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New Entry C & C++ Macros (#824)
* Create macros.md * Update preprocessors.md * Update macros.md * Update macros.md * Create macros.md * yarn format * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/cpp/concepts/macros/macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> * Update content/c/concepts/macros/macros.md Co-authored-by: KTom101 <kyra.thompson@nyu.edu> * Update content/cpp/concepts/macros/macros.md Co-authored-by: KTom101 <kyra.thompson@nyu.edu> * Update macros.md * Update macros.md * yarn format * Update macros.md * Update macros.md Co-authored-by: Brandon Dusch <brandondusch@gmail.com> Co-authored-by: KTom101 <kyra.thompson@nyu.edu>
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‎content/c/concepts/macros/macros.md‎

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---
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Title: 'Macros'
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Description: 'A macro is a label defined in the source code that is replaced by its value by the preprocessor before compilation.'
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Subjects:
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- 'Code Foundations'
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- 'Computer Science'
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Tags:
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- 'Command Line'
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- 'Developer Tools'
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CatalogContent:
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- 'learn-c'
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- 'paths/computer-science'
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---
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A **macro** is a label defined in the source code that is replaced by its value by the [preprocessor](https://www.codecademy.com/resources/docs/c/preprocessors) before [compilation](https://www.codecademy.com/resources/docs/c/compiling). Macros are initialized with the `#define` preprocessor command and can be undefined with the `#undef` command.
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There are two types of macros: object-like macros and function-like macros.
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## Object-Like Macros
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These macros are replaced by their value in the source code before compilation. Their primary purpose is to define constants to be used in the code.
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**Note**: Macro definitions are not followed by a semicolon `;`.
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### Example
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In the following example, `PI` is defined as an object-like macro:
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```c
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#include <stdio.h>
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#define PI 3.1416
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int main() {
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float radius = 3;
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float area;
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area = PI * radius * radius;
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printf("Area is: %f",area);
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return 0;
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}
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```
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This example outputs the following:
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```shell
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Area is: 28.274401
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```
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## Function-Like Macros
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These macros behave like [functions](https://www.codecademy.com/resources/docs/c/functions), in that they take arguments that are used in the replaced code. Note that in defining a function-like macro, there cannot be a space between the macro name and the opening parenthesis.
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### Example
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In the following example, `AREA` is defined as a function-like macro. Note that other macros can be used in defining a subsequent macro. The inner macro is replaced by its value before the outer macro is replaced.
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```c
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#include <stdio.h>
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#define PI 3.1416
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#define AREA(r) r * r * PI
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int main() {
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float radius = 5;
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float result;
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result = AREA(radius);
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printf("Area is: %f",result);
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return 0;
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}
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```
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This example outputs the following:
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```shell
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Area is: 78.540001
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```
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## Predefined Macros
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C has a number of predefined macros, including the following:
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- `__DATE__`: Current date formatted as `MMM DD YYYY`.
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- `__TIME__`: Current time formatted as `HH:MM:SS`.
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- `__FILE__`: Current filename.
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- `__LINE__`: Current line number.
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### Example
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The following example uses the above predefined macros:
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```c
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#include <stdio.h>
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int main() {
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char file[] = __FILE__;
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char date[] = __DATE__;
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char time[] = __TIME__;
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int line = __LINE__;
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printf("File name: %s\n", file);
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printf("Date: %s\n", date);
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printf("Time: %s\n", time);
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printf("Line number: %d\n", line);
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}
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```
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The output resembles the following:
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```shell
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File name: main.c
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Date: Jun 25 2022
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Time: 14:05:33
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Line number: 7
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```
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## Undefining a Macro
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Once defined, a macro can be undefined with the `#undef` command. Using the macro after that point will result in a compile error.
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### Example
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```c
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#include <stdio.h>
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#define TEST 1
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int main() {
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#ifdef TEST
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printf("TEST defined\n");
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#else
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printf("TEST undefined\n");
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#endif
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#undef TEST
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#ifdef TEST
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printf("TEST defined\n");
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#else
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printf("TEST undefined\n");
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#endif
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}
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```
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This results in the output:
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```shell
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TEST defined
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TEST undefined
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```

‎content/c/concepts/preprocessors/preprocessors.md‎

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### Defining Macros
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When the preprocessor encounters a macro in the source code, it will replace the macro with the value it has been assigned. Macros are defined by using `#define` and can be undefined using `#undef`.
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When the preprocessor encounters a [macro](https://www.codecademy.com/resources/docs/c/macros) in the source code, it will replace the macro with the value it has been assigned. Macros are defined by using `#define` and can be undefined using `#undef`.
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```c
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#define PI 3.1416

‎content/cpp/concepts/macros/macros.md‎

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---
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Title: 'Macros'
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Description: 'A macro is a label defined in the source code that is replaced by its value by the preprocessor before compilation.'
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Subjects:
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- 'Code Foundations'
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- 'Computer Science'
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Tags:
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- 'Command Line'
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- 'Developer Tools'
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CatalogContent:
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- 'learn-c-plus-plus'
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- 'paths/computer-science'
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---
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A **macro** is a label defined in the source code that is replaced by its value by the preprocessor before compilation. Macros are initialized with the `#define` preprocessor command and can be undefined with the `#undef` command.
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There are two types of macros: [object](https://www.codecademy.com/resources/docs/cpp/objects)-like macros and [function](https://www.codecademy.com/resources/docs/cpp/functions)-like macros.
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## Object-Like Macros
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These macros are replaced by their value in the source code before compilation. Their primary purpose is to define constants to be used in the code.
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**Note**: Macro definitions are not followed by a semicolon `;`.
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### Example
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In the following example, `PI` is defined as an object-like macro:
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```cpp
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#include <iostream>
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using namespace std;
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#define PI 3.1416
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int main() {
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float radius = 3;
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float area;
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area = PI * radius * radius;
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cout << "Area is " << area;
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return 0;
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}
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```
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This example outputs the following:
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```shell
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Area is 28.2744
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```
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## Function-Like Macros
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These macros behave like functions, in that they take arguments that are used in the replaced code.
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**Note**: When defining a function-like macro, there cannot be a space between the macro name and the opening parenthesis.
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### Example
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In the following example, `AREA` is defined as a function-like macro. Note that other macros can be used in defining a subsequent macro. The inner macro is replaced by its value before the outer macro is replaced.
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```cpp
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#include <iostream>
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using namespace std;
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#define PI 3.1416
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#define AREA(r) r * r * PI
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int main() {
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float radius = 5;
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float result;
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result = AREA(radius);
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cout << "Area is " << result;
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return 0;
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}
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```
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This example outputs the following:
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```shell
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Area is 78.54
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```
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## Predefined Macros
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C++ has a number of predefined macros, including the following:
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- `__DATE__`: Current date formatted as `MMM DD YYYY`.
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- `__TIME__`: Current time formatted as `HH:MM:SS`.
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- `__FILE__`: Current filename.
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- `__LINE__`: Current line number.
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### Example
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The following example uses the predefined macros mentioned above:
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```cpp
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#include <iostream>
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using namespace std;
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int main() {
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char file[] = __FILE__;
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char date[] = __DATE__;
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char time[] = __TIME__;
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int line = __LINE__;
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cout << "File name: " << file << "\n";
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cout << "Date: " << date << "\n";
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cout << "Time: " << time << "\n";
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cout << "Line number: " << line << "\n";
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}
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```
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The output resembles the following:
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```shell
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File name: main.cpp
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Date: Jun 25 2022
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Time: 14:17:50
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Line number: 9
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```
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## Undefining a Macro
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Once defined, a macro can be undefined with the `#undef` command. Using the macro after that point will result in a compile error.
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### Example
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```cpp
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#include <iostream>
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using namespace std;
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#define TEST 1
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int main() {
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#ifdef TEST
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cout << "TEST defined\n";
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#else
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cout << "TEST undefined\n";
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#endif
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#undef TEST
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#ifdef TEST
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cout << "TEST defined\n";
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#else
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cout << "TEST undefined\n";
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#endif
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}
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```
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This results in the output:
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```shell
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TEST defined
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TEST undefined
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```

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