line_iterator()' isn't something that (as far as I know) is defined in the C++ standard library, but there's nothing preventing it.