#include<stdio.h>                                        //头文件

void TtoT(char num1);                                    //10 to 2 函数声明
void TtoE(char num2);                                    //10 to 8 函数声明
void TtoS(char num3);                                    //10 to 16 函数声明
                                    

void main()                                                //主函数
{   
    char cs,num;                                    //定义变量cs num
    while(1)                                            //死循环
    {
        printf("10 TO 2     请按1\n");                    //输出10 TO 2  请按1
        printf("10 TO 8     请按2\n");                    //输出10 TO 8  请按2
        printf("10 TO 16    请按3\n");                    //输出10 TO 16  请按3
        scanf("%d",&cs);                                //输入cs的值
        switch(cs)                                        //switch语句
        {
            case 1:
                printf("请输入要转换的十进制数：");        //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoT(num);                                //调用函数TtoT
                break;                                    //跳出switch语句
            case 2:
                printf("请输入要转换的十进制数：");     //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoE(num);                                //调用函数TtoE
                break;                                    //跳出switch语句
            case 3:
                printf("请输入要转换的十进制数：");     //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoS(num);                                //调用函数TtoS
                break;                                    //跳出switch语句
        }
        printf("\n");
        printf("\n");                                    //换行   目的是第二次进入switch之前换行避免与上一次重行
    }
}

void TtoT(char num1)                                        //函数TtoT
{
    int    n;
    char a;                                            //定义变量
    char b[100];                                            //定义数组
    for(n=0;num1!=0;n++)                                //for循环
    {
        a = num1/2;                                        //取商
        b[n] = num1%2;                                    //取余
        num1 = a;                                        //将a的值给num1
    }
    printf("10 TO 2 值：");                                //输出10 TO 2 值：
    for(;n+1>0;n--)                                        //for循环
    {
        printf("%d",b[n]);                              //输出结果
    }
}

void TtoE(char num2)                                    //函数TtoE
{
    int  n;                                            //定义变量
    char a,b[100];                                        //定义数组
    for(n=0;num2!=0;n++)                                //for循环
    {
        a = num2/8;                                        //取商
        b[n] = num2%8;                                    //取余
        num2 = a;                                        //将a的值给num2
    }
    printf("10 TO 8 值：");                                //输出10 TO 8 值：
    for(;n+1>0;n--)                                        //for循环
    {
        printf("%d",b[n]);                                //输出结果   
    }
}

void TtoS(char num3)                                    //函数TtoS
{
    int    n,d;                                            //定义变量
    char a,b[100],c[]={'A','B','C','D','E','F'};        //定义数组
    for(n=0;num3!=0;n++)                                //for循环
    {
        a = num3/16;                                    //取商
        b[n] = num3%16;                                    //取余
        if(b[n]>9)                                        //判断b[n]是否大于9
        {
            d=b[n]-10;                                    //取差值
            b[n]=c[d];                                    //将c对应的值给b
        }
        num3 = a;                                        //将a的值给num3
    }
    printf("10 TO 16 值：0x");                            //输出10 TO 16 值：0x
    for(;n+1>0;n--)                                        //for循环
    {
        if(b[n]>9)                                        //判断b[n]是否大于9
        {
            printf("%c",b[n]);                            //输出结果
        }
        else
        {
            printf("%d",b[n]);                            //输出结果
        }
    }
}

代码太长了，只是改了一个二进制的样板，八进制的按照这个修改就行了~主要是数据类型匹配问题
至于16进制的字母表示，要好好斟酌一下~

#include<stdio.h>                                        //头文件

void TtoT(int num1);                                    //10 to 2 函数声明
void TtoE(int num2);                                    //10 to 8 函数声明
void TtoS(int num3);                                    //10 to 16 函数声明
                                    

void main()                                                //主函数
{    
    int cs;
    int num;                                    //定义变量cs num
    while(1)                                            //死循环
    {
        printf("10 TO 2     请按1\n");                    //输出10 TO 2  请按1
        printf("10 TO 8     请按2\n");                    //输出10 TO 8  请按2
        printf("10 TO 16    请按3\n");                    //输出10 TO 16  请按3
        scanf("%d",&cs);                                //输入cs的值
        switch(cs)                                        //switch语句
        {
            case 1:
                printf("请输入要转换的十进制数：");        //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoT(num);                                //调用函数TtoT
                break;                                    //跳出switch语句
            case 2:
                printf("请输入要转换的十进制数：");     //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoE(num);                                //调用函数TtoE
                break;                                    //跳出switch语句
            case 3:
                printf("请输入要转换的十进制数：");     //输出请输入要转换的十进制数：
                scanf("%d",&num);                        //输入数字
                TtoS(num);                                //调用函数TtoS
                break;                                    //跳出switch语句
        }
        printf("\n");
        printf("\n");                                    //换行   目的是第二次进入switch之前换行避免与上一次重行
    }
}

void TtoT(int num1)//数据类型与int型不匹配                                        //函数TtoT
{
    int  n;
    int a;                                           //定义变量
    int b[100];                                            //定义数组
    for(n=0;num1!=0;n++)                                //for循环
    {
        a = num1/2;                                        //取商
        b[n] = num1%2;                                    //取余
        num1 = a;
                                                         //将a的值给num1
    }
    n--;//这里这里，由于最后一次循环要执行n++，固n要-1才能匹配~
    printf("10 TO 2 值：");                                //输出10 TO 2 值：
    for(;n+1>0;n--)                                        //for循环
    {
        printf("%d",b[n]);                              //输出结果
    }
}

感谢大神！！！改好了！！！改了好久原来是n出了问题！祝大神好运！

试模拟 itoa 写了一个转2、8、16进制

#include <stdio.h>

char *_iitoa(unsigned int value, char *string, int radix)
{
    if (radix!=2 && radix!=8 && radix!=16)
        return NULL;
    if (value == 0)
    {
        *string = '0';
        *(string+1) = 0;
        return string;
    }
    char *num="0123456789ABCDEF";
    char *p1=string, *p2;
    for(; value>0; value/=radix)
        *p1++ = num[value%radix];
    *p1 = 0;
    p2 = --p1;
    p1 = string;
    for(; p1<p2; p1++, p2--)
    {
        *p1 ^= *p2;
        *p2 ^= *p1;
        *p1 ^= *p2;
    }
    return string;
}

main ()
{
    char s[33];
    printf("3456(10) == %s(2)\n", _iitoa(3456, s, 2));
    printf("3456(10) == %s(8)\n", _iitoa(3456, s, 8));
    printf("3456(10) == %s(16)\n", _iitoa(3456, s, 16));
    printf("-3456(10) == %s(2)\n", _iitoa(-3456, s, 2));
    printf("-3456(10) == %s(8)\n", _iitoa(-3456, s, 8));
    printf("-3456(10) == %s(16)\n", _iitoa(-3456, s,16));
}