入门C语言求来教教我！

#include <stdio.h>
int main()
{
char s[20]={0};
int i;
gets(s);
while(s[i] != '\0'){
if( ( s[i]>='a' && s[i]<='v') || (s[i]>='A' && s[i]<='V'))
s[i] += 4;
else if ( s[i]>'v' && s[i]<='z' )
s[i] = 'a' + s[i] - 'w';
else if(s[i]>='V' && s[i]<='Z')
s[i] = 'A' + s[i] - 'W';
else{printf("error\n");return 0;}
i++;
}
printf("%s\n",s);
return 0;
}

这个根据ascii码的性质，给每个字母+4就行。遇到最后的Z 返回重置。

#include<iostream>
using namespace std;
int main()
{
    char a[5]={'C','h','i','n','a'};
    for(int i=0;i<5;i++)
     {
         a[i]+=4;
         cout<<a[i];
    }        
}

遍历字符串，对每个字符+4然后输出。

你题目的解答代码如下：（如有帮助，望采纳！谢谢! 点击我这个回答右上方的【采纳】按钮）

#include<stdio.h>

int main()
{
    char s[21], ch;
    int i,f=1;
    for (i = 0; i<20 && (ch=getchar()) != '\n'; i++) {
        if (ch >= 'A' && ch <= 'Z') {
            s[i] = (ch-'A'+4)%26 + 'A';
        } else if (ch >= 'a' && ch <= 'z') {
            s[i] = (ch-'z'+4)%26 + 'z';
        } else {
            f=0;
            break;
        }
    }
    s[i] = '\0';
    if (i==0 || f==0)
        printf("error");
    else
        printf("%s",s);
    return 0;
}

供参考：

#include<iostream>
using namespace std;
int main()
{
    int  i = 0;
    char a[21] = { 0 };
    while ((a[i] = cin.get()) != '\n')
    {    
        if (a[i] >= 'a' && a[i] <= 'z')
            a[i] = (a[i] - 'a' + 4) % 26 + 'a';
        else if (a[i] >= 'A' && a[i] <= 'Z')
            a[i] = (a[i] - 'A' + 4) % 26 + 'A';
        else {
            i = 0;
            break;
        }
        i++;
        if (i == 20) break;
    }
    a[i] = '\0';
    if (i == 0)
        cout<<"error"<<endl;
    else
        cout<< a<<endl;
    return 0;
}