C++ Sign function - From MatLab?

Question

I'm trying to re-write some MatLab code in C++ and I've come across this:

currentsign = sign(vector(i));

I have looked on the internet and found this link: http://www.mathworks.co.uk/help/techdoc/ref/sign.html

I'm just wondering if there's a sign function in C++? If not, can anyone suggest any tutorials on creating it.

Thank you :)

score 6 · Accepted Answer · edited May 23 '17 at 11:48

6

template <typename T>
int sign (const T &val) { return (val > 0) - (val < 0); }

Credit due to Ambroz Bizjak.

template <typename T>
std::vector<int> sign (const std::vector<T> &v) {
    std::vector<int> r(v.size());
    std::transform(v.begin(), v.end(), r.begin(), (int(*)(const T&))sign);
    return r;
}

Full example on ideone.

edited May 23 '17 at 11:48

Community

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answered Aug 16 '12 at 15:06

jxh

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1

And doesn't answer the question. – Pete Becker Aug 16 '12 at 15:36

juanchopanza · Answer 2 · 2012-08-17T08:11:51.480

4

I would suggest

First, write a function of functor that takes a single element and returns 1, -1 or 0 depending on the element's value

Second, use std::transform together with this function/functor to take an input container and fill a second container with the desired values

template <typename T>
int signum(const T& val) {
  // implement signum logic
}

#include <vector>
#include <algorithm>

int main() {

  std::vector<int> data = ....;
  std::vector<int> signs(data.size());
  std::transform(data.begin(), data.end(), signs.begin(), signum<int>);

}

edited Aug 17 '12 at 08:11

answered Aug 16 '12 at 15:06

juanchopanza

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Thank you. I'm an idiot and very tired. Didn't realise it was this simple ha - How do I mark these as solved? – Phorce Aug 16 '12 at 15:11
@user1582478 you have to select an answer. There is a tick symbol. next to each answer's score. When you select it in becomes green. – juanchopanza Aug 16 '12 at 15:28

score 1 · Answer 3 · answered Aug 16 '12 at 15:11

#include <algorithm>
#include <iostream>
#include <iomanip>
#include <vector>

inline int get_signum(int val) {
    return val < 0 ? -1
        : val == 0 ? 0
        : 1;
}

int main() {
    std::vector<int> values;
    for (int i = -5; i < 6; ++i)
        values.push_back(i);
    std::vector<int> signum(values.size());
    std::transform(values.begin(), values.end(), signum.begin(), get_signum);
    for (int i = 0; i < values.size(); ++i) {
        std::cout << std::setw(2) << values[i] << ' ' << signum[i] << std::endl;
    }
    return 0;
}

Neel Basu · Answer 4 · 2012-08-16T15:22:05.120

Well You can do It at compile time using template Specialization.
You can use sign<n>::Positive, sign<n>::Negetive and sign<n>::Zero also you can use sign<n>::Sign which is 1|0|-1 which is same as sign of Matlab.

#include <iostream>

template<int n>
struct sign{
    enum{
      Positive = (n > 0),
      Negetive = (n < 0),
      Zero     = 0,
      Sign     = ((n > 0) ? 1 : -1)
    };
};

template<>
struct sign<0>{
    enum{
      Positive = 0,
      Negetive = 0,
      Zero     = 1,
      Sign     = 0
    };
};

int main(){
    std::cout << sign<0>::Positive  << sign<0>::Negetive  << sign<0>::Zero  << sign<0>::Sign  << std::endl;
    std::cout << sign<1>::Positive  << sign<1>::Negetive  << sign<1>::Zero  << sign<1>::Sign  << std::endl;
    std::cout << sign<-1>::Positive << sign<-1>::Negetive << sign<-1>::Zero << sign<-1>::Sign << std::endl;


    return 0;
}

You used to do sign(n) there and here you will do sign<n>::Sign.

score 0 · Answer 5 · answered Aug 16 '12 at 15:11

0

C99 has signbit() and copysign(), which seem to be implemented in glibc on Linux. You didn't specify what platform you're on though, so I'm not sure that helps...

answered Aug 16 '12 at 15:11

twalberg

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Those functions are also available in C++11. – Pete Becker Aug 16 '12 at 15:36

C++ Sign function - From MatLab?

5 Answers5