assign integer value given boolean expression on C++

Question

i'm trying to understand a piece of algorithm taken from a c++ code , here's the C++ code

inline void DBGame::make_chain( Edge* &newedge, Edge *edges[], Node *node ){ newedge->length = edges[0]->length + edges[1]->length;

for (int j = 0; j < 2; j++) {

edges[j]->parent = newedge; edges[j]->remove();

  // replace the old edge with the new edge at the other end
  // -------------------------------------------------------
  int k = (edges[j]->node[0] == node);
  newedge->node[j] = edges[j]->node[k];
  newedge->pself[j] = edges[j]->pself[k];
  *newedge->pself[j] = newedge;
  }

}

fyi, node is an object and not an integer.
I don't understand how to assign an integer value to int k by giving an true-false expression.

Tom Tanner · Accepted Answer · 2012-08-15 12:30:12Z

2

There's an implicit cast of bool to 0 or 1. It doesn't greatly add to readability or efficiency, and is at least partly a result of C not having a bool type.

This line:

int k = (edges[j]->node[0] == node);

is equivalent to this:

int k = edges[j]->node[0] == node ? 1 : 0;

And juut to add to this, as the original questioner seems to have decided to use the comments to extend the question a lot, there are also implicit casts from numeric types and pointers to bool.

So

if (node)

is equivalent to

if (node != 0)

which is another shortcut I'm not exactly enamoured of.

edited Aug 15, 2012 at 12:30

answered Aug 15, 2012 at 9:47

Tom Tanner

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5 Comments

ignisc3 Over a year ago

ok, got it , thanks sorry, I have another question , here's the code if (node->edge[j] && node->edge[j] != pedge) or Node *node = edge->node[0]; if (node == edge->node[1]) { if (node) { node->degree = 2; do you mind explain the mean of 'if (node->edge[j] ` or ' if (node)` , how' the rule an mean of assigning an object to an if statement , instead of assign a true-false expression

Tom Tanner Over a year ago

sorry? I need some english in that statement to make sense of it

ignisc3 Over a year ago

what I mean is usually we give a true-false expression to an if statement,like if (A==B). but in the code the there's no boolean expression, it use an object as an expression , if(node) , node is an object not an expression

ignisc3 Over a year ago

how does assigning an object to an if statement works in c++?

matt Over a year ago

Node is a pointer, so if (node) will evaluate to false if the pointer is null

Andrey Chernukha · Accepted Answer · 2012-08-15 09:47:12Z

0

false is 0, true is everything except 0

answered Aug 15, 2012 at 9:47

Andrey Chernukha

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Comments

Andriy · Accepted Answer · 2012-08-15 09:48:04Z

0

The type of the result of edges[j]->node[0] == node is bool. It can be implicitly converted to int, false becoming 0 and true becoming a non-zero value, usually, 1.

answered Aug 15, 2012 at 9:48

Andriy

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5 Comments

ignisc3 Over a year ago

ok, thanks.. got it .sorry .. I have another question, how does assigning an object to an if statement works in c++? example if(node[a][b]) {...} what I know, usually Iwe assign a true-false expression to an if instead of an object

Andriy Over a year ago

Here it works other way around. An expression is implicitly converted to bool, equivalent to if(node[a][b] != 0)

ignisc3 Over a year ago

nice , thanks, your explaination really healpful for a newbie like me :)

juanchopanza Over a year ago

@Andrey I think there is a minimum reputation score required to up-vote, unfortunately.

Andriy Over a year ago

@juanchopanza: does that apply to answers for his own question?

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