if (test expression){
       statement/s to be executed if test expression is true;
}

#include <stdio.h>
      int main(){
      int num;
      printf("Enter a number to check.\n");
      scanf("%d",&num);
      if(num<0)       /* checking whether number is less than 0 or not. */ 
            printf("Number=%d\n",num);   
/*If test condition is true, statement above will be executed, otherwise it will not be executed */
      printf("The if statement in C programming is easy.");
return 0;
}

Enter a number to check.
-2
Number=-2
The if statement in C programming is easy.

Enter a number to check.
5
The if statement in C programming is easy.

if (test expression)
     statements to be executed if test expression is true;
else
     statements to be executed if test expression is false;

int main(){
      int num;
      printf("Enter a number you want to check.\n");
      scanf("%d",&num);
      if((num%2)==0)          //checking whether remainder is 0 or not.
           printf("%d is even.",num);
      else
           printf("%d is odd.",num);
      return 0;
}

Enter a number you want to check.
25
25 is odd.

Enter a number you want to check.
2
2 is even.

if (test expression)
     statements to be executed if test expression is true;
else
     if(test expression 1)
          statements to be executed if test expressions 1 is true;
       else 
          if (test expression 2)
           .
           .
           .
            else
              statements to be executed if all test expressions are false;

#include <stdio.h>
int main(){ 
     int numb1, numb2;
     printf("Enter two integers to check.\n");
     scanf("%d %d",&numb1,&numb2); 
     if(numb1==numb2) //checking whether two integers are equal.
          printf("Result: %d=%d",numb1,numb2); 
     else 
        if(numb1>numb2) //checking whether numb1 is greater than numb2. 
          printf("Result: %d>%d",numb1,numb2); 
        else 
          printf("Result: %d>%d",numb2,numb1); 
return 0; 
}

Enter two integers to check.
5
3
Result: 5>3

Enter two integers to check.
-4
-4
Result: -4=-4

for(initial expression; test expression; update expression)
{
       code/s to be executed; 
}

#include <stdio.h>
int main(){
    int n, count, sum=0;
    printf("Enter the value of n.\n");
    scanf("%d",&n);
    for(count=1;count<=n;++count)  //for loop terminates if count>n
    {
        sum+=count;    /* this statement is equivalent to sum=sum+count */
    }
    printf("Sum=%d",sum);
    return 0;
}

Enter the value of n.
19
Sum=190

while (test expression)
{
     statements to be executed.  
}

 (n= 1*2*3*……*n) .

/*C program to demonstrate the working of while loop*/
#include <stdio.h>
     int main(){
     int number,factorial;
     printf("Enter a number.\n");
     scanf("%d",&number);
     factorial=1;
     while (number>0){      /* while loop continues util test condition number>0 is true */
           factorial=factorial*number;
           --number;
}
printf("Factorial=%d",factorial);
return 0;
}

Enter a number.
5
Factorial=120

do {
   some code/s;
}
while (test expression);

/*C program to demonstrate the working of do...while statement*/
#include <stdio.h>
int main(){
   int sum=0,num;
   do             /* Codes inside the body of do...while loops are at least executed once. */
   {                                    
        printf("Enter a number\n");
        scanf("%d",&num);
        sum+=num;      
   }
   while(num!=0);
   printf("sum=%d",sum);
return 0;
}

Enter a number
3
Enter a number
-2
Enter a number
0
sum=1

break;

/* C program to demonstrate the working of break statement by terminating a loop, if user inputs negative number*/
# include <stdio.h>
int main(){
   float num,average,sum;
   int i,n;
   printf("Maximum no. of inputs\n");
   scanf("%d",&n);
   for(i=1;i<=n;++i){
       printf("Enter n%d: ",i);
       scanf("%f",&num);
       if(num<0.0)
       break;                     //for loop breaks if num<0.0
       sum=sum+num;
}
  average=sum/(i-1);       
  printf("Average=%.2f",average);
  return 0;
}

Maximum no. of inputs
4
Enter n1: 1.5
Enter n2: 12.5
Enter n3: 7.2
Enter n4: -1
Average=7.07

continue;

//program to demonstrate the working of continue statement in C programming 
# include <stdio.h>
int main(){
    int i,num,product;
    for(i=1,product=1;i<=4;++i){
        printf("Enter num%d:",i);
        scanf("%d",&num);
        if(num==0)
            continue;  / *In this program, when num equals to zero, it skips the statement product*=num and continue the loop. */
        product*=num;
}
    printf("product=%d",product);
return 0;
}

Enter num1:3
Enter num2:0
Enter num3:-5
Enter num4:2
product=-30

switch (expression)
{
case constant1:
   codes to be executed if expression equals to constant1;
   break;
case constant2:
   codes to be executed if expression equals to constant3;
   break;
   .
   .
   . 
default:
   codes to be executed if expression doesn't match to any cases;
}

/* C program to demonstrate the working of switch...case statement */
/* Program to create a simple calculator for addition, subtraction, multiplication and division */
# include <stdio.h>
int main(){
     char operator;
     float num1,num2;
     printf("Enter operator +, - , * or / :\n");
     operator=getche();
     printf("\nEnter two operands:\n");
     scanf("%f%f",&num1,&num2);
     switch(operator)                              
     {
     case '+':
              printf("num1+num2=%.2f",num1+num2);
              break;
     case '-':
              printf("num1-num2=%.2f",num1-num2);
              break;
     case '*':
              printf("num1*num2=%.2f",num1*num2);
              break;
     case '/':
              printf("num2/num1=%.2f",num1/num2);
              break;
     default:                                 
/* if operator is other than +, -, * or /, error message is shown */
              printf(Error! operator is not correct");
              break;
     }
     return 0;
}

Enter operator +, -, * or / :
/
Enter two operators:
34
3
num2/num1=11.33

goto label;
.............
.............
.............
label: 
statement;

/* C program to demonstrate the working of goto statement.*/
# include <stdio.h>
int main(){
   float num,average,sum;
   int i,n;
   printf("Maximum no. of inputs: ");
   scanf("%d",&n);
   for(i=1;i<=n;++i){
       printf("Enter n%d: ",i);
       scanf("%f",&num);
       if(num<0.0)
       goto jump;             /* control of the program jumps to label jump */ 
       sum=sum+num;
}
jump:
  average=sum/(i-1);       
  printf("Average: %.2f",average);
  return 0;
}

Maximum no. of inputs: 4
Enter n1: 1.5
Enter n2: 12.5
Enter n3: 7.2
Enter n4: -1
Average: 7.07

main()

printf()

scanf()

#include <stdio.h>
void function_name(){
................
................
}
int main(){
...........
...........
function_name();
...........
...........
}

/*Program to demonstrate the working of user defined function*/
#include <stdio.h>
int add(int a, int b);           //function prototype(declaration)
int main(){
     int num1,num2,sum;
     printf("Enters two number to add\n");
     scanf("%d %d",&num1,&num2);
     sum=add(num1,num2);         //function call 
     printf("sum=%d",sum); 
     return 0;
}
int add(int a,int b)            //function declarator
{             
/* Start of function definition. */
     int add;
     add=a+b;
     return add;                  //return statement of function 
/* End of function definition. */   
}

return_type function_name(type(1) argument(1),....,type(n) argument(n));

 int add(int a, int b);

function_name(argument(1),....argument(n));

 add(num1,num2);

return_type function_name(type(1) argument(1),..,type(n) argument(n))
{
                //body of function
}

return_type function_name(type(1) argument(1),....,type(n) argument(n))

 int add(int a,int b);

return (expression);
          OR
     return;

return;
return a;
return (a+b);