Show Lecture.CopyIf as a slide show.

CS253 Copy If

Introduction

• copy_if() is like copy(), except that it doesn’t copy everything.
• Instead it takes a predicate that determines whether or not to copy a given element.
  • A predicate is a function that returns bool.
• copy_if() takes three iterators and a predicate:

    copy_if(source-begin, source-end, dest-begin, predicate)

• Why isn’t there a dest-end?
  • It isn’t needed. source-begin and source-end say how much to copy.

First attempt

Let’s ensure that we know how to use copy() before moving on to copy_if():

string foo = "This is serious—I have to ration my Diet Mountain Dew!";
cout << foo << "\n";
string bar;
copy(foo.begin(), foo.end(), bar.begin());
cout << bar << "\n";

SIGSEGV: Segmentation fault

Why did that fail? No space allocated in bar.

Second attempt

string foo = "This is serious—I have to ration my Diet Mountain Dew!";
cout << foo << "\n";
string bar(foo.size(), 'X');
copy(foo.begin(), foo.end(), bar.begin());
cout << bar << "\n";

This is serious—I have to ration my Diet Mountain Dew!
This is serious—I have to ration my Diet Mountain Dew!

• There—we allocated enough space in bar:
• The string(size_t, char) constructor filled bar with the appropriate number of 'X' characters.

Third attempt

string foo = "This is serious—I have to ration my Diet Mountain Dew!";
cout << foo << "\n";

string bar(foo.size(), 'X');
// Don’t copy vowels:
copy_if(foo.begin(), foo.end(), bar.begin(),
        [](char c){return "aeiouyAEIOUY"s.find(c)==string::npos;} );

cout << bar << "\n";

This is serious—I have to ration my Diet Mountain Dew!
Ths s srs— hv t rtn m Dt Mntn Dw!XXXXXXXXXXXXXXXXXXXXX

• Hooray, copy_if() worked!
• Hey, what’s with those X’s? bar.size() is unchanged.

Fourth attempt

string foo = "This is serious—I have to ration my Diet Mountain Dew!";
cout << foo << "\n";

string bar(foo.size(), 'X');
// Don’t copy vowels:
auto it = copy_if(foo.begin(), foo.end(), bar.begin(),
                  [](char c){return "aeiouyAEIOUY"s.find(c)==string::npos;} );

// Make bar the correct size:
bar.resize(it-bar.begin());
cout << bar << "\n";

This is serious—I have to ration my Diet Mountain Dew!
Ths s srs— hv t rtn m Dt Mntn Dw!

We resized bar to the correct size.

In-place

string foo = "This is serious—I have to ration my Diet Mountain Dew!";
cout << foo << "\n";

auto it = copy_if(foo.begin(), foo.end(), foo.begin(),
                  [](char c){return "aeiouyAEIOUY"s.find(c)==string::npos;} );
// Make foo the correct size:
foo.resize(it-foo.begin());
cout << foo << "\n";

This is serious—I have to ration my Diet Mountain Dew!
Ths s srs— hv t rtn m Dt Mntn Dw!

We can copy from & to the same location.