# C 程序：檢查數字是正數還是負數 > 原文： [https://www.programiz.com/c-programming/examples/negative-positive-zero](https://www.programiz.com/c-programming/examples/negative-positive-zero) #### 在此示例中，您將學習檢查數字（由用戶輸入）是負數還是正數。 要理解此示例，您應該了解以下 [C 編程](/c-programming "C tutorial")主題： * [C 編程運算符](/c-programming/c-operators) * [C `if...else`語句](/c-programming/c-if-else-statement) * * * 該程序從用戶那里獲取一個號碼，并檢查該號碼是`positive`還是`negative`或`zero`。 * * * ## 使用`if...else`檢查正負 ```c #include <stdio.h> int main() { double num; printf("Enter a number: "); scanf("%lf", &num); if (num <= 0.0) { if (num == 0.0) printf("You entered 0."); else printf("You entered a negative number."); } else printf("You entered a positive number."); return 0; } ``` 您還可以使用[嵌套](https://www.programiz.com/c-programming/c-if-else-statement) `if else` [語句](https://www.programiz.com/c-programming/c-if-else-statement)來解決此問題。 * * * ## 使用嵌套`if else`檢查正數或負數 ```c #include <stdio.h> int main() { double num; printf("Enter a number: "); scanf("%lf", &num); if (num < 0.0) printf("You entered a negative number."); else if (num > 0.0) printf("You entered a positive number."); else printf("You entered 0."); return 0; } ``` * * * **輸出 1** ```c Enter a number: 12.3 You entered a positive number. ``` **輸出 2** ```c Enter a number: 0 You entered 0. ```