09. Control Statements

Control Statements

The statements that are used to control flow of execution of program are called as control statements.

Types of Control Statements :

  1. Selection Control Structures
  2. Loop Control Structures

Selection Control Structures :

The control structures that allows us to execute actions based on a condition, provided by us, are called as selection control structures.

Or, When the selected statements are executed based on some condition then selection control structures are used.

Types of Selection Control Structures :

  1. if....else
  2. switch...case
  3. '?:' (terniary)

if else selection control structure :

if else control structure is used, when we need to execute some tasks, based on the expression, that results zero or non-zero value.

Syntax of if :

int main() {

  // syntax :
  if(expression_resulting_zero_or_non_zero){
    // Note : Every non- zero number is treated as 1.
    // multiple expressions are allowed, seperated with commas.
  }

  // usage :
  if(0){
    // statements inside body of if, will not execute.
  }

  // usage :
  if(1){
    // statements inside body of if, will execute.
  }

  return 0;
}

Examples of if :

int main() {
  int login_status = 0;

  // only if.
  if(login_status != 1){
    printf("You are not allowed to access this system.\n");
  }  

  return 0;
}

int main() {
  int login_status = 0;

  //if with else
  if(login_status != 1){
    printf("You are not allowed to access this system.\n");
  }
  else {
    printf("Access granted..\n");
  }

  return 0;
}

int main() {
  int login_status = 0;

  //if without else
  if(login_status != 1){
    printf("You are not allowed to access this system.\n");
  }

  if(login_status == 1)
  {
    printf("Access granted..\n");
  }

  return 0;
}

int main() {
  int login_status = 0;

  //if with else
  if(login_status == 0){
    printf("You are not allowed to access this system.\n");
  }
  else if(login_status == 1){
    printf("Access granted..\n");
  }
  else{
    printf("We have secured our system, from this hack.\n");
    printf("Your identity have been reported to security agency.\n");
  }

  return 0;
}

Notable Points about if...else in above example:

  1. else is a reserved keyword that is optional to use with if.
  2. else is used to connect another if with previous if.

Multiple Expressions with if :

We could use multiple expressions with if :

Body of if is always evaluated as per the last value.

int main() {
  if(0, 1, 0){
    printf("I will not get printed.\n");
  }

  if(1, 0, 1){
    printf("I will get printed.\n");
  }

  return 0;
}

Expression is always evaluated from left to right because, rule of Associativity of comma is left to right.

int main() {

  if(printf("First\n"), printf("Second\n"), printf("Third\n")){
    printf("Let's have a look in depth.\n");
  }

  return 0;
}

Nested if

When if statements is used inside body of another if, it is called nested if.

int main() {
  if(1) {

    if(1){ // this is nested if
      printf("I am present inside body of if.\n");
    }

  }
}

Switch Case Control Structure :

switch case control structure is used, when we have possible combinations, prior to writing our c program.

Example : Cities of students, studying in a school.

Syntax :

int main() {

  switch(expression_with_some_value) {
    case value : statements_to_execute; break;
    case value : statements_to_execute; break;
    default:
      statements_to_execute
  }

  return 0;
}

Example :

int main() {

  switch(1){
    case 1: printf("Value is 1\n"); break;
    case 2: printf("Value is 2\n"); break;
    default :
      printf("Value does not match any above case.\n");
  }

  return 0;
}

Explanation of each part in switch case :

int main() {
  int x = 10;

  switch(x){
    case 1: printf("Hello");
  }

  // here x, is the value that will be used to match with every case.
  // os tries to match value of x with 1 and if both of them are found to be same, then printf("Hello"); will be executed.

  return 0;
}

int main() {
    int roll_no = 2;

    switch(roll_no){
      case 2 : printf("Student Name: Arnav\n"); break;
      // In above case break keyword is used to exit execution of switch case.
      // Means other cases after matching case will not be evaluated.
    }

  return 0;
}

int main() {
  int record_no = 10;

  switch(record_no){
    case 1: printf("Name: Rahul, Salary : 1000\n"); break;
    case 2: printf("Name: Amit, Salary : 1000\n"); break;
    case 3: printf("Name: Suraj, Salary : 1000\n"); break;
    default :
      printf("Sorry, record not found.\n");
  }

  return 0;
}

break is used to exit out of switch body.

statements associated with default are executed only if, any of the above cases do not match.

Ternary Control Structure :

Ternary control structure is used, in place of if..else, In fact, it is short form of if..else. It is ternary operator.

int main() {
  int marks = 10;

  int interview_eligibility = (marks > 60) ? 1 : 0;

  if(interview_eligibility)
    printf("Please visit administrator block at 6, for interview.\n");
  else
    printf("You are not eligible for interview.\n");

  return 0;
}