Question:Write a c program for finding gcd (greatest common divisor) of two given numbers 

#include<stdio.h>
 
int main(){

    int x,y,m,i;

    printf("Insert any two number: ");

    scanf("%d%d",&x,&y);
    if(x>y)
         m=y;
    else
         m=x;

    for(i=m;i>=1;i--){
         if(x%i==0&&y%i==0){
             printf("\nHCF of two number is : %d",i) ;
             break;
         }
    }
    return 0;
}
Alternate Solution:
#include<stdio.h>
int main(){
int n1,n2;
printf("\nEnter two numbers:");
scanf("%d %d",&n1,&n2);
while(n1!=n2){
if(n1>=n2-1)
n1=n1-n2;
else
n2=n2-n1;
}
printf("\nGCD=%d",n1);
return 0;
}

Question:HCF  program with multiple numbers in c 

#include<stdio.h>
int main(){
    int x,y=-1;
    printf("Insert numbers. To exit insert zero: ");
   
    while(1){
         scanf("%d",&x);
         if(x<1)
             break;
         else if(y==-1)
             y=x;
         else if (x<y)
             y=gcd(x,y);
         else
             y=gcd(y,x);
    }

    printf("GCD is %d",y);
   
    return 0;
}

int gcd(int x,int y){
    int i;
    for(i=x;i>=1;i--){
         if(x%i==0&&y%i==0){
             break;
         }
    }
    return i;
}

Question:How to swap two numbers in c without using third variable 

#include<stdio.h>

int main(){

    int a=5,b=10;

    //process one
    a=b+a;
    b=a-b;
    a=a-b;
    printf("a= %d  b=  %d",a,b);

    //process two
    a=5;b=10;
    a=a+b-(b=a);
    printf("\na= %d  b=  %d",a,b);

    //process three
    a=5;b=10;
    a=a^b;
    b=a^b;
    a=b^a;
    printf("\na= %d  b=  %d",a,b);

    //process four
    a=5;b=10;
    a=b-~a-1;
    b=a+~b+1;
    a=a+~b+1;
    printf("\na= %d  b=  %d",a,b);

    //process five
    a=5,b=10;
    a=b+a,b=a-b,a=a-b;
    printf("\na= %d  b=  %d",a,b);

    return 0;
}

Question:Write a c program for swapping of two arrays 

#include<stdio.h>
int main(){
  int a[10],b[10],c[10],i;
  printf("Enter First array->");
  for(i=0;i<10;i++)
  scanf("%d",&a[i]);
  printf("\nEnter Second array->");
  for(i=0;i<10;i++)
            scanf("%d",&b[i]);
  printf("Arrays before swapping");
  printf("\nFirst array->");
  for(i=0;i<10;i++){
            printf("%d",a[i]);
  }
  printf("\nSecond array->");
  for(i=0;i<10;i++){
            printf("%d",b[i]);
  }
  for(i=0;i<10;i++){
            //write any swapping technique
            c[i]=a[i];
            a[i]=b[i];
            b[i]=c[i];
  }
  printf("\nArrays after swapping");
  printf("\nFirst array->");
  for(i=0;i<10;i++){
            printf("%d",a[i]);
  }
  printf("\nSecond array->");
  for(i=0;i<10;i++){
            printf("%d",b[i]);
  }
  return 0;
}

Question:Swapping of strings using c programming language 

#include<stdio.h>
int main(){
  int i=0,j=0,k=0;
  char str1[20],str2[20],temp[20];
  puts("Enter first string");
  gets(str1);
  puts("Enter second string");
  gets(str2);
  printf("Before swaping the strings are\n");
  puts(str1);
  puts(str2);
  while(str1[i]!='\0'){
             temp[j++]=str1[i++];
  }
  temp[j]='\0';
  i=0,j=0;
  while(str2[i]!='\0'){
              str1[j++]=str2[i++];
  }
  str1[j]='\0';
  i=0,j=0;
  while(temp[i]!='\0'){
              str2[j++]=temp[i++];
  }
  str2[j]='\0';
  printf("After swaping the strings are\n");
  puts(str1);
  puts(str2);
  return 0;
}

Question:Decimal to binary conversion in c programming language. 

#include<stdio.h>

int main(){

    long int decimalNumber,remainder,quotient;

    int binaryNumber[100],i=1,j;

    printf("Enter any decimal number: ");

    scanf("%ld",&decimalNumber);

    quotient = decimalNumber;

    while(quotient!=0){

         binaryNumber[i++]= quotient % 2;

         quotient = quotient / 2;
    }

    printf("Equivalent binary value of decimal number %d: ",decimalNumber);

    for(j = i -1 ;j> 0;j--)

         printf("%d",binaryNumber[j]);

    return 0;
}
Sample output:
Enter any decimal number: 50
Equivalent binary value of decimal number 50: 110010

Question:C code for decimal to octal converter 

#include<stdio.h>

int main(){

  long int decimalNumber,remainder,quotient;
  int octalNumber[100],i=1,j;

  printf("Enter any decimal number: ");
  scanf("%ld",&decimalNumber);

  quotient = decimalNumber;

  while(quotient!=0){
      octalNumber[i++]= quotient % 8;
      quotient = quotient / 8;
  }

  printf("Equivalent octal value of decimal number %d: ",decimalNumber);
  for(j = i -1 ;j> 0;j--)
      printf("%d",octalNumber[j]);

  return 0;
}
Sample output:
Enter any decimal number: 50
Equivalent octal value of decimal number 50: 62

Question:Easy way to convert decimal number to octal number in c 

#include<stdio.h>
int main(){

  long int decimalNumber;

  printf("Enter any decimal number : ");
  scanf("%d",&decimalNumber);

  printf("Equivalent octal number is: %o",decimalNumber);

  return 0;
}
Sample output:
Enter any decimal number: 25
Equivalent octal number is: 31

Question:C code to convert decimal to hexadecimal 

#include<stdio.h>
int main(){
    long int decimalNumber,remainder,quotient;
    int i=1,j,temp;
    char hexadecimalNumber[100];

    printf("Enter any decimal number: ");
    scanf("%ld",&decimalNumber);

    quotient = decimalNumber;

    while(quotient!=0){
         temp = quotient % 16;

      //To convert integer into character
      if( temp < 10)
           temp =temp + 48;
      else
         temp = temp + 55;

      hexadecimalNumber[i++]= temp;
      quotient = quotient / 16;
  }

    printf("Equivalent hexadecimal value of decimal number %d: ",decimalNumber);
    for(j = i -1 ;j> 0;j--)
      printf("%c",hexadecimalNumber[j]);

    return 0;
}
Sample output:
Enter any decimal number: 45
Equivalent hexadecimal value of decimal number 45: 2D

Question:Easy way to convert decimal number to hexadecimal number 

#include<stdio.h>
int main(){

  long int decimalNumber;

  printf("Enter any decimal number: ");
  scanf("%d",&decimalNumber);

  printf("Equivalent hexadecimal number is: %X",decimalNumber);

  return 0;
}
Sample output:
Enter any decimal number: 45
Equivalent hexadecimal number is: 2D