sort a vector or pair alphabetically.

Natural_Bugger

#include <iostream>     // std::cout
#include <algorithm>    // std::sort
#include <vector>       // std::vector

bool compareFunction (std::string a, std::string b) {return a<b;} 
//compare any way you like, here I am using the default string comparison

int main () 
{
  std::string myNames[] = {"Henry","Tom","Jafar","Alice","Bob","Cindy","Clara","Michael"};

  std::vector<std::string> myvector (myNames, myNames+8); //create vector from array

  std::sort(myvector.begin(),myvector.end(),compareFunction);//sort the vector


  std::cout << "Sorted vector:";
  for (std::vector<std::string>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
        std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

http://cpp.sh/7hrvf

how would this apply to my vector?

vector< pair <string, string> > data; // name / email

takes 2 arguments.

bool compareFunction (std::string a, std::string b) {return a<b;}

but i don't see these arguments applied here.

std::sort(myvector.begin(),myvector.end(),compareFunction);//sort the vector

i need the "key" value of my pairs fully alphabetically sorted up to the last char of the name.

regards.

Pablo J. Rogina

@Natural_Bugger sorry for asking, but how is this issue related to Qt specifically?

It seems like an initial assignment in a programming course

Natural_Bugger

@Pablo-J-Rogina

well, I'm solving some online programming "riddles" and since i use Qt creator, because i like it.
been rusty for years.

J.Hilk

@Natural_Bugger

bool compareFunction (std::pair<std::string, std::string> a, std::pair<std::string, std::string> b) {return a.first<b.first;}

btw, this should, even for std::string be const references

bool compareFunction (const std::pair<std::string, std::string> &a, const std::pair<std::string, std::string> &b) {return a.first<b.first;}

JonB

@Natural_Bugger
If you wish to store std::pair() elements in your vector, you need a compareFunction() which takes std::pair() instead of std::string as its arguments (well, const references to them really, but that's not the point).

Natural_Bugger

@J-Hilk said in sort a vector or pair alphabetically.:

bool compareFunction (const std::pair<std::string, std::string> &a, const std::pair<std::string, std::string> &b) {return a.first<b.first;}

indeed, thank you very much.
i thought i had to expand the function and use some more "std::.." functions, but it worked perfectly.

but I'm still a little clueless on how the function receives it arguments.

@JonB

also to you, thnx

J.Hilk

@Natural_Bugger said in sort a vector or pair alphabetically.:

but I'm still a little clueless on how the function receives it arguments.

this is taken from the documentation

The argument for the compare, expects a function that returns a bool and accepts 2 arguments

JonB

@Natural_Bugger said in sort a vector or pair alphabetically.:

but I'm still a little clueless on how the function receives it arguments.

When you call std::sort() on a vector of objects, internally it will repeatedly call the function you pass as compareFunction on pairs of elements in the vector as it goes along, using its return result to re-order these elements in the vector, till they are fully sorted. All you need to know is that the argument types to compareFunction must match exactly the type of the elements in the vector, else you will get a compile-time type error.

Natural_Bugger

@JonB

thnx, i placed the sort function at the right spot in the first moment and it worked out perfectly first time.

it just wasn't obvious on how the "compareFunction" receives it's arguments.
that's clever, it wouldn't be obvious to most at first sight.

std::sort(myvector.begin(),myvector.end(),compareFunction);//sort the vector

no arguments are passed to "compareFunction", whilst requiring 2 arguments

bool compareFunction (std::string a, std::string b) {return a<b;}

thnx @J-Hilk and @JonB

JonB

@Natural_Bugger
compareFunction is passed to std::sort() without any following parentheses, i.e. not compareFunction(...). That means at this point is does not call the function, it merely passes the address of the function. std::sort() works through the vector element range you passed, taking pairs of elements and now calling compareFunction(element1, element2).