Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: question about function object
Top Forums Programming question about function object Post 302497011 by homeboy on Wednesday 16th of February 2011 04:24:34 AM
Old 02-16-2011
question about function object
I have a code as following:
Code:
#include <iostream>
#include <algorithm>
#include <list>
using namespace std;

//the class Nth is a predicates
class Nth{
    private:
         int nth;
         int count;
    public:
         Nth(int n):nth(n),count(0){}
         bool operator()(int){
             return ++count == nth;
        }
};

int main()
{
       list<int> lst;
 
       for(int i=1;i<10;i++)
           lst.push_back(i);

       copy(lst.begin(),lst.end(),ostream_iterator<int>(cout," "));
       cout<<endl;

       list<int>::iterator pos = remove_if(lst.begin(),lst.end(),Nth(3));
       lst.erase(pos,lst.end());

       copy(lst.begin(),lst.end(),ostream_iterator<int>(cout," "));
       cout<<endl;
}

the output is:
1 2 3 4 5 6 7 8 9
1 2 4 5 7 8 9
Not as I expect:
1 2 3 4 5 6 7 8 9
1 2 4 5 6 7 8 9
The book explained:
This happens because the usual implementation of the algorithm copies the predicate internally during the algorithm:
Code:
template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       ForwIter pos = find_if(beg,end,op);
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            return remove_copy_if(++next,end,beg,op);
       }
}

and it says it is possible to avoid this surprise behavior like this:
Code:
template<class ForwIter,class Predicate>
ForwIter std::remove_if(ForwIter beg,ForeIter end,Predicate op)
{
       while(beg != end && !op(*beg))
            beg++;
       if(pos == end)
            return beg;
       else{
            ForwIter next = beg;
            return remove_copy_if(++next,end,beg,op);
       }
}

I don't know what the difference between the two implementations is.I think i was able to understand the first one,but why the second one works fine?I thought they were the same.Any help will be appreciated.
 

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Question about function

/ (6 Replies)
Discussion started by: jy2728
6 Replies

2. Programming

Listing function exports from object file

Is it possible to view all the functions exported by a given object file? "dump -tv" comes the closest, but what exactly am I looking for to determine whether the symbol exists in the object file? Essentially, I have a library that requires a call to "xdr_sizeof" and the compile is failing... (5 Replies)
Discussion started by: DreamWarrior
5 Replies

3. Programming

Question about the system() function in C

Hello all ! Could someone throw some light on whether there's a limit to the number of characters contained in the command string that is passed to the system() call in C. Is it OS dependent? If yes, what are the limits for each? Thanks. (4 Replies)
Discussion started by: vsanjit
4 Replies

4. UNIX for Dummies Questions & Answers

Object reference not set to an instance of an object

I am new to PHP and UNIX. I am using Apache to do my testing on a Windows Vista machine. I am getting this error when I am trying to connect to a web service. I did a search and did not see any posts that pertain to this. Here is my function: <?php function TRECSend($a, $b, $c, $d,... (0 Replies)
Discussion started by: EddiRae
0 Replies

5. Shell Programming and Scripting

Question about Function calls

Hello everyone, here's my problem: I want to create two shell scripts. one of them should includes some functions, the other one just the function calls. Is this possible? Can i call a function which is placed in a scriptfile eg functions.sh out of another script eg call.sh? :confused: And if... (2 Replies)
Discussion started by: Sebi0815
2 Replies

6. Shell Programming and Scripting

How to Call external function in .C or .So (Shared Object)

Hi, Anybody know any way to Call with Shell Script an external function wrote in .C or .So (Shared Object) on AIX enviroment and returning parameters of .C or .SO to Shell Script? Tks!! (6 Replies)
Discussion started by: rdgsantos
6 Replies

7. Programming

c++ object constructor question

I have the following code class Param{ public: Param(int aa, int bb){ a=aa; b=bb; } int a,b; }; void function(Param); int main(){ function(2,3); return 0; } (6 Replies)
Discussion started by: santiagorf
6 Replies

8. Programming

Shared Object Question

Hello, I am new to programming shared objects and I was hoping someone could tell me if what I want to do is possible, or else lead me in the right direction. I have a main program that contains an abstract base class. I also have a subclass that I'm compiling as a shared object. The subclass... (13 Replies)
Discussion started by: dorik
13 Replies

9. Programming

passing object to function, columns class

I am working on a small columns class, since I use allot of tabular data. I am trying to set up code to allow me to efficiently read in tabular data, manipulate it, and write to output files. I more or less know what I need to do, but there are many options to sort through. I have the beginnings... (14 Replies)
Discussion started by: LMHmedchem
14 Replies

10. Programming

How to initialize an object with another object of different class?

How to initialize an object of class say "A", with an object of type say "B". The following code give the error message "error: conversion from âAâ to non-scalar type âBâ requested" #include <iostream> using namespace std; class B; class A{ public: A() { cout <<"\nA()" << endl; } ... (1 Reply)
Discussion started by: techmonk
1 Replies

LEARN ABOUT MINIX

ppmtosixel

ppmtosixel(1)                                                 General Commands Manual                                                ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.  The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
              pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order  of  magni-
              tude larger than a compressed file and prints much slower.

       -margin
              If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
              fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater  than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

                                                                   26 April 1991                                                     ppmtosixel(1)
All times are GMT -4. The time now is 12:30 AM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy