#include<stdio.h>
#include<stdlib.h>

// basic implementation
void underscorify(char* s) {
  char* p = s;
  while (*p != 0) {
    if (*p == ' ') {
      *p = '_';
    }
    p++;
  }
}

// how a developer might code it
void underscorify2(char* s) {
  char* p;
  for (p = s; *p ; p++) {
    if (*p == ' ') {
      *p = '_';
    }
  }
}

// how a kernel hacker might code it
void underscorify3(char* s) {
  for ( ; *s ; s++) {
    if (*s == ' ') *s = '_';
  }
}


void underscorify_bad(char* s) {
  char* p = s;
  while (*p != '0') {
    if (*p == 0) {
      *p = '_';
    }
    p++;
  }
}

void underscorify_bad2(char* s) {
  char* p = s;
  while (*p != '0') {
    if (*p == ' ') {
      *p = '_';
    }
    p++;
  }
}


int main() {
  char msg[] = "Here are words";
  puts(msg);
  underscorify_bad2(msg);
  puts(msg);
}