getchar() 在创建字符串数组时不读取空格？

Question

I am trying to create a reversed string using getchar() to first read a string in array format. I did not write the reversing part yet. The code works for any strings without space. Ex. "HelloWorld!" will output "HelloWorld!" but "Hello World!" will only output "Hello".

#include <stdio.h>

#define MAX_SIZE 100

int main() 
{
    char temp;
    char my_strg[MAX_SIZE];
    int length;

    printf("Please insert the string you want to reverse: ");
    scanf("%s", my_strg);

    while((temp = getchar()) != '\n')
    {
        my_strg[length] = temp;
        length++;
    } 

    printf("%s\n", my_strg);

    return 0;
}

Answer 1

Your issue is scanf . scanf only reads until whitespace is encountered. To fix this, you can use fgets (and strcspn to remove the newline):

if(fgets(my_strg, sizeof(my_strg), stdin) == NULL)
{
    perror("fgets");
    return EXIT_FAILURE;
}
my_strg[strcspn(my_strg, "\n")] = '\0';

Or with scanf ...

if(scanf("%99[^\n]", my_strg) != 1)
{
    fprintf(stderr, "scanf() failed to read\n");
    return EXIT_FAILURE;
}

After these changes you can entirely remove your getchar loop.

In your program you need to check for I/O errors. I've shown you how to do this above. If you don't do this, your program could give incorrect output or in certain cases cause undefined behavior (which usually results in a crash).

Bonus: here's how to reverse a string:

size_t l = strlen(my_strg) / 2;
char *s = my_strg, *e = s + l;
while(l--)
{
    char tmp = *--e;
    *e = *s;
    *s++ = tmp;
}

Answer 2

Your program has undefined behaviour , since you are attempting to access an element of an array, my_strg[length] with an unitialized value of length .

To fix this, move your declaration of length to after the scanf call and initialize it to the length of the string that scanf reads:

scanf("%s", my_strg);
size_t length = strlen(my_strg);

Alternatively, drop the scanf call completely and initialize length to zero:

    char my_strg[MAX_SIZE] = { 0, }; // Note: make sure you ALSO initialize your array!!
    printf("Please insert the string you want to reverse: ");
    size_t length = 0;
    while ((temp = getchar()) != '\n') {
    //..

Note: If you (for some reason) don't want to initialize your entire array to zeros (as the first line in my second code block will do), then make sure to add a zero ( nul ) character at the end of the string, before printing it (or doing anything else with it). You could simply add this line after the while loop:

    my_strg[length] = '\0'; // "length" will already point to one-beyond-the-end

EDIT: To address the very good points made by David C. Rankin in the comments section, you could (should) improve your while loop control to: (a) prevent buffer overflow and (b) handle input error conditions. Something like this:

while ((length < MAXSIZE - 1) && (temp = getchar()) != '\n' && temp != EOF) {
    //..

but the exact tests and controls you use would depend on how you wish to handle such issues.

Answer 3

By definition, scanf("%s", my_strg) reads a string until the first white space (and a blank counts as white space). So "Hello world" will be read until the first blank, ie my_strg will contain "Hello" then. To read until a new line (including the newline), use fgets .

BTW: variable length is uninitialized, such that you get undefined behaviour.