C语言实现4位数字素数位追加+1值的代码错误排查

• 十位数字计算错误：当前代码的十位计算语句tensplace = ((userinput-onesplace/10))%10;存在运算符优先级问题，/运算优先级高于-，会先计算onesplace/10，个位数字取值范围为0-9，除以10的结果是0，因此这行代码实际等价于tensplace = userinput%10，和个位数值完全相同。比如输入2345时，你计算得到的十位是5、个位也是5，自然输出结果和预期不符。
• 次要问题：提示语的换行符写为/n，正确的转义换行符应为\n；各位数字拆分逻辑过于冗余，完全不需要先减去低位再计算，直接用整除+取模的组合即可，更简洁也不容易出错。

对于4位正整数，各位拆分可简化为：

• 千位：userinput / 1000 % 10
• 百位：userinput / 100 % 10
• 十位：userinput / 10 % 10
• 个位：userinput % 10

#include <stdio.h>

int main() {
    unsigned int userinput;
    unsigned int onesplace;
    unsigned int tensplace;
    unsigned int hundredsplace;
    unsigned int thousandsplace;
    printf("Please enter your number \n");
    scanf("%u", &userinput);
    // 简化各位拆分逻辑
    thousandsplace = userinput / 1000 % 10;
    hundredsplace = userinput / 100 % 10;
    tensplace = userinput / 10 % 10;
    onesplace = userinput % 10;

    printf("%u", thousandsplace);
    if ((thousandsplace == 2)||(thousandsplace == 3)||(thousandsplace == 5)||(thousandsplace == 7)) {
        printf("%u", thousandsplace + 1);
    }
    printf("%u", hundredsplace);
    if ((hundredsplace == 2)||(hundredsplace == 3)||(hundredsplace == 5)||(hundredsplace == 7)) {
        printf("%u", hundredsplace + 1);
    }
    printf("%u", tensplace);
    if ((tensplace == 2)||(tensplace == 3)||(tensplace == 5)||(tensplace == 7)) {
        printf("%u", tensplace + 1);
    }
    printf("%u", onesplace);
    if ((onesplace == 2)||(onesplace == 3)||(onesplace == 5)||(onesplace == 7)) {
        printf("%u", onesplace + 1);
    }
    return 0;
}

修正后输入2345，输出结果为2334456，完全符合需求。

内容的提问来源于stack exchange，提问作者A J