mix.c

#include <stdio.h>
/*-*/

char *mix(const char *);

int main(){
 char *p;
 p = "hello world";
 printf("%s\n", mix(p));
 return(0);
}

char *mix(const char *p){
 int c;
 char *r, *s;
 void *malloc();
 
 r = s = (char *)malloc(sizeof p);
 for(c = 0; *p != 0; c++, p++, r++){
  if(c%2 == 0){
   if(*p > 96 && *p < 123)
    *r = *p -32;
   else
    *r = *p;
  }else{
   if(*p < 64 && *p > 91)
    *r = *p +32;
   else
    *r = *p;
  }
 }
 *++r = 0;
 return(s);
}

OUTPUT:
HeLlO WoLrD