C语言编程中碰到一个问题，求大神指教

代码如下：

#include<stdio.h>
int main()
{
    char a[200];//a数组用来存储字符串
    int i,t,m;
    int sum1=0;//sum1用来计算各个字母出现的次数
    int b[26];//用来存储各个字母出现的次数
    char c[26]={"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
    char *d, n=0;//指针变量指向数组c
    d=c;
    for(i=0; a[i]!=""; i++)
        scanf("%c", &a[i]);//输入各个字符
    if(a[i]>="A"&&a[i]<="Z")
            a[i]+=32;//把字符全部变为小写的字母
    for(t=0; t<26; t++)
    {
    {
        for(i=0; a[i]!=""; i++)
            if(a[i]==c[t])
                sum1++;
    }
                b[t]=sum1;//计算出各个小写字母的次数
    }
    for(i=0; i<25; i++)
        for(t=0; t<25-i; t++)
            if(b[t]<b[t+1])
            {
                m=b[t];
                n=*d;
                b[t]=b[t+1];
                *d=*(d+1);
                b[t+1]=m;
                *(d+1)=n;

            }//将次数从大到小排序
    for(t=0; t<26; t++)
    {    printf("%c:%d", *(d+t), b[t]);
        printf("
");
    }
    return 0;
}