C程序打印换行符

Question

below is my C program for K&R exercise 1-19.

#include <stdio.h>
#define MAXLINE 999

int main (void) {
    printf("Line Reverser.\n");
    printf("This program reverses the input per line.\n");
    middle ();
}
int middle (void) {
    reverse ();
    printf("\n");
    middle ();
}
int reverse (void) {
    int i, c;
    char s[MAXLINE] = {0};
    for (i = 0; ((c = getchar()) != '\n') && (i <= MAXLINE); ++i) {
        s[i] = c;
    };
    if (c == '\n') {
        s[i] = c;
        ++i;
    };
    for (i = MAXLINE; (s[i] != '0') && i >= 0; --i) {
        printf("%c", s[i]);
    };
    return 0;
}

When I run it using Code::Blocks, I get the following output if input is asdf.

Line Reverser.
This program reverses the input per line.
asdf










fdsa

I expected the output to be just fdsa, without the 10 newlines. I don't understand why there are 10 newlines that are being printed. What am I missing?

Edit: I know this can be done way more efficiently, but this was my first attempt and I'm taking it as a learning experience as to why the newlines are being printed.

Answer 1

Your code is mostly correct. At the end of reading the input, the variable i has saved the length of the input string, so just use that as the point to start reversing. You likely don't need to save the carriage return at the end of s since you don't want it to be at the beginning of the output. Note also that you need to declare reverse before you call it from main. So you can put the function declaration above main and leave the definition below, or move the entire function definition above main. So, something like this is correct:

#include <stdio.h>
#define MAXLINE 999

int reverse (void);

int main (void) {
    printf("Line Reverser.\n");
    printf("This program reverses the input per line.\n");
    reverse ();
}

int reverse (void) {
    int i, j, c;
    char s[MAXLINE] = {0};
    for (i = 0; ((c = getchar()) != '\n') && (i <= MAXLINE); ++i) {
        s[i] = c;
    }

    for (j = i; (s[j] != '0') && j >= 0; --j) {
        printf("%c", s[j]);
    }
    printf("\n");
    return 0;
}

Answer 2

When I run your program, I get a single newline.

$ ./test
Line Reverser.
This program reverses the input per line.
foo

oof$ 

There's a bunch of issues. First, the final prinf("\\n") never runs because it's immediately after a return . Thus the reversed line has no newline on it.

for (i = MAXLINE; (s[i] != '0') && i >= 0; --i) {
    printf("%c", s[i]);
};
return 0;
printf("\n"); // never runs

Second, the code explicitly puts the trailing newline back onto the s . This is why there's an extra newline between the input and its reversal.

if (c == '\n') {
    s[i] = (char)c;
    ++i;
};

We can see this with a simple debugging printf("s = '%s'\\n", s); .

$ ./test
Line Reverser.
This program reverses the input per line.
foo
s = 'foo
'

oof$

The simple fix is to not put the final newline one, and print the newline before the return.

int reverse (void) {
    int i, c;
    char s[MAXLINE] = {0};
    for (i = 0; ((c = getchar()) != '\n') && (i <= MAXLINE); ++i) {
        s[i] = (char)c;
    };
    for (i = MAXLINE; (s[i] != '0') && i >= 0; --i) {
        printf("%c", s[i]);
    };
    printf("\n");
    return 0;
}

$ ./test
Line Reverser.
This program reverses the input per line.
foo
oof
$

There's another bug, though it's not the cause of the problem. (s[i] != '0') is checking to see if s[i] is the character 0 not the number 0. With your test data, it effectively does nothing.

for (i = MAXLINE; (s[i] != '0') && i >= 0; --i) {
    printf("%d: %c\n", i, s[i]);
}

This loop only works because s is initialized to be all null bytes. Every character is printed, but they're all null. We can see this by printing i as well.

for (i = MAXLINE; (s[i] != '0') && i >= 0; --i) {
    printf("%d: %c\n", i, s[i]);
}

$ ./test
Line Reverser.
This program reverses the input per line.
foo
999: 
998: 
...
4: 
3: 
2: o
1: o
0: f

---

This can be made simpler and more efficient by reading from the end of the line, not the end of the buffer. We can remember how large the string is while we read.

#include <stdio.h>

void reverse(void) {
    // Use BUFSIZ from stdio
    char buf[BUFSIZ];

    // Read a line, but not the newline.
    // buf_idx remembers where the end of the line is.
    size_t buf_idx;
    int c;
    for(
        buf_idx = 0;
        ((c = getchar()) && (c != EOF) && (c != '\n') );
        buf_idx++
    ) {
        buf[buf_idx] = (char)c;
    }

    // A for loop always executes all its statements, even when the
    // condition fails, so we need to back up one.
    // Stick a null on the end of buf. It's not necessary, but just
    // in case we want to print it for debugging.
    buf[buf_idx--] = '\0';


    // Print the characters from the end to the start.
    for( int i = (int)buf_idx; i >= 0; i-- ) {
        printf("%c", buf[i]);
    }

    // Print a trailing newline.
    printf("\n");
}

BUFSIZ is a good size for a read buffer decided by the operating system. It's provided by stdio.h .

It's not necessary to spend the runtime initializing buf because we only ever read from it what we write. Though it's probably a good idea to stick a null on the end anyway.