打印数组元素 [英] Printing array elements

问题描述

下面的C方案的预期输出打印的数组元素。但是，当实际运行，它没有这样做。

The expected output of the following C program is to print the array elements. But when actually run, it doesn't do so.

#include<stdio.h>

#define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))

int array[] = {23,34,12,17,204,99,16};

int main()
{
    int d;

    for(d=-1;d <= (TOTAL_ELEMENTS-2);d++)
        printf("%d\n",array[d+1]);

    return 0;
}

这是什么原因？

推荐答案

当你做的比较 D＆LT; =（TOTAL_ELEMENTS-2），一个类型转换执行。 D 的类型为符号int ，而（TOTAL_ELEMENTS-2）的类型为size_t ，这是一个无符号的类型。 C的规则说，当操作员有一个符号和无符号参数，无符号的参数是大于或等于大小的参数签署的，则签订参数转换为无。

When you do the comparison d <= (TOTAL_ELEMENTS-2), a type conversion is performed. d is of type signed int while (TOTAL_ELEMENTS-2) is of type size_t, which is an unsigned type. The rules of C say that when an operator has a signed and an unsigned argument, and the unsigned argument is of greater or equal size to the signed argument, then the signed argument is converted to unsigned.

即，作为比较结束

(size_t) d <= (TOTAL_ELEMENTS-2)

而因为为size_t 是无符号，（为size_t）-1 是一个非常，非常大的数量，而不是 - 1了。对于32位为size_t 这将是2 ³² - 1 = 4,294,967,295

And because size_t is unsigned, (size_t) -1 is a really, really large number, not -1 any more. For a 32-bit size_t it would be 2³² - 1 = 4,294,967,295.

要解决这个问题，你可以明确地投的右手边符号int：

To fix this, you can explicitly cast the right-hand side to signed int:

d <= (int) (TOTAL_ELEMENTS-2)

或者，更好的，刚刚摆脱怪异负索引和等。

Or, better, just get rid of the weird negative indexing and such.

有关将来参考，打开所有的编译器警告即可。 GCC，例如，会如果打开打印警告 -Wall -Wextra ：

For future reference, turn on all the compiler warnings you can. gcc, for instance, will print a warning if you turn on -Wall -Wextra:

$ gcc -o arrayprint -Wall -Wextra -ansi arrayprint.c 
arrayprint.c: In function ‘main’:
arrayprint.c:11: warning: comparison between signed and unsigned

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