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Initialize an array of doubles at compile time

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11

1

static constexpr unsigned num_points 7810 ;
std::array< double, num_points > axis;

for(int i = 0; i < num_points; ++i) 

 axis[i] = 180 + 0.1 * i;

axis is a class-wide constant. I want to avoid initializing it like any other global variable. Can it be done in compile time?

c++ c++17 compile-time

share|improve this question

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, see stackoverflow.com/a/56376301/2466431
  
  – JVApen
  8 hours ago
  

  
  
  
  

add a comment | 

11

1

static constexpr unsigned num_points 7810 ;
std::array< double, num_points > axis;

for(int i = 0; i < num_points; ++i) 

 axis[i] = 180 + 0.1 * i;

axis is a class-wide constant. I want to avoid initializing it like any other global variable. Can it be done in compile time?

c++ c++17 compile-time

share|improve this question

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, see stackoverflow.com/a/56376301/2466431
  
  – JVApen
  8 hours ago
  

  
  
  
  

add a comment | 

static constexpr unsigned num_points 7810 ;
std::array< double, num_points > axis;

for(int i = 0; i < num_points; ++i) 

 axis[i] = 180 + 0.1 * i;

axis is a class-wide constant. I want to avoid initializing it like any other global variable. Can it be done in compile time?

c++ c++17 compile-time

share|improve this question

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

static constexpr unsigned num_points 7810 ;
std::array< double, num_points > axis;

for(int i = 0; i < num_points; ++i) 

 axis[i] = 180 + 0.1 * i;

share|improve this question

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

share|improve this question

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

edited 8 hours ago

Barry

191k21347630

edited 8 hours ago

Barry

191k21347630

asked 8 hours ago

VoracVorac

3,30353572

asked 8 hours ago

VoracVorac

3,30353572

2 Answers
2

active

oldest

votes

13

Here is the full compileable code:

#include <array>

template<int num_points>
static constexpr std::array<double, num_points> init_axis() 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
;

struct Z 
 static constexpr int num_points = 10;
 static constexpr auto axis = init_axis<num_points>();
;

share|improve this answer

edited 7 hours ago

answered 8 hours ago

SergeyASergeyA

46.2k54194

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Suggest substituting std::array<double,num_points> for auto so the reader doesn't have to check the initializing function to know the type.
  
  – doug
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug I find it to be the matter of preference. Some people like to know the type, some people prefer less typeing (pun natural!).
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I prefer auto too, for the same reason. But when the type isn't obvious for someone reading the line of code, including me a month later, I'll put in the extra typing. Probably helpful for readers coming to the question as well.
  
  – doug
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug your code editor doesn't give you the type and doesn't allow you to jump to function in a single click?
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course. But when just scanning a screen of code it doesn't. Have to put the cursor on the variable. I prefer to avoid that.
  
  – doug
  7 hours ago
  

  
  
  
  

add a comment | 

7

For completeness' sake, here's a version that does not require the definition of a function but instead uses a constexpr lambda:

static constexpr auto axis = []() constexpr 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

(Note the () in the last line, which calls the lambda right away.)

If you don't like the auto in the axis declaration because it makes it harder to read the actual type, but you don't want to repeat the type inside the lambda, you can instead do:

static constexpr std::array<double, num_points> axis = []() constexpr 
 auto a = decltype(axis);
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

share|improve this answer

edited 7 hours ago

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

add a comment | 

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13

Here is the full compileable code:

#include <array>

template<int num_points>
static constexpr std::array<double, num_points> init_axis() 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
;

struct Z 
 static constexpr int num_points = 10;
 static constexpr auto axis = init_axis<num_points>();
;

share|improve this answer

edited 7 hours ago

answered 8 hours ago

SergeyASergeyA

46.2k54194

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Suggest substituting std::array<double,num_points> for auto so the reader doesn't have to check the initializing function to know the type.
  
  – doug
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug I find it to be the matter of preference. Some people like to know the type, some people prefer less typeing (pun natural!).
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I prefer auto too, for the same reason. But when the type isn't obvious for someone reading the line of code, including me a month later, I'll put in the extra typing. Probably helpful for readers coming to the question as well.
  
  – doug
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug your code editor doesn't give you the type and doesn't allow you to jump to function in a single click?
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course. But when just scanning a screen of code it doesn't. Have to put the cursor on the variable. I prefer to avoid that.
  
  – doug
  7 hours ago
  

  
  
  
  

add a comment | 

13

Here is the full compileable code:

#include <array>

template<int num_points>
static constexpr std::array<double, num_points> init_axis() 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
;

struct Z 
 static constexpr int num_points = 10;
 static constexpr auto axis = init_axis<num_points>();
;

share|improve this answer

edited 7 hours ago

answered 8 hours ago

SergeyASergeyA

46.2k54194

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Suggest substituting std::array<double,num_points> for auto so the reader doesn't have to check the initializing function to know the type.
  
  – doug
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug I find it to be the matter of preference. Some people like to know the type, some people prefer less typeing (pun natural!).
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I prefer auto too, for the same reason. But when the type isn't obvious for someone reading the line of code, including me a month later, I'll put in the extra typing. Probably helpful for readers coming to the question as well.
  
  – doug
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @doug your code editor doesn't give you the type and doesn't allow you to jump to function in a single click?
  
  – SergeyA
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course. But when just scanning a screen of code it doesn't. Have to put the cursor on the variable. I prefer to avoid that.
  
  – doug
  7 hours ago
  

  
  
  
  

add a comment | 

Here is the full compileable code:

#include <array>

template<int num_points>
static constexpr std::array<double, num_points> init_axis() 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
;

struct Z 
 static constexpr int num_points = 10;
 static constexpr auto axis = init_axis<num_points>();
;

share|improve this answer

edited 7 hours ago

answered 8 hours ago

SergeyASergeyA

46.2k54194

#include <array>

template<int num_points>
static constexpr std::array<double, num_points> init_axis() 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
;

struct Z 
 static constexpr int num_points = 10;
 static constexpr auto axis = init_axis<num_points>();
;

share|improve this answer

edited 7 hours ago

answered 8 hours ago

SergeyASergeyA

46.2k54194

answered 8 hours ago

SergeyASergeyA

46.2k54194

answered 8 hours ago

SergeyASergeyA

46.2k54194

7

For completeness' sake, here's a version that does not require the definition of a function but instead uses a constexpr lambda:

static constexpr auto axis = []() constexpr 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

(Note the () in the last line, which calls the lambda right away.)

If you don't like the auto in the axis declaration because it makes it harder to read the actual type, but you don't want to repeat the type inside the lambda, you can instead do:

static constexpr std::array<double, num_points> axis = []() constexpr 
 auto a = decltype(axis);
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

share|improve this answer

edited 7 hours ago

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

add a comment | 

7

For completeness' sake, here's a version that does not require the definition of a function but instead uses a constexpr lambda:

static constexpr auto axis = []() constexpr 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

(Note the () in the last line, which calls the lambda right away.)

If you don't like the auto in the axis declaration because it makes it harder to read the actual type, but you don't want to repeat the type inside the lambda, you can instead do:

static constexpr std::array<double, num_points> axis = []() constexpr 
 auto a = decltype(axis);
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

share|improve this answer

edited 7 hours ago

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

add a comment | 

For completeness' sake, here's a version that does not require the definition of a function but instead uses a constexpr lambda:

static constexpr auto axis = []() constexpr 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

(Note the () in the last line, which calls the lambda right away.)

If you don't like the auto in the axis declaration because it makes it harder to read the actual type, but you don't want to repeat the type inside the lambda, you can instead do:

static constexpr std::array<double, num_points> axis = []() constexpr 
 auto a = decltype(axis);
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

share|improve this answer

edited 7 hours ago

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

static constexpr auto axis = []() constexpr 
 std::array<double, num_points> a;
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

static constexpr std::array<double, num_points> axis = []() constexpr 
 auto a = decltype(axis);
 for(int i = 0; i < num_points; ++i) 
 
 a[i] = 180 + 0.1 * i;
 
 return a;
();

share|improve this answer

edited 7 hours ago

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069

answered 8 hours ago

Nikos C.Nikos C.

36.5k54069