#include<stdio.h>

int main(){

    int number,i=0,j,digit;
    char * word[1000];

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

    while(number){

    digit = number %10;
    number = number /10;

         switch(digit){
             case 0: word[i++] = "zero"; break;
             case 1: word[i++] = "one"; break;
             case 2: word[i++] = "two"; break;
             case 3: word[i++] = "three"; break;
             case 4: word[i++] = "four"; break;
             case 5: word[i++] = "five"; break;
             case 6: word[i++] = "six"; break;
             case 7: word[i++] = "seven"; break;
             case 8: word[i++] = "eight"; break;
             case 9: word[i++] = "nine"; break;

         }
    }
   
    for(j=i-1;j>=0;j--){
         printf("%s ",word[j]);
    }

    return 0;

}
Sample output:
Enter any integer: 23451208
two three four five one two zero eight

#include<stdio.h>
#include<string.h>

void toWord(int,int);
char * getPositionValue(int);
char * digitToWord(int);

char  word[100][30];
int i =0;

int main(){

    int j,k,subnumer;
    unsigned long int number;

    printf("Enter any postive number: ");
    scanf("%lu",&number);
   
    if(number ==0){
         printf("Zero");
         return 0;
    }

    while(number){

         if(i==1){
             toWord(number %10,i);
             number = number/10;
         }else{
             toWord(number %100,i);
             number = number/100;
         }

         i++;
        
    }

    printf("Number in word: ");
    *word[i-1] = *word[i-1] - 32;
    for(j=i-1;j>=0;j--){
         printf("%s",word[j]);
    }

    return 0;

}

void toWord(int number,int position){

    char  numberToword[100]={" "};

    if(number ==0){
    }else if (number < 20 ||number %10==0){
         strcpy(numberToword,digitToWord(number));
    }else{
         strcpy(numberToword,digitToWord((number/10)*10));
         strcat(numberToword,digitToWord(number%10));
    }
   
    strcat(numberToword,getPositionValue(position));
    strcpy(word[i],numberToword);
}

char * getPositionValue(int postion){

    static char positionValue[10]=" ";
   
    switch(postion){

         case 1: strcpy(positionValue,"hundreds "); break;
         case 2: strcpy(positionValue,"thousand "); break;
         case 3: strcpy(positionValue,"lakh "); break;
         case 4: strcpy(positionValue,"crore "); break;
         case 5: strcpy(positionValue,"arab "); break;
         case 6: strcpy(positionValue,"kharab "); break;
         case 7: strcpy(positionValue,"neel "); break;
         case 8: strcpy(positionValue,"padam "); break;
    }
    
    return positionValue;
}

char * digitToWord(int digit){

     static char digitInWord[10]=" ";

    switch(digit){
         case 1: strcpy(digitInWord , "one "); break;
         case 2: strcpy(digitInWord , "two "); break;
         case 3: strcpy(digitInWord , "three "); break;
         case 4: strcpy(digitInWord , "four "); break;
         case 5: strcpy(digitInWord , "five "); break;
         case 6: strcpy(digitInWord , "six "); break;
         case 7: strcpy(digitInWord , "seven "); break;
         case 8: strcpy(digitInWord , "eight "); break;
         case 9: strcpy(digitInWord , "nine ");break;
         case 10: strcpy(digitInWord , "ten "); break;
         case 11: strcpy(digitInWord , "eleven "); break;
         case 12: strcpy(digitInWord , "twelve "); break;
         case 13: strcpy(digitInWord , "thirteen "); break;
         case 14: strcpy(digitInWord , "fourteen "); break;
         case 15: strcpy(digitInWord , "fifteen "); break;
         case 16: strcpy(digitInWord , "sixteen "); break;
         case 17: strcpy(digitInWord , "seventeen "); break;
         case 18: strcpy(digitInWord , "eighteen "); break;
         case 19: strcpy(digitInWord , "nineteen "); break;
         case 20: strcpy(digitInWord , "twenty "); break;
         case 30: strcpy(digitInWord , "thirty "); break;
         case 40: strcpy(digitInWord , "fourty "); break;
         case 50: strcpy(digitInWord , "fifty "); break;
         case 60: strcpy(digitInWord , "sixty "); break;
         case 70: strcpy(digitInWord , "seventy "); break;
         case 80: strcpy(digitInWord , "eighty "); break;
         case 90: strcpy(digitInWord,"ninety "); break;
    }
   
    return digitInWord;
}

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

int main(){
   
    char fromUnit,toUnit;
    char *fUnit,*tUnit;
    long double fromValue,meterValue,toValue;
    int power =0;

    printf("To convert the 12 Inch to Foot\n");
    printf("enter the the unit in the format : dc12\n");

    printf("Ell: a\n");
    printf("Femi: b\n");
    printf("Foot: c\n");
    printf("Inch: d\n");
    printf("Light year: e\n");
    printf("Metre: f\n");
    printf("Mile: g\n");
    printf("Nano meter: h\n");
    printf("Pace: i\n");
    printf("Point: j\n");
    printf("Yard: k\n");
    printf("Mili meter: l\n");
    printf("Centi meter: m\n");
    printf("Deci meter: n\n");
    printf("Deca meter: o\n");
    printf("Hecto meter: p\n");
    printf("Kilo meter: q\n");

    scanf("%c%c%Lf",&fromUnit,&toUnit,&fromValue); 

    switch(fromUnit){
         case 'a': meterValue = fromValue * 1.143; fUnit="ell"; break;
         case 'b': meterValue = fromValue ; power = -15; fUnit="fm"; break;
         case 'c': meterValue = fromValue * 0.3048; fUnit="ft"; break;
         case 'd': meterValue = fromValue * 0.0254; fUnit="in"; break;
         case 'e': meterValue = fromValue * 9.4607304725808; power =15; fUnit="ly"; break;
         case 'f': meterValue = fromValue; fUnit="m"; break;
         case 'g': meterValue = fromValue * 1609.344; fUnit="mi"; break;
         case 'h': meterValue = fromValue; fUnit="nm"; power = -9; break;
         case 'i': meterValue = fromValue * 0.762 ; fUnit="pace"; break;
         case 'j': meterValue = fromValue * 0.000351450; fUnit="pt"; break;
         case 'k': meterValue = fromValue * 0.9144; fUnit="yd"; break;
         case 'l': meterValue = fromValue * 0.001; fUnit="mm"; break;
         case 'm': meterValue = fromValue * 0.01; fUnit="cm"; break;
         case 'n': meterValue = fromValue * 0.1; fUnit="deci meter"; break;
         case 'o': meterValue = fromValue * 10; fUnit="deca meter"; break;
         case 'p': meterValue = fromValue * 100; fUnit="hm"; break;
         case 'q': meterValue = fromValue * 1000; fUnit="km"; break;
         default:
             printf("Invalid input"); exit(0);
    }

    switch(toUnit){
         case 'a': toValue = meterValue/1.143; tUnit="ell"; break;
         case 'b': toValue = meterValue; tUnit="fm"; break;
         case 'c': toValue = meterValue/0.3048; tUnit="ft"; break;
         case 'd': toValue = meterValue/0.0254; tUnit="in"; break;
         case 'e': toValue = meterValue/9.4607304725808; tUnit="ly"; break;
         case 'f': toValue = meterValue; tUnit="m";break;
         case 'g': toValue = meterValue/1609.344; tUnit="mi"; break;
         case 'h': toValue = meterValue; tUnit="nm"; break;
         case 'i': toValue = meterValue/0.762; tUnit="pace"; break;
         case 'j': toValue = meterValue/0.000351450; tUnit="pt"; break;
         case 'k': toValue = meterValue/0.9144; tUnit="yd"; break;
         case 'l': toValue = meterValue/0.001; tUnit="mm"; break;
         case 'm': toValue = meterValue/0.01; tUnit="cm"; break;
         case 'n': toValue = meterValue/0.1; tUnit="deci meter"; break;
         case 'o': toValue = meterValue/10; tUnit="deca meter"; break;
         case 'p': toValue = meterValue/100; tUnit="hm"; break;
         case 'q': toValue = meterValue/1000; tUnit="km"; break;
         default:
             printf("Invalid input"); exit(0);
    }
   
    if(power==0)
         printf("%.4Lf %s = %.4Lf %s",fromValue,fUnit,toValue,tUnit);
    else{
         while(tovalue > 10 
         printf("%.4Lf %s = %.4Lf*10^%d %s",fromValue,fUnit,toValue,power,tUnit);
    }


    return 0;
}

#include<stdio.h>
#include<string.h>
int main(){
  char str[20];
  int i;
  printf("Enter any string->");
  scanf("%s",str);
  printf("The string is->%s",str);
  for(i=0;i<=strlen(str);i++){
      if(str[i]>=65&&str[i]<=90)
       str[i]=str[i]+32;
  }
  printf("\nThe string in lower case is->%s",str);
  return 0;
}

#include<stdio.h>
int main(){
  char str[20];
  int i;
  printf("Enter any string->");
  scanf("%s",str);
  printf("The string is->%s",str);
  for(i=0;i<=strlen(str);i++){
            if(str[i]>=97&&str[i]<=122)
            str[i]=str[i]-32;
  }
  printf("\nThe string in lowercase is->%s",str);
  return 0;
}