如果我输入负数，我应该通过忽略否定答案来获得输出？

Question

如果用户输入一个负数，你可以忽略它的符号并考虑它对应的正值。如果用户输入零，那么程序应该终止。

#include <stdio.h>

int main() {
    long input;
    long reverse = 0;
    long temp;
    printf("Enter the input number: ");
    scanf("%ld", &input); //Read input from user
    temp = input;         //Save input to another variable
    while (temp > 0) {
        // extract last digit from temp and add it as last digit in reverse
        // by moving existing digits one position towards left
        reverse = reverse * 10 + (temp % 10);
        temp = temp / 10; // removes last digit and move other digits to right
    }
    if (input == reverse) // check if input and reverse are same
        printf("Number is a palindrome\n");
    else
        printf("Number is not palindrome\n");
    return 0;
}

输出是这样的

Enter an integer number: 12321
12321 is a Palindrome.
Enter an integer number: 2312
2312 is not a Palindrome.
Enter an integer number: -45566554
45566554 is a Palindrome.
Enter an integer number: t
Enter an integer number: 36298100189263
36298100189263 is a Palindrome.
Enter an integer number: 362981089263
362981089263 is not a Palindrome.
Enter an integer number: 0
Goodbye! 

Answer 1

如果你想忽略一个数字的符号，将它移到另一个数字并强制符号为正：

temp = (input < 0) ? -input : input;

这是简洁的三元版本，但如果您愿意，可以恢复为更经典的版本：

if (input < 0) {
    temp = -input;
} else {
    temp = input;
}

Answer 2

您可以测试该数字是否为负并取其相反。只有一种特殊情况，LONG_MIN，可能没有积极的对应物。使用无符号算术可以优雅地处理这种特殊情况：

#include <stdio.h>

int main() {
    long input;
    unsigned long uinput;
    unsigned long reverse = 0;
    unsigned long temp;
    printf("Enter the input number: ");
    if (scanf("%ld", &input) != 1) // Read input from user
        return 1;
    if (input >= 0) {
        uinput = input;
    } else {
        uinput = -(unsigned long)input;
    }
    temp = uinput;
    while (temp > 0) {
        // extract last digit from temp and add it as last digit in reverse
        // by moving existing digits one position towards left
        reverse = reverse * 10 + temp % 10;
        temp = temp / 10; // removes last digit and move other digits to right
    }
    if (uinput == reverse) // check if input and reverse are same
        printf("Number is a palindrome\n");
    else
        printf("Number is not palindrome\n");
    return 0;
}