Exercise 1:

I would like to have a software with this output:

	Calculator 0.1 - GPL
	Write 'help' for the help menu, 'quit' for exit.
	Cmd: 

If the user writes 'help', this menu appears:

	help - this menu
	quit - exit
	a mathematical question like: 12 + 54

If the user writes 'quit', the software exists. If the user writes:

	Cmd: 45 - 2
	Output: 43
	Cmd:

The software must return the correct value of the mathematical operation. The mathematical operations are: +, -, * and /. I would like to have every single operation in a different functions. 

A bit of tip: if you want convert a string in a number you can use the 'atoi'. This is a little example:
---
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int
main (void)
{
  char buf[512];
  int numb;

  /* I put 123 in the buf string: */
  strcpy (buf, "123");
  numb = atoi (buf);

  printf ("%d\n", numb);
  return 0;
}
---
Another important tip is: the scanf can receive more variables in the same time like this code:
---
#include <stdio.h>

int
main (void)
{
  char buf[512];
  int numb;
  int ret;

  ret = scanf ("%s%d", buf, &numb);
  printf ("The user has inserted %d variables: %s %d\n", ret, buf, numb);

  return 0;
}
---


Exercise 2:

Our first library. I would like to have 4 new functions from our next softwares. These functions have these prototypes:

	int my_string_length(char *);
	int my_string_compare(char *, char *);
	int my_string_contains(char *, char);
	char *my_string_revert(char *);

my_string_length returns the length of the string. my_string_compare must be exactly like the strcmp function from the string.h. my_string_contains return 0 or 1 if the string (first argument) contains the specified character (second argument). The last function, my_string_revert returns the same string but reverted.

You can test these functions with a simple main function. I don't want these functions based on string.h header or other string functions!