跳转到内容

数组步长

本页使用了标题或全文手工转换
维基百科,自由的百科全书

数组步长(stride of an array,也称increment, pitch或step size)是程序设计时,相邻数组元素在内存中的开始地址的距离,度量单位可以是字节或者数组元素个数。步长不可小于数组元素的尺寸,但可以大于,表示有填充的字节。

数组步长如果等于数组元素的尺寸,则数组在内存中是连续的。这可称为单位步长(unit stride)。非单位步长适用于二维数组多维数组

非单位步长的存在理由

填充

许多程序语言允许数据结构对齐。例如:

struct A {
    int a;
    char b;
};

struct A myArray[100];

myArray可能具有步长为8,而不是5。这用于优化处理时间而不是优化最少使用内存。

平行数组的重叠

重叠的平行数组:

#include <stdio.h>

struct MyRecord {
    int value;
    char *text;
};

/*
    Print the contents of an array of ints with the given stride.
    Note that size_t is the correct type, as int can overflow.
*/
void print_some_ints(const int *arr, int length, size_t stride)
{
    int i;
    printf("Address\t\tValue\n");
    for (i=0; i < length; ++i) {
        printf("%p\t%d\n", arr, arr[0]);
        arr = (int *)((unsigned char *)arr + stride);
    }
}

int main(void)
{
    int ints[100] = {0};
    struct MyRecord records[100] = {0};

    print_some_ints(&ints[0], 100, sizeof ints[0]);
    print_some_ints(&records[0].value, 100, sizeof records[0]);
    return 0;
}

这是一种类型双关

数组剖面

某些程序设计语言如PL/I允许从数组中选项某些列或行作为数组剖面(array cross-section)。[1]:p.262 例如,对于二维数组:

  declare some_array (12,2)fixed;

只考虑第二列的一个导出数组可引用为

  some_array(*,2)

非单位步长多维数组例子

非单位步长特别适用于图像。这允许创建子图像而不必复制像素。Java示例:

  public class GrayscaleImage {
    private final int width, height, widthStride;
    /** Pixel data. Pixel in single row are always considered contiguous in this example. */
    private final byte[] pixels;
    /** Offset of the first pixel within pixels */
    private final int offset;

    /** Constructor for contiguous data */
    public Image(int width, int height, byte[] pixels) {
      this.width = width;
      this.height = height;
      this.pixels = pixels;
      this.offset = 0;
      this.widthStride = width;
    }

    /** Subsection constructor */
    public Image(int width, int height, byte[] pixels, int offset, int widthStride) {
      this.width = width;
      this.height = height;
      this.pixels = pixels;
      this.offset = offset;
      this.widthStride = widthStride;
    }

    /** Returns a subregion of this Image as a new Image. This and the new image share
        the pixels, so changes to the returned image will be reflected in this image. */
    public Image crop(int x1, int y1, int x2, int y2) {
      return new Image(x2 - x1, y2 - y1, pixels, offset + y1*widthStride + x1, widthStride);
    }

    /** Returns pixel value at specified coordinate */
    public byte getPixelAt(int x, int y) {
      return pixels[offset + y * widthStride + x];
    }
  }

参考文献

  1. ^ Hughes, Joan K. PL/I Structured Programming (second ed.). New York: John Wiley and Sons. 1979. ISBN 0-471-01908-9.