Compound statement

Compound statement is a set of statements or no statements enclosed within set braces. It is also called as blocks.  Compounds statements are usually used in control flow statements(if, if-else, nested if, for, while, do-while etc).

Example 1: 
if (a < b) {
printf("a is less than b");
printf("I am inside compound statement\n");
}
There are two statements inside the if block which are compound statements.

Example 2: 
for (i = 0; i < 5; i++) {

}
There is no statement present inside the for loop.  It is also called as compound statement.

Example 3: 
for (i = 0; i < 5; i++) {
      int j = 10;  // variable declaration
}
Variable declaration is allowed inside blocks.  But, the scope(lifetime) of the variable is only inside the block where it is declared.

Example c program using compound statements

 
  #include <stdio.h>
  int main() {
        int i = 20;

        /* two statements inside if block */
        if (i > 10) {
                printf("Value of i is greater than 20\n");
                printf("Compound statement\n");
        }

        printf("\nBlock with empty statement!! - start\n");
        /* zero statements inside for loop */
        for (i = 0; i < 5; i++) {

        }
        printf("Block with empty statement!! - End\n\n");

        /* variable declaration inside for loop */
        for (i = 0; i < 5; i++) {
                int j = i;
                printf("Value of j is %d\n", j);
        }
        return 0;
  }

  Output:

  jp@jp-VirtualBox:~/$ ./a.out 
  Value of i is greater than 20
  Compound statement

  Block with empty statement!! - start
  Block with empty statement!! - End

  Value of j is 0
  Value of j is 1
  Value of j is 2
  Value of j is 3
  Value of j is 4

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