新人学C。怎的让这语句其功能是检查ch是否字母字符

新人学C。怎样让这语句其功能是检查ch是否字母字符。
#include<stdio.h>
int main ()
{
int isletter(char ch);
int a,b;
scanf("%d",&a);
b=isletter(a);
if(b==1)
printf("yes\n");
else
printf("no\n");
}
int isletter(char ch)
{
if('a'<=ch<='z'||'Z'>=ch>='A')
return 1;
else
return 0;
}

[解决办法]
'a'<=ch&&ch<='z'||'Z'>=ch&&ch>='A
[解决办法]

C/C++ code

#include<stdio.h>int main (){    int isletter(char ch);    //int a, b;    //scanf("%d",&a);       char a, b;    scanf("%c",&a);    b=isletter(a);    if(b==1)        printf("yes\n");     else        printf("no\n");}int isletter(char ch){           if('a'<=ch && ch <='z'||'Z'>=ch && ch >='A')        return 1;    else        return 0;}