The C Language Basics Questions and Answers
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What is a local block?

A local block is any portion of a C program that is enclosed by the left brace ({) and the right brace (}). A C function contains left and right braces, and therefore anything between the two braces is contained in a local block. An if statement or a switch statement can also contain braces, so the portion of code between these two braces would be considered a local block.

Additionally, you might want to create your own local block without the aid of a C function or keyword construct. This is perfectly legal. Variables can be declared within local blocks, but they must be declared only at the beginning of a local block. Variables declared in this manner are visible only within the local block. Duplicate variable names declared within a local block take precedence over variables with the same name declared outside the local block. Here is an example of a program that uses local blocks:


void main(void);
void main()
     /* Begin local block for function main() */
     int test_var = 10;
     printf("Test variable before the if statement: %dn", test_var);
     if (test_var > 5)
          /* Begin local block for "if" statement */
          int test_var = 5;
          printf("Test variable within the if statement: %dn",
               /* Begin independent local block (not tied to
                  any function or keyword) */
               int test_var = 0;
               "Test variable within the independent local block:%dn",
          /* End independent local block */
     /* End local block for "if" statement */
     printf("Test variable after the if statement: %dn", test_var);
/* End local block for function main() */


This example program produces the following output:

Test variable before the if statement: 10

Test variable within the if statement: 5

Test variable within the independent local block: 0

Test variable after the if statement: 10

Notice that as each test_var was defined, it took precedence over the previously defined test_var. Also notice that when the if statement local block had ended, the program had reentered the scope of the original test_var, and its value was 10.



Should variables be stored in local blocks?


The use of local blocks for storing variables is unusual and therefore should be avoided, with only rare exceptions. One of these exceptions would be for debugging purposes, when you might want to declare a local instance of a global variable to test within your function. You also might want to use a local block when you want to make your program more readable in the current context.

Sometimes having the variable declared closer to where it is used makes your program more readable. However, well-written programs usually do not have to resort to declaring variables in this manner, and you should avoid using local blocks.


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