P进制数x→十进制数y

int y = 0, product = 1;//y十进制数，product记录权重
while(x != 0){y += (x % 10) * product;//x%10获取x的个位数x /= 10;//去掉x的个位product *= p;//下一权重
}

8. 九进制转十进制

#include<iostream>
using namespace std;
int main(){int a = 2022, b = 0, p = 1;while(a){b += (a % 10) * p;p *= 9;a /= 10;}printf("%d", b);return 0;
} 

十进制y→Q进制z

int z[40], num = 0;//数组z存放Q进制数y的每一位，num为位数
do{//z数组从高位z[num-1]到低位z[0]即为Q进制z，进制转换完成z[num++] = y % Q;//除基取余y /= Q;
}while(y != 0);//当商非零时循环

3612. 八进制

#include<iostream>
using namespace std;
int main(){int ans[40], x, num = 0, q = 8;while(scanf("%d", &x) != EOF){num = 0;do{ans[num++] = x % q;x /= q;}while(x);for(int i = num - 1; i >= 0; i--) printf("%d", ans[i]);printf("\n");}return 0;
}