Simultaneous Integer Init & Assign

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5.3 Combined Initialization and Assignment Functions

For convenience, GMP provides a parallel series of initialize-and-set functions which initialize the output and then store the value there. These functions' names have the form mpz_init_set...

Here is an example of using one:

     {
       mpz_t pie;
       mpz_init_set_str (pie, "3141592653589793238462643383279502884", 10);
       ...
       mpz_sub (pie, ...);
       ...
       mpz_clear (pie);
     }

Once the integer has been initialized by any of the mpz_init_set... functions, it can be used as the source or destination operand for the ordinary integer functions. Don't use an initialize-and-set function on a variable already initialized!

— Function: void mpz_init_set (mpz_t rop, const mpz_t op)
— Function: void mpz_init_set_ui (mpz_t rop, unsigned long int op)
— Function: void mpz_init_set_si (mpz_t rop, signed long int op)
— Function: void mpz_init_set_d (mpz_t rop, double op)

Initialize rop with limb space and set the initial numeric value from op.

— Function: int mpz_init_set_str (mpz_t rop, const char *str, int base)

Initialize rop and set its value like mpz_set_str (see its documentation above for details).
If the string is a correct base base number, the function returns 0; if an error occurs it returns −1. rop is initialized even if an error occurs. (I.e., you have to call mpz_clear for it.)