由putchar在最后一个字符后打印的垃圾

Question

My putchar() function is returning garbage after the sum is made.

Here's my code snippet:

scanf("%d", & keys);
getchar();
while ((c = getchar()) != EOF)
{
    c = c + keys;
    putchar(c);
}
puts("");

Answer 1

If I understand where you are going, your problem (the funny characters at the end) are due to your addition of c = c + keys; resulting in a character value greater than 126 (eg the '~' char). For example, if your keys is greater than 4 and you input 'z' , then your resulting c + keys is outside the valid range of ASCII characters. See ASCII Table and Description

Depending on what you want to do, you can simply use % (modulo) to ensure you always have a valid ASCII character before calling putchar() . Something like:

    while ((c = getchar()) != EOF) {
        c = (c + keys) % ('~' - ' ' + 1) + ' '; /* use modulo to ensure ASCII */
        putchar (c);
    }

( note: '~' - ' ' + 1 is simply the printable range of ASCII values -- 95 chars - thanks Roland)

Putting together a guess at a short example program that appears to be where you are going, you could do:

#include <stdio.h>

/* simple helper function to remove to end of line */
void empty_line (void)
{
    int c = getchar();

    while (c != '\n' && c != EOF)
        c = getchar();
}

int main (void) {

    int c, keys;

    if (scanf("%d", & keys) != 1 || keys < 0) { /* Validate EVERY Input! */
        fputs ("error: invalid or negative integer input.\n", stderr);
        return 1;
    }
    empty_line();

    while ((c = getchar()) != EOF) {
        c = (c + keys) % ('~' - ' ' + 1) + ' '; /* use modulo to ensure ASCII */
        putchar (c);
    }
    putchar ('\n');     /* not puts, you need 1-char, not string */
}

( note: you must Validate every input, especially when using scanf to perform a conversion to int -- or any other type)

Example Use/Output

$ ./bin/putcharmod
33
My dog has zero fleas, my cat has none :)
/[aFQIaJCUa\GTQaHNGCUmaO[aECVaJCUaPQPGa{jK

Above, despite having keys = 33 , inputting 'z' results in no funny characters as the total c + keys is reduced to within the printable range of characters.

Of course adjust the scheme to meet whatever your end objective happens to be, but regardless, if you are outputting to stdout with putchar() , you will need to do something similar to make sure what you output is printable.

Let me know if you have further questions.