无法使用从 fgets 输入的字符串搜索字符

Question

我正在编写一个程序，使用 strchr 查找字符串中特定字符的位置。当使用 fgets 从用户输入字符串时，程序无法正常执行。

然而，当使用 gets 时一切正常。

使用 gets() 的代码：

#include <stdio.h>
#include <string.h>

int main() {
    char let;
    char input[50];
    char *ptr;
    printf("what is the string that you would like to check?\n");
    gets(input);
    printf("what is the character that you would like to find?\n");
    let = getchar();
    printf("in string \"%s\"...\n", input);
    ptr = strchr(input, let);
    while (ptr != NULL) {
        printf("the letter %c was found at character %d\n", let, ptr - input + 1);
        ptr = strchr(ptr + 1, let);
    }
    return 0;
}

输出：

what is the string that you would like to check?
what is the character that you would like to find?
in string "why is the world when wondering"...
the letter w was found at character 1
the letter w was found at character 12
the letter w was found at character 18
the letter w was found at character 23

无效的代码usgin fgets():

#include <stdio.h>
#include <string.h>

int main() {
    char let;
    char input[50];
    char *ptr;
    printf("what is the string that you would like to check?\n");
    fgets(input, 16, stdin);
    printf("what is the character that you would like to find?\n");
    let = getchar();
    printf("in string \"%s\"...\n", input);
    ptr = strchr(input, let);
    while (ptr != NULL) {
        printf("the character is found at %d \n", ptr - input + 1);
        ptr = strchr(ptr + 1, let);
    }
    return 0;
}

输出：

what is the string that you would like to check?
what is the character that you would like to find?
in string "abcdefghijklmno"...

Answer 1

改变

fgets(input, 16, stdin)

到

fgets(input, sizeof(input), stdin)

当您将 16 的参数传递给 fgets() 时，您是在指示它读取 不超过 15 个字符 。为什么？

来自 fgets() 联机帮助页：

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s.

如果您提供多于size -1个字符，剩余的字符留在输入缓冲区。

然后当程序后续调用

let = getchar();

let 会被分配输入缓冲区中的下一个字符 - 甚至无需等待您输入任何其他内容。然后它会在您提供的字符串中搜索此字符 - 但在您提供的示例中找不到它。

Answer 2

在第一个代码片段中调用

gets (input);

不限制用户可以输入的字符数。考虑到函数 gets 不受 C 标准支持并且不安全。

在第二个代码片段中，您将变量 input 声明为

char input[50];

所以用下面的方式调用fgets就更自然了

fgets (input,sizeof( input ),stdin );

而不是

fgets (input,16,stdin);

标准函数 getchar 读取任何字符，包括白色 spaces。

所以最好使用

scanf( " %c", &let );

否则getchar可以读取输入缓冲区中留下的任何字符（包括例如换行符）。 scanf 的调用将跳过任何白色 space 字符。

无法使用从 fgets 输入的字符串搜索字符

Unable to search character with string input from fgets

c

gets

fgets

char