Toggle case of string

/* TOGGLE CASE OF STRING */

#include<stdio.h>

int main()
{
 char str[100];
 int i;
 printf("Please enter a string: ");

 // gets(str);
 // fgets is a better option over gets to read multiword string .

 fgets(str, 100, stdin);

 // Following can be added for extra precaution for '\n' character
 // if(str[length(str)-1] == '\n') str[strlen(str)-1]=NULL;

 for(i=0;str[i]!=NULL;i++)
 {
  if(str[i]>='A'&&str[i]<='Z')
   str[i]+=32;
  else if(str[i]>='a'&&str[i]<='z')
   str[i]-=32;
 }

 printf("String in toggle case is: %s",str);

 return 0;
}