c - scanf ("%f", &weight) 和 scanf ("%f", Weight) 如何确定何时添加前缀 &

Question

我阅读了以下代码:

//talkback.c -- nosy, informative program
#include <stdio.h>
#include <string.h>  //for strlen() prototype
#define DENSITY 62.4  //human density in lbs per cu ft

int main() {

    float weight, volume;
    int size, letters;
    char name[40];  //name is an array of 40 characters

    printf("Hi! What's your first name?\n");
    scanf("%s", name);
    printf("%s, what's your weight in pounds?\n", name);
    scanf("%f", &weight);
    size  = sizeof name;
    letters = strlen(name);
    volume = weight / DENSITY;
    printf("Well, %s, your volumn is %2.2f cubit feet.\n", name, volume);
    printf("Also, your first name has %d letters, \n", letters);
    printf("and we have %d bytes to store it.\n", size);

}

运行并输出:

In [13]: !./a.out
Hi! What's your first name?
Hery
Hery, what's your weight in pounds?
100
Well, Hery, your volumn is 1.60 cubit feet.
Also, your first name has 4 letters, 
and we have 40 bytes to store it.

我注意到 scanf("%s", name); 和 scanf("%f", &weight); ，第二个在前面有一个前缀 &重量。

如何区分哪种情况需要加前缀？

Answer 1

scanf() 代表扫描格式化字符串。现在，在从标准输入流扫描输入时，scanf() 需要将该输入数据放入某处。为了存储格式化的输入数据，scanf()需要知道相同数据类型的变量的内存位置。这就是为什么 scanf() 需要一个指针(C 中的指针存储变量或表达式的内存位置)来存储输入。变量前面的地址运算符 (&)，即&var 指示变量“var”的内存位置。

int var;
scanf("%d",&var);

char str[20];
scanf("%s",str);

对于第二个示例，我们不需要取址运算符，因为C 将数组名称变量视为指针。

printf() 是 scanf() 的逆函数。它将格式化的字符串打印到标准输出。 Printf 不需要任何内存位置来打印输出，它只需要变量来获取数据并根据格式说明符对其进行格式化。

printf("%c in ASCII is %d",65,65);

The output will be: A in ASCII is 65