[英]Detecting signed integer multiplication overflow in C
I'm writing this code for more than 3 hours already..我写这段代码已经超过 3 个小时了..
I gave up about the overflow thing and tried to google and look it up on stackoverflow.我放弃了关于溢出的事情并试图谷歌并在stackoverflow上查找它。
I did not find any solution besides the one that I wrote in my code as you can see in lines 27-28
(where it returns 0).除了我在代码中编写的解决方案之外,我没有找到任何解决方案,正如您在
lines 27-28
(它返回 0)中看到的那样。 But this condition also does not work.但是这个条件也行不通。
#include <stdio.h>
int reverse(int x) {
int pos = 0;
int reversed = 0;
int numOfDigits = 0;
int tenPower = 1;
if (x < 0) {
pos = -x;
} else
pos = x;
while (pos > 0) {
pos = (pos - (pos % 10)) / 10;
numOfDigits++;
}
while (numOfDigits > 0) {
for (int i = numOfDigits - 1; i > 0; i--) {
if (numOfDigits == 1)
tenPower = 1;
else
tenPower *= 10;
}
//overflow check - does not work
if (x % 10 != 0 && ((x % 10) * tenPower) / (x % 10) != tenPower)
return 0;
reversed += (x % 10) * tenPower;
numOfDigits--;
x = (x - (x % 10)) / 10;
tenPower = 1;
}
if (x < 0)
return -reversed;
else
return reversed;
}
int main() {
int arr[5] = {-30, 120, 1501, 321, 0};
for (int i = 0; i < 5; i++) {
printf("Original number is: %d \n", arr[i]);
printf("Reversed number is: %d \n", reverse(arr[i]));
}
}
The input that is not working due to overflow is:由于溢出而无法工作的输入是:
1534236469
The error code on leetcode is leetcode 上的错误代码是
Line 25: Char 28: runtime error: signed integer overflow:
1000000000 * 9 cannot be represented in type 'int' [solution.c]
Line:
if (x%10 != 0 &&((x%10)*tenPower) / (x%10) != tenPower)
行:
if (x%10 != 0 &&((x%10)*tenPower) / (x%10) != tenPower)
Other than that, the code is working and every number (positive & negative numbers) is being successfully reversed.除此之外,代码正在运行,并且每个数字(正数和负数)都被成功反转。
I'll be glad to hear you out about a possible solution and also let me know what do you think about my code and the way I decided to complete this task, I know that's the most basic and naïve way to do it, but I'll be glad to know how could I improve it.我很高兴听到你关于可能的解决方案的消息,也让我知道你对我的代码的看法以及我决定完成这项任务的方式,我知道这是最基本和最天真的方法,但我'会很高兴知道我该如何改进它。
The task is:任务是:
Given a signed 32-bit integer x, return x with its digits reversed.
给定一个带符号的 32 位 integer x,返回 x 并反转其数字。 If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.
如果反转 x 导致 go 的值超出有符号的 32 位 integer 范围 [-231, 231 - 1],则返回 0。
Assume the environment does not allow you to store 64-bit integers (signed or unsigned).
假设环境不允许您存储 64 位整数(有符号或无符号)。
Examples:
例子:
Input: x = 123 Output: 321, Input: x=-120 Output = -21
输入:x = 123 Output:321,输入:x=-120 Output = -21
The main thing you want to check for overflow is this:您要检查溢出的主要内容是:
reversed += (x % 10) * tenPower;
So what you want to know is if this is true:所以你想知道的是这是否属实:
((x % 10) * tenPower) > INT_MAX
Or this is true:或者这是真的:
(reversed + (x % 10) * tenPower) > INT_MAX
Of course these can never be true in code due to overflow, but we can rearrange the terms:当然,由于溢出,这些在代码中永远不会成立,但我们可以重新排列这些术语:
if ((x % 10) != 0 && (tenPower > INT_MAX / (x % 10)) ||
(((x % 10) * tenPower) > INT_MAX - reversed))
return 0;
You can do something like this.你可以做这样的事情。
int reverse(int n) {
// 1st overflow checking...
// Check if the absolute value is greater than INT32_MAX.
// I converted "int" to "long" to get correct overflow value.
if (n < 0 && (long) n * -1l > INT32_MAX) {
return 0;
}
int res = 0;
// Convert to absolute value.
int tmp = n < 0 ? n * -1 : n;
while (tmp > 0) {
// Get the right most digit and add it to the current result.
res += tmp % 10;
// Remove the right most digit.
tmp /= 10;
// "tmp" still has remaining numbers.
if (tmp > 0) {
// 2nd overflow checking...
// Check if reversed value will be greater than INT32_MAX when appending 0 to right most.
// I converted "int" to "long" to get correct overflow value.
if ((long) res * 10l > INT32_MAX) {
return 0;
}
// Append 0 to right most value of result.
// If result is equal to 0, do not append 0.
res *= res == 0 ? 1 : 10;
}
}
// Return result.
// If original value is negative, return negative result value..
return n < 0 ? res * -1 : res;
}
OP's code is subject to int
overflow in various places OP的代码在各个地方都会发生
int
溢出
if (x < 0) { pos = -x; }
...
(x % 10) * tenPower
....
reversed += (x % 10) * tenPower;
...
if (x < 0) return -reversed;
A simple overflow pre-test involves INT_MAX/INT_MIN
一个简单的溢出预测试涉及
INT_MAX/INT_MIN
if (x < 0) {
if (x < -INT_MAX) { puts("Overflow"); return 0; }
pos = -x;
}
// x is >= 0 here, tenPower >= 1
int digit = x % 10;
if (digit > INT_MAX/tenPower) { puts("Overflow"); return 0; }
int digit10 = digit * tenPower;
if (reversed > INT_MAX - digit10) { puts("Overflow"); return 0; }
reversed += digit10;
Or use stand-alone full range tests .或者使用独立的全方位测试。
// Return 1 on overflow
int is_undefined_mult1(int a, int b) {
if (a > 0) {
if (b > 0) {
return a > INT_MAX / b; // a positive, b positive
}
return b < INT_MIN / a; // a positive, b not positive
}
if (b > 0) {
return a < INT_MIN / b; // a not positive, b positive
}
return a != 0 && b < INT_MAX / a; // a not positive, b not positive
}
int digit = x % 10;
if (is_undefined_mult1(digit, tenPower)) { puts("Overflow"); return 0; }
int digit10 = digit * tenPower;
if (is_undefined_add1(reversed, digit10)) { puts("Overflow"); return 0; }
reversed += digit10;
You can use bit_length() to check integer size您可以使用bit_length()检查 integer 大小
if digit.bit_length() >= 32:
return 0
else:
return reversed
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