#include "string.h"
main( )
{char str[30];
int i,k=0;
scanf("%s",str);
for(i=0; str[i]!='\0';i++)
if(str[i]>=a&&str[i]<=z||str[i]>=A&&str[i]<=Z)
k++;
prinf("%d",k);}

warning C4013: 'scanf' undefined; assuming extern returning int
e:\study\c++\test for bccn\0810.c(7) : error C2065: 'a' : undeclared identifier
e:\study\c++\test for bccn\0810.c(7) : error C2065: 'z' : undeclared identifier
e:\study\c++\test for bccn\0810.c(7) : error C2065: 'A' : undeclared identifier
e:\study\c++\test for bccn\0810.c(7) : error C2065: 'Z' : undeclared identifier
e:\study\c++\test for bccn\0810.c(9) : warning C4013: 'prinf' undefined; assuming extern returning int

啊 对哦  忘记了 谢谢LZ

加#include<stdio.h>
字符都加个 单引号 ''...

谢谢LZ的提醒  下次一定注意  再也不犯这种低级错误