C Program to Convert A String From Lower TO Upper Letters And Vice-Versa

/* C Program to Convert A String From Lower TO Upper Letters And Vice-Versa */

#include< stdio.h>
#include< ctype.h>
#include< conio.h>
void upper(char instr[])
{
int i=0;
while(instr[i])
{
instr[i]=toupper(instr[i]);
i++;
}
}

void lower(char instr[])
{
int i=0;
while(instr[i])
{
instr[i]=tolower(instr[i]);
i++;
}
}

void main()
{
char instr[100],inchar;
clrscr();
printf("input the string\n");
gets(instr);
printf("To upper-or-lower case(u/l)?\n");
scanf("%c",&inchar);

switch(inchar)
{
case 'u':
case 'U':
upper(instr);
break;
case 'l':
case 'L':
lower(instr);
break;
default:
printf("invalid");

}

printf("the converted string is...\n");
printf("%s\n",instr);
getch();
}