loops_and_conditional_statements.md

loops and conditional statements

The 'C' programming language provides the conditional statements if, else and else if to perform various functions. For ex: run the program until this condition satisfy and exit if it does not. This would be the case for the algorithms. They always check for conditions that satisfy and execute and proceed to the next stages.

The if conditional checks for a truth statement. Any number other than 0 is true in C (Yes it means even the negative numbers).

The if condition looks like the below.

 if (condition) {
    // execute staement(s)
 }

if the condition is evaluated to true, then the statements inside the if are executed. Otherwise the statements enclosed in the if are not going to be run.

The otherwise part we call it the else conditional. The else conditional executed if the conditional in the if statement is not executed. Very simple.

The else condition looks like the below.

 if (condition) {
     // execute code path 1
 } else {
     // execute code path 2
 }

Ex:

  int a = -1;
  
  if (a) {
      printf("a value is %d\n", a);
  } else {
      printf("a value is zero\n");
  }

Remember that the else conditional always follow the if.

The compiler always give an error if the else conditional does not follow the if.

If there are more than one conditions to be executed based on some truth conditionals, we use else if.

The else if conditional looks like the below.

    if (condition1) {
        // execute conde path 1
    } else if (condition2) {
        // execute code path 2
    } else {
        // execute code path 3
    }

Ex:

     int a = 2;
    
     if (a == 1) {
         printf("a value is %d\n", a);
     } else if (a == 2) {
         printf("a value is %d\n", a);
     }

An else will always be at the last of the if else if conditional.

A series of if and else if conditional statements is also called an else if ladder.

For ex: an else if ladder would look as below.

if (fruit_name == FRUIT_APPLE) {
    printf("Apple\n");
} else if (fruit_name == FRUIT_ORANGE) {
    printf("Orange\n");
} else if (fruit_name == FRUIT_PINEAPPLE) {
    printf("Pine Apple\n");
} else if (fruit_name == FRUIT_GRAPE) {
    printf("Grape\n");
} else {
    printf("Noooo .. i dont have any Fruits\n");
}

Always make sure that an else statement is always present in an if.. else if conditional. This allows not to miss any case that does not satisfy either if or else if. Most of the software bugs lie around the if, else if and else parts.

Sometimes, the else if ladder is replaced with a switch statement. The switch statement is often and mostly used for data elements of type int or char. On the otherhand, switch can't be used with float, double or strings. The compiler cries if any of them are being used in a switch.

For ex: the switch statement converted fruit example looks as below.

switch (fruit_name) {
	case FRUIT_APPLE:
       printf("Apple\n");
   break;
   case FRUIT_ORANGE:
       printf("Orange\n");
   break;
   case FRUIT_PINEAPPLE:
       printf("Pine Apple\n");
   break;
   case FRUIT_GRAPE:
       printf("Grape\n");
   break;
   default:
       printf("Noooo .. i dont have any Fruits\n");
}

With in the switch statement, a series of case blocks with the default block can be used. Consider the case blocks as the if conditionals followed by the break statement causing the next case statement NOT to execute. If there is no break statement, the next conditions may execute and the result will be entirely different. We will see about this in the following paragraphs. The default statement is like an else conditional. If any of the case statements does not satisfy the condition given in the switch the default statement gets executed.

The conditionals statements are very important in the coding.

There are cases where the switch statement is not always if .. else if. Consider the following example from the stackoverflow website here

There comes a situation where you need to loop until some condition is evaluated to true. You can do with the while and for statements. There is also do .. while statement. We will see about these below.

The while statement loops till the condition evaluates to false. It looks like the following..

while (condition) {
    // series of statements    
}

When the condition fails, the while conditional will not get executed.

The series of statements are evaulated again and again till the condition becomes invalid.

For ex: let us consider a case where we print 10 number from 1.

int i = 1;

while (i <= 10) {
     printf("number %d\n", i);
     i ++;
}

The i value before the while is 1 and the loop starts printing the numbers from 1 till 10 and when i becomes 11 the condition evaulates to false and the loop stops.

The for is similar but it has the following advantages.

for statement has initializer, condition evaluation and incrementer.

 for (init; cond; increment) {
 }

For ex: the while loop is rewritten as

 int i;
 
 for (i = 1; i <= 10; i ++) {
     printf("i value %d\n", i);
 }

The initialiser sets the value of the variable to 1 and it gets executed only once. The condition then executes and if condition becomes satisfied the statements with in the for loop are executed. After the last statement is executed, the increment part of the for loop is executed and then the condition with in the for loop. The initializer will only be executed in the beginning.

There can be cases where one needs to run the program forever. This forever programs we call infinite loops.

The infinite loops can be written as the following:

 while (1) {
    // statements;
 }
 
 for (;;) {
    // statements;
 }

An infinite program, when ran, can be stopped via signalling it by the ctrl + c combination from the Keyboard. We look at the signals more in the upcoming topics.

Infinite loops cause the program to consume all of the CPU causing the system load go high and the temperature of the system. Often, the daemons use the infinite loops in a controlled way to make the program run forever. The controlled way avoids the heavy CPU loads.