字符串模拟大整数加法、大整数减法、大整数乘法

1 大数加法

#include<stdio.h>
#include<string.h>

int judge(char *num){
    char *p = num;
    int c = 0;
    while(*(p + c)){
        if(*(p + c) < '0' || *(p + c) > '9'){
            return -1;
        }
        c ++;
    }
    return c;
}

void reverse(char *p){
    int i;
    int l1 = strlen(p);
    for(i = 0; i < l1 / 2; i ++){
        char c = p[i];
        p[i] = p[l1 - 1 - i];
        p[l1 - 1 - i] = c;
    }
}

void add(char *num1, char *num2, char *sum)
{
    int i;
    reverse(num1);
    reverse(num2);
    char *p1 = num1;
    char *p2 = num2;
    int later = 0;
    char *p3 = sum;
    while(*p1 || *p2){
        int t = later;
        if(*p1){
            t += (*p1 - '0');
            p1 ++;
        }
        if(*p2){
            t += (*p2 - '0');
            p2 ++;
        }
        later = t / 10;
        *p3 = (t % 10) + '0';
        p3 ++;
    }
    if(later > 0){
        *p3 = (later + '0');
        p3 ++;
    }
    *p3 = '\0';
    reverse(sum);
}
int main(int argc, char *argv[]){
    char num1[10000];
    char num2[10000];
    scanf("%s", num1);
    scanf("%s", num2);
    printf("%s + %s = ?\n", num1, num2);
    int num1_len = judge(num1);
    if(num1_len < 0){
        printf("num1 error!\n");
        return -1;
    }
    int num2_len = judge(num2);
    if(num2_len < 0){
        printf("num2 error!\n");
        return -1;
    }
    char sum[10000];
    add(num1, num2, sum);
    printf("%s\n", sum);
    return 0;
}

2 大数减法

#include<stdio.h>
#include<string.h>

int judge(char *num){
    char *p = num;
    int c = 0;
    while(*(p + c)){
        if(*(p + c) < '0' || *(p + c) > '9'){
            return -1;
        }
        c ++;
    }
    return c;
}

void reverse(char *p){
    int i;
    int l1 = strlen(p);
    for(i = 0; i < l1 / 2; i ++){
        char c = p[i];
        p[i] = p[l1 - 1 - i];
        p[l1 - 1 - i] = c;
    }
}

void minus(char *num1, char *num2, char *res)
{
    int i;
    reverse(num1);
    reverse(num2);
    int num1_len = strlen(num1);
    int num2_len = strlen(num2);
    if(num1_len < num2_len){
        for(i = num1_len; i < num2_len; i ++){
            num1[i] = '0';
        }
    }else{
        for(i = num2_len; i < num1_len; i ++){
            num2[i] = '0';
        }
    }
    char *big = num1;
    char *small = num2;
    for(i = (num1_len > num2_len ? num1_len : num2_len) - 1; i >= 0; i ++){
        if(num1[i] < num2[i]){
            big = num2;
            small = num1;
            break;
        }else if(num1[i] > num2[i]){
            big = num1;
            small = num2;
            break;
        }
    }
    int token = 0;
    int a, b;
    for(i = 0; i < (num1_len > num2_len ? num1_len : num2_len); i ++){
        a = big[i] - '0' - token;
        token = 0;
        b = small[i] - '0';
        if(a < b){
            token = 1;
            a += 10;
        }
        res[i] = a - b + '0';
    }
    reverse(res);
}
int main(int argc, char *argv[]){
    char num1[10000];
    char num2[10000];
    memset(num1, 0x0, sizeof(num1));
    memset(num2, 0x0, sizeof(num2));
    scanf("%s", num1);
    scanf("%s", num2);
    printf("|%s - %s| = ?\n", num1, num2);
    int num1_len = judge(num1);
    if(num1_len < 0){
        printf("num1 error!\n");
        return -1;
    }
    int num2_len = judge(num2);
    if(num2_len < 0){
        printf("num2 error!\n");
        return -1;
    }
    char res[10000];
    minus(num1, num2, res);
    char *p = res;
    while(*p == '0'){
        p ++;
    }
    if(!*p){
        printf("0\n");
    }else{
        printf("%s\n", p);
    }
    return 0;
}

3 大数乘法

#include<stdio.h>
#include<string.h>

#define MAX_LEN 1000000

int judge(char *num){
    char *p = num;
    int c = 0;
    while(*(p + c)){
        if(*(p + c) < '0' || *(p + c) > '9'){
            return -1;
        }
        c ++;
    }
    return c;
}

void reverse(char *p){
    int i;
    int l1 = strlen(p);
    for(i = 0; i < l1 / 2; i ++){
        char c = p[i];
        p[i] = p[l1 - 1 - i];
        p[l1 - 1 - i] = c;
    }
}

void multi(char *num, char c, int I, char *res){
    int a;
    int i, ii;
    int later = 0;
    for(ii = 0; ii < I; ii ++){
        res[ii] = '0';
    }
    for(i = 0; i < strlen(num); i ++){
        a = (c - '0') * (num[i] - '0') + later;
        res[ii ++] = a % 10 + '0';
        later = a / 10;
    }
    while(later){
        res[ii ++] = later % 10 + '0';
        later /= 10;
    }
    res[ii] = '\0';
}

void add(char *num1, char *sum){
    char *p1 = num1;
    int later = 0;
    char *p3 = sum;
    while(*p1 || *p3){
        int t = later;
        if(*p1){
            t += (*p1 - '0');
            p1 ++;
        }
        if(*p3){
            t += (*p3 - '0');
        }
        later = t / 10;
        *p3 = (t % 10) + '0';
        p3 ++;
    }
    if(later > 0){
        *p3 = (later + '0');
        p3 ++;
    }
    *p3 = '\0';
}

char t[MAX_LEN];
void compute(char *num1, char *num2, char *sum)
{
    int i;
    reverse(num1);
    reverse(num2);
    for(i = 0; i < strlen(num2); i ++){
        memset(t, 0x0, sizeof(t));
        multi(num1, num2[i], i, t);
        add(t, sum);
    }
    reverse(sum);
}

char num1[MAX_LEN / 2 - 1];
char num2[MAX_LEN / 2 - 1];
char res[MAX_LEN];
int main(int argc, char *argv[]){
    int i;
    scanf("%s", num1);
    scanf("%s", num2);
    printf("%s * %s = ?\n", num1, num2);
    int num1_len = judge(num1);
    if(num1_len < 0){
        printf("num1 error!\n");
        return -1;
    }
    int num2_len = judge(num2);
    if(num2_len < 0){
        printf("num2 error!\n");
        return -1;
    }
    memset(res, 0x0, sizeof(res));
    compute(num1, num2, res);
    int is = 1;
    for(i = 0; i < strlen(res); i ++){
        if(res[i] != '0' && is){
            is = 0;
        }
        if(!is){
            printf("%c", res[i]);
        }
    }
    if(is){
        printf("0");
    }
    printf("\n");
    return 0;
}

备注：写出来做个备份。