R3D57R34K R3D57R34K - 1 month ago 8
C Question

Read input into a dynamic sized int?

I need to read a huge number from the stdin, here is the code i have so far:

int main() {
int x;
int* n1;
scanf("%d", &x); // I get the number of digits the integer will have

n1 = malloc(x * sizeof(int)); // I resize the integer pointer
}


So my question is how can I read this possibly huge sized int from stdin?

Answer

the most reliable method of handling very large numbers is to use the 'bignum.c', which is available at: http://www3.cs.stonybrook.edu/~skiena/392/programs/bignum.c

the referenced code is a complete program,. but the functions and struct are free to use.

where is what you would find there:

/*  bignum.c
    Implementation of large integer arithmetic: addition, subtraction,
        multiplication, and division.

    begun: February 7, 2002
    by: Steven Skiena
*/

/*
Copyright 2003 by Steven S. Skiena; all rights reserved. 

Permission is granted for use in non-commerical applications
provided this copyright notice remains intact and unchanged.

This program appears in my book:

"Programming Challenges: The Programming Contest Training Manual"
by Steven Skiena and Miguel Revilla, Springer-Verlag, New York 2003.

See our website www.programming-challenges.com for additional information.

This book can be ordered from Amazon.com at

http://www.amazon.com/exec/obidos/ASIN/0387001638/thealgorithmrepo/

*/


#include <stdio.h>

#define MAXDIGITS   100     /* maximum length bignum */ 

#define PLUS        1       /* positive sign bit */
#define MINUS       -1      /* negative sign bit */

typedef struct {
        char digits[MAXDIGITS];         /* represent the number */
    int signbit;            /* 1 if positive, -1 if negative */ 
        int lastdigit;          /* index of high-order digit */
} bignum;


print_bignum(bignum *n)
{
    int i;

    if (n->signbit == MINUS) printf("- ");
    for (i=n->lastdigit; i>=0; i--)
        printf("%c",'0'+ n->digits[i]);

    printf("\n");
}

int_to_bignum(int s, bignum *n)
{
    int i;              /* counter */
    int t;              /* int to work with */

    if (s >= 0) n->signbit = PLUS;
    else n->signbit = MINUS;

    for (i=0; i<MAXDIGITS; i++) n->digits[i] = (char) 0;

    n->lastdigit = -1;

    t = abs(s);

    while (t > 0) {
        n->lastdigit ++;
        n->digits[ n->lastdigit ] = (t % 10);
        t = t / 10;
    }

    if (s == 0) n->lastdigit = 0;
}

initialize_bignum(bignum *n)
{
    int_to_bignum(0,n);
}


int max(int a, int b)
{
    if (a > b) return(a); else return(b);
}

/*  c = a +-/* b;   */

add_bignum(bignum *a, bignum *b, bignum *c)
{
    int carry;          /* carry digit */
    int i;              /* counter */

    initialize_bignum(c);

    if (a->signbit == b->signbit) c->signbit = a->signbit;
    else {
        if (a->signbit == MINUS) {
            a->signbit = PLUS;
            subtract_bignum(b,a,c);
            a->signbit = MINUS;
        } else {
                        b->signbit = PLUS;
                        subtract_bignum(a,b,c);
                        b->signbit = MINUS;
        }
        return;
    }

    c->lastdigit = max(a->lastdigit,b->lastdigit)+1;
    carry = 0;

    for (i=0; i<=(c->lastdigit); i++) {
        c->digits[i] = (char) (carry+a->digits[i]+b->digits[i]) % 10;
        carry = (carry + a->digits[i] + b->digits[i]) / 10;
    }

    zero_justify(c);
}


subtract_bignum(bignum *a, bignum *b, bignum *c)
{
    int borrow;         /* has anything been borrowed? */
    int v;              /* placeholder digit */
    int i;              /* counter */

    initialize_bignum(c);

    if ((a->signbit == MINUS) || (b->signbit == MINUS)) {
                b->signbit = -1 * b->signbit;
                add_bignum(a,b,c);
                b->signbit = -1 * b->signbit;
        return;
        }

    if (compare_bignum(a,b) == PLUS) {
        subtract_bignum(b,a,c);
        c->signbit = MINUS;
        return;
    }

        c->lastdigit = max(a->lastdigit,b->lastdigit);
        borrow = 0;

        for (i=0; i<=(c->lastdigit); i++) {
        v = (a->digits[i] - borrow - b->digits[i]);
        if (a->digits[i] > 0)
            borrow = 0;
        if (v < 0) {
            v = v + 10;
            borrow = 1;
        }

                c->digits[i] = (char) v % 10;
        }

    zero_justify(c);
}

compare_bignum(bignum *a, bignum *b)
{
    int i;              /* counter */

    if ((a->signbit == MINUS) && (b->signbit == PLUS)) return(PLUS);
    if ((a->signbit == PLUS) && (b->signbit == MINUS)) return(MINUS);

    if (b->lastdigit > a->lastdigit) return (PLUS * a->signbit);
    if (a->lastdigit > b->lastdigit) return (MINUS * a->signbit);

    for (i = a->lastdigit; i>=0; i--) {
        if (a->digits[i] > b->digits[i]) return(MINUS * a->signbit);
        if (b->digits[i] > a->digits[i]) return(PLUS * a->signbit);
    }

    return(0);
}

zero_justify(bignum *n)
{
    while ((n->lastdigit > 0) && (n->digits[ n->lastdigit ] == 0))
        n->lastdigit --;

        if ((n->lastdigit == 0) && (n->digits[0] == 0))
        n->signbit = PLUS;  /* hack to avoid -0 */
}


digit_shift(bignum *n, int d)       /* multiply n by 10^d */
{
    int i;              /* counter */

    if ((n->lastdigit == 0) && (n->digits[0] == 0)) return;

    for (i=n->lastdigit; i>=0; i--)
        n->digits[i+d] = n->digits[i];

    for (i=0; i<d; i++) n->digits[i] = 0;

    n->lastdigit = n->lastdigit + d;
}



multiply_bignum(bignum *a, bignum *b, bignum *c)
{
    bignum row;         /* represent shifted row */
    bignum tmp;         /* placeholder bignum */
    int i,j;            /* counters */

    initialize_bignum(c);

    row = *a;

    for (i=0; i<=b->lastdigit; i++) {
        for (j=1; j<=b->digits[i]; j++) {
            add_bignum(c,&row,&tmp);
            *c = tmp;
        }
        digit_shift(&row,1);
    }

    c->signbit = a->signbit * b->signbit;

    zero_justify(c);
}


divide_bignum(bignum *a, bignum *b, bignum *c)
{
        bignum row;                     /* represent shifted row */
        bignum tmp;                     /* placeholder bignum */
    int asign, bsign;       /* temporary signs */
        int i,j;                        /* counters */

    initialize_bignum(c);

    c->signbit = a->signbit * b->signbit;

    asign = a->signbit;
    bsign = b->signbit;

    a->signbit = PLUS;
        b->signbit = PLUS;

    initialize_bignum(&row);
    initialize_bignum(&tmp);

    c->lastdigit = a->lastdigit;

    for (i=a->lastdigit; i>=0; i--) {
        digit_shift(&row,1);
        row.digits[0] = a->digits[i];
        c->digits[i] = 0;
        while (compare_bignum(&row,b) != PLUS) {
            c->digits[i] ++;
            subtract_bignum(&row,b,&tmp);
            row = tmp;
        }
    }

    zero_justify(c);

    a->signbit = asign;
    b->signbit = bsign;
}



main()
{
    int a,b;
    bignum n1,n2,n3,zero;

    while (scanf("%d %d\n",&a,&b) != EOF) {
        printf("a = %d    b = %d\n",a,b);
        int_to_bignum(a,&n1);
        int_to_bignum(b,&n2);

        add_bignum(&n1,&n2,&n3);
        printf("addition -- ");
        print_bignum(&n3);

        printf("compare_bignum a ? b = %d\n",compare_bignum(&n1, &n2));

        subtract_bignum(&n1,&n2,&n3);
        printf("subtraction -- ");
        print_bignum(&n3);

                multiply_bignum(&n1,&n2,&n3);
        printf("multiplication -- ");
                print_bignum(&n3);

        int_to_bignum(0,&zero);
        if (compare_bignum(&zero, &n2) == 0)
            printf("division -- NaN \n");
                else {
            divide_bignum(&n1,&n2,&n3);
            printf("division -- ");
                    print_bignum(&n3);
        }
        printf("--------------------------\n");
    }
}