Курс C с нуля: часть 6 — переменные

 

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Предыдущая часть — Курс C с нуля: часть 5 – константы.

  • переменная с C — это именованный участок памяти, в котором программа может хранить и управлять данными;
  • значение переменной в C может меняться во время выполнения программы (в отличии от констант);
  • переменная может быть любого типа данных — int, float, char и т.д..

Правила имен переменных в C

  1. имя переменной должно начинаться со знака подчеркивания или буквы;
  2. имена переменных чувствительны к регистру символов;
  3. имя переменной может включать в себя цифры и буквы;
  4. недопустимо использование специальных символов, за исключением знака подчеркивания;
  5. sum, height, _value — вот несколько примеров имен для переменных.

Объявление и инициализация переменных

  • переменные должны быть объявлены перед использованием;
  • Memory space is not allocated for a variable while declaration. It happens only on variable definition.
  • Variable initialization means assigning a value to the variable.
S.No
 Type 
Syntax
Example
1
Variable declaration
data_type variable_name;
int x, y, z; char flat, ch;
2
Variable initialization
data_type variable_name = value;
int x = 50, y = 30; char flag = ‘x’, ch=’l’;

There are three types of variables in C program They are,

  1. Local variable
  2. Global variable
  3. Environment variable

1. Example program for local variable in C:

  • The scope of local variables will be within the function only.
  • These variables are declared within the function and can’t be accessed outside the function.
  • In the below example, m and n variables are having scope within the main function only. These are not visible to test function.
  • Like wise, a and b variables are having scope within the test function only. These are not visible to main function.
void test();
int main()
{
int m = 22, n = 44;
// m, n are local variables of main function
/*m and n variables are having scope
within this main function only.
These are not visible to test funtion.*/
/* If you try to access a and b in this function,
you will get ‘a’ undeclared and ‘b’ undeclared error */
printf(“nvalues : m = %d and n = %d”, m, n);
test();
}
void test()
{
int a = 50, b = 80;
// a, b are local variables of test function
/*a and b variables are having scope
within this test function only.
These are not visible to main function.*/
/* If you try to access m and n in this function,
you will get ‘m’ undeclared and ‘n’ undeclared
error */printf(“nvalues : a = %d and b = %d”, a, b);
}

Output:

values : m = 22 and n = 44
values : a = 50 and b = 80

2. Example program for global variable in C:

  • The scope of global variables will be throughout the program. These variables can be accessed from anywhere in the program.
  • This variable is defined outside the main function. So that, this variable is visible to main function and all other sub functions.
void test();int m = 22, n = 44;
int a = 50, b = 80;
int main()
{
printf(“All variables are accessed from main function”);
printf(“nvalues: m=%d:n=%d:a=%d:b=%d”, m,n,a,b);
test();
}
void test()
{
printf(“nnAll variables are accessed from”
” test function”);
printf(“nvalues: m=%d:n=%d:a=%d:b=%d”, m,n,a,b);
}

Output:

All variables are accessed from main function
values : m = 22 : n = 44 : a = 50 : b = 80

All variables are accessed from test function
values : m = 22 : n = 44 : a = 50 : b = 80

3. Environment variables in C:

  • Environment variable is a variable that will be available for all C  applications and C programs.
  • We can access these variables from anywhere in a C program without declaring and initializing in an application or C program.
  • The inbuilt functions which are used to access, modify and set these environment variables are called environment functions.
  • There are 3 functions which are used to access, modify and assign an environment variable in C. They are,

1. setenv()
2. getenv()
3. putenv()

Example program for getenv() function in C:

      This function gets the current value of the environment variable. Let us assume that environment variable DIR is assigned to “/usr/bin/test/”.

int main()
{
printf(“Directory = %sn”,getenv(“DIR”));
return 0;
}

Output:

/usr/bin/test/

 Example program for setenv() function in C:

      This function sets the value for environment variable. Let us assume that environment variable “FILE” is to be assigned “/usr/bin/example.c”

int main()
{
setenv(“FILE”, “/usr/bin/example.c”,50);
printf(“File = %sn”, getenv(“FILE”));
return 0;
}

Output:

File = /usr/bin/example.c

Example program for putenv() function in C:

      This function modifies the value for environment variable. Below example program shows that how to modify an existing environment variable value.

int main()
{
setenv(“DIR”, “/usr/bin/example/”,50);
printf(“Directory name before modifying = ”
“%sn”, getenv(“DIR”));
putenv(“DIR=/usr/home/”);
printf(“Directory name after modifying = ”
“%sn”, getenv(“DIR”));
return 0;
}

Output:

Directory name before modifying = /usr/bin/example/
Directory name after modifying = /usr/home/

Difference between variable declaration & definition in C:

 S.no 
 Variable declaration
Variable definition
1
Declaration tells the compiler about data type and size of the variable.
Definition allocates memory for the variable.
2
Variable can be declared many times in a program.
It can happen only one time for a variable in a program.
3
The assignment of properties and identification to a variable.
Assignments of storage space to a variable.

На этом, к сожалению — всё. Перевод старый, незавершённый и не сохранил источник.


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