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The C Language Basics - C Interview Questions and Answers

1. 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:


#include <stdio.h>
void main(void);
void main()
{
     /* Begin local block for function main() */
     int test_var = 10;
     printf("Test variable before the if statement: %d\n", test_var);
     if (test_var > 5)
     {
          /* Begin local block for "if" statement */
          int test_var = 5;
          printf("Test variable within the if statement: %d\n",
                 test_var);
          {
               /* Begin independent local block (not tied to
                  any function or keyword) */
               int test_var = 0;
               printf(
               "Test variable within the independent local block:%d\n",
               test_var);
          }
          /* End independent local block */
     }
     /* End local block for "if" statement */
     printf("Test variable after the if statement: %d\n", 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.

2. 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.

3. When is a switch statement better than multiple if statements?

A switch statement is generally best to use when you have more than two conditional expressions based on a single variable of numeric type. For instance, rather than the code

if (x == 1)
     printf("x is equal to one.\n");
else if (x == 2)
     printf("x is equal to two.\n");
else if (x == 3)
     printf("x is equal to three.\n");
else
     printf("x is not equal to one, two, or three.\n");

the following code is easier to read and maintain:

switch (x)
{
     case 1:   printf("x is equal to one.\n");
                    break;
     case 2:   printf("x is equal to two.\n");
                    break;
     case 3:   printf("x is equal to three.\n");
                    break;
     default:  printf("x is not equal to one, two, or three.\n");
                    break;
}

Notice that for this method to work, the conditional expression must be based on a variable of numeric type in order to use the switch statement. Also, the conditional expression must be based on a single variable. For instance, even though the following if statement contains more than two conditions, it is not a candidate for using a switch statement because it is based on string comparisons and not numeric comparisons:

char* name = "Lupto";
if (!stricmp(name, "Isaac"))
     printf("Your name means 'Laughter'.\n");
else if (!stricmp(name, "Amy"))
     printf("Your name means 'Beloved'.\n ");
else if (!stricmp(name, "Lloyd"))
     printf("Your name means 'Mysterious'.\n ");
else
     printf("I haven't a clue as to what your name means.\n");

4. Is a default case necessary in a switch statement?

No, but it is not a bad idea to put default statements in switch statements for error- or logic-checking purposes. For instance, the following switch statement is perfectly normal:

switch (char_code)
{
     case 'Y':
     case 'y': printf("You answered YES!\n");
               break;
     case 'N':
     case 'n': printf("You answered NO!\n");
               break;
}

Consider, however, what would happen if an unknown character code were passed to this switch statement. The program would not print anything. It would be a good idea, therefore, to insert a default case where this condition would be taken care of:

...
     default:  printf("Unknown response: %d\n", char_code);
               break;
...

Additionally, default cases come in handy for logic checking. For instance, if your switch statement handled a fixed number of conditions and you considered any value outside those conditions to be a logic error, you could insert a default case which would flag that condition. Consider the following example:

void move_cursor(int direction)
{
     switch (direction)
     {
          case UP:     cursor_up();
                       break;
          case DOWN:   cursor_down();
                       break;
          case LEFT:   cursor_left();
                       break;
          case RIGHT:  cursor_right();
                       break;
          default:     printf("Logic error on line number %ld!!!\n",
                               __LINE__);
                       break;
     }
}


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