pat b 1034 有理数四则运算

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rational n.有理数；fraction 分数；reduction 化简；

#include<cstdio>#include<algorithm>using namespace std;struct fraction{    long long up,down;    fraction(){        up=0;        down=0;    }};//第二处int gcd(long long a，long long b){    return a%b?gcd(b,a%b):b;}fraction reduction(fraction a){    if(a.down<0){        //abs()求绝对值。第一处        a.up=-a.up;        a.down=-a.down;    }else if(a.down==0){        return a;    }    if(a.up==0){        a.down=1;//注意：输出的时候算整数     }else{        int d=gcd(abs(a.up),a.down);        a.up=a.up/d;        a.down=a.down/d;    }     return a;}void printFraction(fraction a){    fraction b=reduction(a);    if(b.down==0){        printf("Inf");        return ;        }    if(b.up<0)printf("(");    if(b.down==1){        printf("%lld",b.up);        }else if(abs(b.up)>b.down){//注意：负数!!         printf("%lld %lld/%lld",b.up/b.down,abs(b.up)%b.down,b.down);    }else {        printf("%lld/%lld",b.up,b.down);    }    if(b.up<0)printf(")");}fraction add(fraction a,fraction b){    fraction c;    if(a.down!=b.down)    {       c.up=a.up*b.down+b.up*a.down;       c.down=a.down*b.down;    }else{        c.up=a.up+b.up;        c.down=b.down;    }    return reduction(c);}fraction sub(fraction a,fraction b){    fraction c;    if(a.down!=b.down)    {       c.up=a.up*b.down-b.up*a.down;       c.down=a.down*b.down;    }else{        c.up=a.up-b.up;        c.down=b.down;    }    return reduction(c);}fraction multi(fraction a,fraction b){    fraction c;    c.up=a.up*b.up;    c.down=a.down*b.down;    return reduction(c);}fraction div(fraction a,fraction b){    fraction c;    c.up=a.up*b.down;    c.down=a.down*b.up;    return reduction(c);}int main(){    fraction a,b;    scanf("%lld/%lld %lld/%lld",&a.up,&a.down,&b.up,&b.down);    a=reduction(a);    b=reduction(b);    printFraction(a);    printf(" + ");    printFraction(b);    printf(" = ");    printFraction(add(a,b));    printf("\n");    printFraction(a);    printf(" - ");    printFraction(b);    printf(" = ");    printFraction(sub(a,b));    printf("\n");    printFraction(a);    printf(" * ");    printFraction(b);    printf(" = ");    printFraction(multi(a,b));    printf("\n");    printFraction(a);    printf(" / ");    printFraction(b);    printf(" = ");    printFraction(div(a,b));    printf("\n");    return 0;}

第一遍后两个测试点没过，改正了第二处gcd()的参数类型就过了（之前是int）就过了。第一处之前用的是abs()，以为是求相反数，结果是求绝对值，像“-5/-9”这样的就过不了，改正之后就好了。
总结：在大部分答案正确的情况下（分最多的测试用例答案正确），错误的可能有：a.没有注意取值边界的情况b.没有注意特殊取值的情况（要考虑到所有情况）以上两种情况具体会涉及到定义变量的类型，分支语句的判断条件等。

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