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..
Содержание
Правила имен переменных в C
- имя переменной должно начинаться со знака подчеркивания или буквы;
- имена переменных чувствительны к регистру символов;
- имя переменной может включать в себя цифры и буквы;
- недопустимо использование специальных символов, за исключением знака подчеркивания;
- 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,
- Local variable
- Global variable
- 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.
1
|
#include<stdio.h>
|
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();
}
{
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.
1
|
#include<stdio.h>
|
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 All variables are accessed from test function |
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/”.
1
2
|
#include <stdio.h>
#include <stdlib.h>
|
{
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”
1
2
|
#include <stdio.h>
#include <stdlib.h>
|
{
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.
1
2
|
#include <stdio.h>
#include <stdlib.h>
|
{
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.