2

I have the following python code

for m,n in [(-1,1),(-1,0),(-1,-1)] if 0<=i+m<b and 0<=j+n<l and image[i+m][j+n] == '0']

image is array defined and i and j is also defined.

Following is how I have converted this into C++

std::vector<std::pair<int,int> > direction;
direction.push_back(std::make_pair(-1,1));
direction.push_back(std::make_pair(-1,0));
direction.push_back(std::make_pair(-1,-1));
for ( std::vector<std::pair<int,int> >::iterator itr = direction.begin(); 
                   itr != direction.end(); ++itr) {
    int m = (*itr).first;
    int n = (*itr).second;
   if ( (0 <= i + m && i + m < width ) && 
                   (0 <= j + n && j + n < width ) && 
                   image[i + m][j + n ] == 0) {
}

Is this conversion correct?

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4
  • 1
    you can use itr->first syntax instead of (*itr).first, although it's just a matter of taste. Commented Dec 1, 2011 at 17:42
  • image is defined as ['10001','10100','00000','00000','00111','00100','00100'] Commented Dec 1, 2011 at 17:46
  • In the C++ code? What kind of data is that? 2D array with chars ? Commented Dec 1, 2011 at 17:48
  • I am using int and not char like std::vector<std::vector<int> > image; Commented Dec 1, 2011 at 17:54

5 Answers 5

Reset to default
2

As another person remarked, the width used in two places is probably incorrect.

Assuming that, here's a comparision of direct translation from Python versus C++-like code:

#include <iostream>
#include <list>
#include <utility>
#include <vector>
using namespace std;

void likeCPlusPlus()
{
    int i = 666, j = 666, width = 666, height = 666, image[666][666];

    for( int dy = 1;  dy >= -1;  --dy )
    {
        int const   dx  = -1;
        int const   x   = i + dx;
        int const   y   = j + dy;

        if(
            0 <= x && x < width &&
            0 <= y && y < height &&
            image[x][y] == 0
            )
        {}
    }
}

void likePythonInCPlusPlus()
{
    int i = 666, j = 666, width = 666, image[666][666];

    std::vector<std::pair<int,int> > direction;
    direction.push_back(std::make_pair(-1,1));
    direction.push_back(std::make_pair(-1,0));
    direction.push_back(std::make_pair(-1,-1));
    for ( std::vector<std::pair<int,int> >::iterator itr = direction.begin(); 
                       itr != direction.end(); ++itr)
    {
        int m = (*itr).first;
        int n = (*itr).second;
        if ( (0 <= i + m && i + m < width ) && 
                       (0 <= j + n && j + n < width ) && 
                       image[i + m][j + n ] == 0)
        {}
    }
}

int main()
{}
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Comments

2

Almost. You have two differences: in Python, you have i+m<b and j+n<l, which makes me think b!=l.

In your C++ code, you have i + m < width and j + n < width, where width is the same.

If width == b == l, then everything's fine.

Actually, depends on how image is defined. The image[i + m][j + n ] == 0 is what bothers me (the part with ==0)

As the @Avinash comment says, image is vector< vector< int > >, so the code is fine.

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4 Comments

and they are the same as width? Plus, in Python, you compare the image with chars (== '0'), and here with ints ( == 0 ). If this is correct, everything else is fine.
additionally the python snippet is a list comprehension so OP should probably push the m and n into a list if the condition of the if is true
@soulcheck - probably yes, but we don't know what's after the if's condition.
@KirilKirov aye, though we know that the python snippet ends with ] and has list comprehension syntax so whatever he does with m and n will get pushed some list
1

You don't need to build that vector at runtime, if it's really a hardcoded constant. Just do:

const std::pair<int,int> list[] = { {-1,1}, {-1,0}, {-1,-1} };
for (int index = 0; index < sizeof(list)/sizeof(*list); ++index)
{
    int m = list[index].first;
    int n = list[index].second;
    ...
}

if you're allowed C++0x, or

const struct { int first, second; } list[] = { {-1,1}, {-1,0}, {-1,-1} };
...

if not. Otherwise, the translation looks plausible.

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4 Comments

error C4430: missing type specifier - int assumed. Note: C++ does not support default-int for list declaration
Which version, (I posted two), which line, and with or without C++0x support? NB, I just edited to qualify std::pair, since I omitted using namespace std; along with the includes etc.
following is the real error : error C2552: 'list' : non-aggregates cannot be initialized with initializer list
The first version, which specifically requires C++0x support for extended initializer lists? Try the second version of the declaration instead then, it's marginally more verbose but doesn't require C++0x support
1

If you don't try to reproduce Python idioms in C++, the code can be simplified to:

for (int n = 1; n >= -1; --n) {
    const int m = -1;
    if (...
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Comments

1

The following works under c++1z:

#include <vector>
using namespace std;
for( auto [m,n] : vector<tuple<int,int> >{{-1,1}, {-1,0}, {-1,-1}})
  if(0<=i+m<b and 0<=j+n<l and image[i+m][j+n] == '0'){}
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