for_each and transform

Question

for_each(ivec.begin(),ivec.end(),
        []( int& a)->void{ a = a < 0 ? -a : a; 
    });

transform(ivec.begin(),ivec.end(),ivec.begin(),
        [](int a){return a < 0 ? -a : a;
    });

I am currently learning lambdas and I am curious how the two implementations, that I have posted above, differ?

Answer 1

The two implementations you show do not differ logically (assuming you get the first version right by adding a return). The first one modifies elements in place while the last one overwrites its elements with new values.

The biggest difference I see, with transform you just can pass abs instead of a lambda that reimplements it.

Answer 2

transform is what would, in a functional language, be called map. That is, it applies a function to every element in the input range, and stores the output into an output range. (So it is generally intended to not modify the inputs, and instead store a range of outputs)

for_each simply discards the return value from the applied function (so it might modify the inputs).

That's the main difference. They are similar, but designed for different purposes.