10. More on Control Statements

Loop Control Structures :

Loop control structures are used to perform repetitive tasks.

Example : to print table for any user provided number.

Loops can be infinite, make sure to end loop at some point.

Types of Loop Control Structures :

1. while
2. do...while
3. for

while control structure :

while control structure is used to execute repetitive tasks, till the condition provided evaluates to non-zero

Syntax :

int main() {

  while(expressions_that_evaluates_to_zero_or_non_zero){
    // statements to execute.
  }

  return 0;
}

Example :

int main() {
  int number = 10;
  int index = 1;

  // loop to print table of any number.
  while(number <= 10){
    printf("%d x %d = %d\n", number, index, number * index);
    index++;
  }

  return 0;
}

int main() {
  while(1, 2, 0){
      printf("Try to predict the output!\n");
  }

  return 0;
}

do...while control structure :

do while control structure is used when we need to execute some task prior to checking condition and till the condition is met.

Syntax :

int main() {

  do{
    // statements to execute.
  }while(expression_that_results_zero_or_non_zero_value);

  return 0;
}

Example :

int main() {
  int user_choice = 0;

  do{

    printf("Choose a random number :\t");
    scanf("%d",&user_choice);

  }while(user_choice != rand() % 100);

  // rand() % 100 generates a random number between 0 and 100;

  return 0;
}

for Control Structure :

for control structure is allows us to define initiation and final condition along with operation in same line.

In older versions of C, initialization of variable is allowed only at top, but in ANSI C17, variable can be declared at time of usage.

Syntax :

int main() {

  for(initialization_expression; termination_expression; operation_expression){
    // statements to execute.
  }

  return 0;
}

Example :

int main() {

  // prints counting from 0 to 99.
  for(int i=0; i<100; i++){
    printf("%d\t",i);
  }

  return 0;
}

All three initiation, termination and operation section of for are optional.

for without initialization section :

int main() {
  int i=0;

  for(; i<100; i++){
    printf("%d\t",i);
  }

  return 0;
}

for without termination section :

int main() {

  for(int number = 0; ; number++){
    printf("%d\t", number);

    if(number >=99)
      break;
  }

  return 0;
}

for without operation section :

int main() {

  for(int i=0; i<100; ){
    printf("%d\t",i);
    i++;
  }

  return 0;
}

for without any section :

int main() {
  int i = 0;

  for(;;){
    if(i < 100)
      printf("%d\t",i++);
  }

  return 0;
}

Multiple expressions in for :

int main() {
  int i=0;

  for(printf("initialization\n"), i = 0; printf("Condition Check\n"), i<5; printf("Operation\n"), i++){
    printf("Inside body of if\n");
  }

  return 0;
}

The above program verifies that we could have multiple expressions in if.

the expressions are evaluated from left to right due to associativity of comma.

Working of for :

1. initialization Occurs
2. Termination Condition is Checked
3. Body is executed
4. Operation section is executed, means value is incremented or decremented.

int main() {
  int i=0;

  for(printf("initialization\n"), i = 0; printf("Condition Check\n"), i<5; printf("Operation\n"), i++){
    printf("Inside body of if\n");
  }

  return 0;
}