pat 1108. Finding Average

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本文针对PAT-A-1108题“FindingAverage”，详细解析了如何从一组可能包含非法输入的实数中筛选出合法数值并计算其平均值。介绍了合法数值的定义、输入输出规范及样例，并提供了完整的C++实现代码。

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原题链接：https://www.patest.cn/contests/pat-a-practise/1108

1108. Finding Average (20)

时间限制

400 ms

内存限制

65536 kB

代码长度限制

16000 B

判题程序

Standard

作者

CHEN, Yue

The basic task is simple: given N real numbers, you are supposed to calculate their average. But what makes it complicated is that some of the input numbers might not be legal. A "legal" input is a real number in [-1000, 1000] and is accurate up to no more than 2 decimal places. When you calculate the average, those illegal numbers must not be counted in.

Input Specification:

Each input file contains one test case. For each case, the first line gives a positive integer N (<=100). Then N numbers are given in the next line, separated by one space.

Output Specification:

For each illegal input number, print in a line "ERROR: X is not a legal number" where X is the input. Then finally print in a line the result: "The average of K numbers is Y" where K is the number of legal inputs and Y is their average, accurate to 2 decimal places. In case the average cannot be calculated, output "Undefined" instead of Y. In case K is only 1, output "The average of 1 number is Y" instead.

Sample Input 1:

7
5 -3.2 aaa 9999 2.3.4 7.123 2.35

Sample Output 1:

ERROR: aaa is not a legal number
ERROR: 9999 is not a legal number
ERROR: 2.3.4 is not a legal number
ERROR: 7.123 is not a legal number
The average of 3 numbers is 1.38

Sample Input 2:

2
aaa -9999

Sample Output 2:

ERROR: aaa is not a legal number
ERROR: -9999 is not a legal number
The average of 0 numbers is Undefined

#include <iostream>
#include <cstdio>
#include <cstring>
#include <map>
#include <set>

using namespace std;

char s[1000];

int main()
{
    int n,i,j,flag,cnt,cc=0;
    double sum=0.0;
    scanf("%d",&n);
    for(i=0; i<n; i++)
    {
        cnt=0;
        flag=0;
        scanf("%s",s);
        int len=strlen(s);
        if(s[0]=='-')//注意负号的判断
            j=1;
        else
            j=0;
        for(; j<len; j++)
        {
            if(s[j]=='.')//小数点最多只能是一个
            {
                if(len-1-j>2)
                {
                    flag=1;
                    break;
                }
                cnt++;
                if(cnt==2)
                {
                    flag=1;
                    break;
                }
                continue;
            }
            if(s[j]<'0'||s[j]>'9')
            {
                flag=1;
                break;
            }
        }
        if(flag){
            printf("ERROR: %s is not a legal number\n",s);
            continue;
        }
        if(s[0]=='-')
            j=1;
        else
            j=0;
        flag=0;
        double sum1=0,sum2=0;
        for(; j<len; j++)
        {
            if(s[j]=='.')
            {
                flag=1;
                continue;
            }
            if(flag==0)
                sum1=sum1*10+s[j]-'0';
            else
                sum2=sum2*10+s[j]-'0';
        }
        if(sum1>1000||(sum1==1000&&sum2!=0)){
            printf("ERROR: %s is not a legal number\n",s);
            continue;
        }
        cc++;
        while(sum2>=1)
            sum2/=10;
        double ss=sum1+sum2;
        if(s[0]=='-')//注意符号一定要在加完之后判断，不能在判断完符号之后再加，这样数值错误
            ss=-ss;
        sum+=ss;
    }
    if(cc==0)
        printf("The average of 0 numbers is Undefined\n");
    else if(cc==1)
        printf("The average of 1 number is %.2f\n",sum);//注意只有一个数字时，输出number而不是numbers（坑点）
    else
        printf("The average of %d numbers is %.2f\n",cc,sum/cc);
    return 0;
}