#include<stdio.h>
#define MAX 1000
int main(){
   
    char octalNumber[MAX];
    long int i=0;

    printf("Enter any octal number: ");
    scanf("%s",octalNumber);

    printf("Equivalent binary value: ");
    while(octalNumber[i]){
        switch(octalNumber[i]){
             case '0': printf("000"); break;
             case '1': printf("001"); break;
             case '2': printf("010"); break;
             case '3': printf("011"); break;
             case '4': printf("100"); break;
             case '5': printf("101"); break;
             case '6': printf("110"); break;
             case '7': printf("111"); break;
             default:  printf("\nInvalid octal digit %c ",octalNumber[i]); return 0;
    }
    i++;
 }

    return 0;
}
Sample output:
Enter any octal number: 123
Equivalent binary value: 001010011

#include<stdio.h>
#include<math.h>

int main(){
   
    long int octal,decimal =0;
    int i=0;

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

    while(octal!=0){
         decimal = decimal + (octal % 10) * pow(8,i++);
         octal = octal/10;
    }

    printf("Equivalent decimal value: %ld",decimal);

   return 0;
}
Sample output:
Enter any octal number: 346
Equivalent decimal value: 230

#include<stdio.h>

int main(){
   
    long int octalNumber;

    printf("Enter any octal number: ");
    scanf("%o",&octalNumber);

    printf("Equivalent decimal number is: %d",octalNumber);

    return 0;
}
Sample output:
Enter any octal number: 17
Equivalent decimal number is: 15

#include<stdio.h>
#define MAX 1000

int main(){
    char binaryNumber[MAX],hexaDecimal[MAX];
    long int i=0;

    printf("Enter any hexadecimal number: ");
    scanf("%s",hexaDecimal);

    printf("\nEquivalent binary value: ");
    while(hexaDecimal[i]){
         switch(hexaDecimal[i]){
             case '0': printf("0000"); break;
             case '1': printf("0001"); break;
             case '2': printf("0010"); break;
             case '3': printf("0011"); break;
             case '4': printf("0100"); break;
             case '5': printf("0101"); break;
             case '6': printf("0110"); break;
             case '7': printf("0111"); break;
             case '8': printf("1000"); break;
             case '9': printf("1001"); break;
             case 'A': printf("1010"); break;
             case 'B': printf("1011"); break;
             case 'C': printf("1100"); break;
             case 'D': printf("1101"); break;
             case 'E': printf("1110"); break;
             case 'F': printf("1111"); break;
             case 'a': printf("1010"); break;
             case 'b': printf("1011"); break;
             case 'c': printf("1100"); break;
             case 'd': printf("1101"); break;
             case 'e': printf("1110"); break;
             case 'f': printf("1111"); break;
             default:  printf("\nInvalid hexadecimal digit %c ",hexaDecimal[i]); return 0;
         }
         i++;
    }

    return 0;
}
Sample output:
Enter any hexadecimal number: 2AD5
Equivalent binary value: 0010101011010101

#include<stdio.h>
int main(){
   
    long int binaryNumber,octalNumber=0,j=1,remainder;

    printf("Enter any number any binary number: ");
    scanf("%ld",&binaryNumber);

    while(binaryNumber!=0){
         remainder=binaryNumber%10;
        octalNumber=octalNumber+remainder*j;
        j=j*2;
        binaryNumber=binaryNumber/10;
    }

    printf("Equivalent octal value: %lo",octalNumber);

    return 0;
}
Sample output:
Enter any number any binary number: 1101
Equivalent hexadecimal value: 15