利用字符串进行大数字的进制转换，转换后第一个字符为"?"

代码如下，将16进制转换为2进制

#include"stdio.h"
#include"string.h"
#include"stdlib.h"

int main()
{
    int n,i;char *a,*b;
    b=(char *)malloc(100000);
    a=(char *)malloc(100000);
    scanf("%s",a);
    getchar();
    for(i=0;i<strlen(a);i++)
        switch(a[i])
    {
        case'0':strcat(b,"0000");break;
        case'1':strcat(b,"0001");break;
        case'2':strcat(b,"0010");break;
        case'3':strcat(b,"0011");break;
        case'4':strcat(b,"0100");break;
        case'5':strcat(b,"0101");break;
        case'6':strcat(b,"0110");break;
        case'7':strcat(b,"0111");break;
        case'8':strcat(b,"1000");break;
        case'9':strcat(b,"1001");break;
        case'A':strcat(b,"1010");break;
        case'B':strcat(b,"1011");break;
        case'C':strcat(b,"1100");break;
        case'D':strcat(b,"1101");break;
        case'E':strcat(b,"1110");break;
        case'F':strcat(b,"1111");break;
    }
    printf("%s",b);
}

调试结果如下

望解疑，谢谢！

第9行下添加初始化语句
malloc分配空间后必须初始化

memset(b, 0, 100000);