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What does a comma mean inside an 'if' statement? [duplicate]

What does the comma operator , do?comma operator in if conditionWhat is the difference between #include <filename> and #include “filename”?What are the differences between a pointer variable and a reference variable in C++?What does the explicit keyword mean?What is the “-->” operator in C++?What is The Rule of Three?What is the meaning of prepended double colon “::”?What are the basic rules and idioms for operator overloading?C++11 introduced a standardized memory model. What does it mean? And how is it going to affect C++ programming?Why are elementwise additions much faster in separate loops than in a combined loop?Why is my program slow when looping over exactly 8192 elements?

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19

This question already has an answer here:

• 
  What does the comma operator , do?
  
  9 answers
  
  

Consider:

for (auto i = 0; i < g.size(); ++i)
 for (auto j = 0; j < g.size(); ++j) if (g[i][j] == 0) dfs(g, i, j), ++regions;
return regions;

I don't like one line code. What does the code execute in the if()?

I am confused by the "," sign.

Usually I would write it as:

 for (auto i = 0; i < g.size(); ++i)
 
 for (auto j = 0; j < g.size(); ++j)
 
 if (g[i][j] == 0)
 
 dfs(g, i, j)
 
 ,++regions; // I am not sure what to do here. Inside the "if" scope??
 
 
 return regions;

c++

share|improve this question

edited yesterday

Gilad

asked May 11 at 20:44

GiladGilad

3,51383992

marked as duplicate by phuclv, David Z, Community♦ May 12 at 4:11

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The second code snippet is not balanced - there are three opening s and two closing s - it is missing a }.
  
  – Peter Mortensen
  May 11 at 22:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  See also here: stackoverflow.com/questions/16475032/…
  
  – Valentino
  May 11 at 23:07
  

  
  
  
  

add a comment | 

19

This question already has an answer here:

• 
  What does the comma operator , do?
  
  9 answers
  
  

Consider:

for (auto i = 0; i < g.size(); ++i)
 for (auto j = 0; j < g.size(); ++j) if (g[i][j] == 0) dfs(g, i, j), ++regions;
return regions;

I don't like one line code. What does the code execute in the if()?

I am confused by the "," sign.

Usually I would write it as:

 for (auto i = 0; i < g.size(); ++i)
 
 for (auto j = 0; j < g.size(); ++j)
 
 if (g[i][j] == 0)
 
 dfs(g, i, j)
 
 ,++regions; // I am not sure what to do here. Inside the "if" scope??
 
 
 return regions;

c++

share|improve this question

edited yesterday

Gilad

asked May 11 at 20:44

GiladGilad

3,51383992

marked as duplicate by phuclv, David Z, Community♦ May 12 at 4:11

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  The second code snippet is not balanced - there are three opening s and two closing s - it is missing a }.
  
  – Peter Mortensen
  May 11 at 22:53
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  See also here: stackoverflow.com/questions/16475032/…
  
  – Valentino
  May 11 at 23:07
  

  
  
  
  

add a comment | 

This question already has an answer here:

• 
  What does the comma operator , do?
  
  9 answers
  
  

Consider:

for (auto i = 0; i < g.size(); ++i)
 for (auto j = 0; j < g.size(); ++j) if (g[i][j] == 0) dfs(g, i, j), ++regions;
return regions;

I don't like one line code. What does the code execute in the if()?

I am confused by the "," sign.

Usually I would write it as:

 for (auto i = 0; i < g.size(); ++i)
 
 for (auto j = 0; j < g.size(); ++j)
 
 if (g[i][j] == 0)
 
 dfs(g, i, j)
 
 ,++regions; // I am not sure what to do here. Inside the "if" scope??
 
 
 return regions;

c++

share|improve this question

edited yesterday

Gilad

asked May 11 at 20:44

GiladGilad

3,51383992

for (auto i = 0; i < g.size(); ++i)
 for (auto j = 0; j < g.size(); ++j) if (g[i][j] == 0) dfs(g, i, j), ++regions;
return regions;

 for (auto i = 0; i < g.size(); ++i)
 
 for (auto j = 0; j < g.size(); ++j)
 
 if (g[i][j] == 0)
 
 dfs(g, i, j)
 
 ,++regions; // I am not sure what to do here. Inside the "if" scope??
 
 
 return regions;

share|improve this question

edited yesterday

Gilad

asked May 11 at 20:44

GiladGilad

3,51383992

share|improve this question

edited yesterday

Gilad

asked May 11 at 20:44

GiladGilad

3,51383992

asked May 11 at 20:44

GiladGilad

3,51383992

asked May 11 at 20:44

GiladGilad

3,51383992

1 Answer
1

active

oldest

votes

27

The programmer has used the comma operator to provide two unrelated expressions in a single statement. Because it's a single statement, both expressions are "inside" the if condition.

It's a poor hack, which would be better done with actual braces surrounding two statements.

Your example is not equivalent; it should be:

if (g[i][j] == 0) 

 dfs(g, i, j);
 ++regions;

share|improve this answer

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  ohh god, why why why would anyone write this code. Thanks
  
  – Gilad
  May 11 at 20:50
  

  
  
  
  


• 
  
  
  

  
  8
  

  
  
  
  
  
  

  
  
  @Gilad Trying to be clever, most likely! And failing.
  
  – Lightness Races in Orbit
  May 11 at 20:51
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LightnessRacesinOrbit This is a habit of old school programmers due to small monitor resolution (25 rows 80 columns) code was writing as short as possible. More code on one page better readability. Try read modern code on such monitor and you will see how many empty rows in it.
  
  – Andrey Sv
  May 11 at 21:14
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Of course, that hack is especially useful for macros, which one also should avoid assiduously.
  
  – Deduplicator
  May 11 at 21:33
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @AndreySv Such programmers have had some thirty years to break that habit, i.e. since before C++ even existed.
  
  – Lightness Races in Orbit
  May 11 at 22:59
  

  
  
  
  

 | 
show 15 more comments

27

The programmer has used the comma operator to provide two unrelated expressions in a single statement. Because it's a single statement, both expressions are "inside" the if condition.

It's a poor hack, which would be better done with actual braces surrounding two statements.

Your example is not equivalent; it should be:

if (g[i][j] == 0) 

 dfs(g, i, j);
 ++regions;

share|improve this answer

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  ohh god, why why why would anyone write this code. Thanks
  
  – Gilad
  May 11 at 20:50
  

  
  
  
  


• 
  
  
  

  
  8
  

  
  
  
  
  
  

  
  
  @Gilad Trying to be clever, most likely! And failing.
  
  – Lightness Races in Orbit
  May 11 at 20:51
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LightnessRacesinOrbit This is a habit of old school programmers due to small monitor resolution (25 rows 80 columns) code was writing as short as possible. More code on one page better readability. Try read modern code on such monitor and you will see how many empty rows in it.
  
  – Andrey Sv
  May 11 at 21:14
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Of course, that hack is especially useful for macros, which one also should avoid assiduously.
  
  – Deduplicator
  May 11 at 21:33
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @AndreySv Such programmers have had some thirty years to break that habit, i.e. since before C++ even existed.
  
  – Lightness Races in Orbit
  May 11 at 22:59
  

  
  
  
  

 | 
show 15 more comments

27

The programmer has used the comma operator to provide two unrelated expressions in a single statement. Because it's a single statement, both expressions are "inside" the if condition.

It's a poor hack, which would be better done with actual braces surrounding two statements.

Your example is not equivalent; it should be:

if (g[i][j] == 0) 

 dfs(g, i, j);
 ++regions;

share|improve this answer

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

• 
  
  
  

  
  7
  

  
  
  
  
  
  

  
  
  ohh god, why why why would anyone write this code. Thanks
  
  – Gilad
  May 11 at 20:50
  

  
  
  
  


• 
  
  
  

  
  8
  

  
  
  
  
  
  

  
  
  @Gilad Trying to be clever, most likely! And failing.
  
  – Lightness Races in Orbit
  May 11 at 20:51
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @LightnessRacesinOrbit This is a habit of old school programmers due to small monitor resolution (25 rows 80 columns) code was writing as short as possible. More code on one page better readability. Try read modern code on such monitor and you will see how many empty rows in it.
  
  – Andrey Sv
  May 11 at 21:14
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Of course, that hack is especially useful for macros, which one also should avoid assiduously.
  
  – Deduplicator
  May 11 at 21:33
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  @AndreySv Such programmers have had some thirty years to break that habit, i.e. since before C++ even existed.
  
  – Lightness Races in Orbit
  May 11 at 22:59
  

  
  
  
  

 | 
show 15 more comments

The programmer has used the comma operator to provide two unrelated expressions in a single statement. Because it's a single statement, both expressions are "inside" the if condition.

It's a poor hack, which would be better done with actual braces surrounding two statements.

Your example is not equivalent; it should be:

if (g[i][j] == 0) 

 dfs(g, i, j);
 ++regions;

share|improve this answer

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

if (g[i][j] == 0) 

 dfs(g, i, j);
 ++regions;

share|improve this answer

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831

answered May 11 at 20:46

Lightness Races in OrbitLightness Races in Orbit

299k56484831