小C程序不起作用

Question

I tried to make a program that gets a user input(lines) and prints the longest line that is over 80 characters long. I made the program , but when i ran it , it outputed some very weird symbols. Could you please tell me what might be wrong with my code ?

#include <stdio.h>
#define MINLINE 80
#define MAXLINE 1000

int getline(char current[]);
void copy(char from[], char to[]);

int main(void)
{
    int len; // current input line lenght
    int max; // the lenght of a longest line that's over 80 characters
    char current[MAXLINE]; // current input line
    char over80[MAXLINE]; // input line that's over 80 characters long

    while (len = (getline(current)) > 0) {
        if (len > MINLINE) {
            max = len;
            copy(current, over80);
        }
    }

    if (max > 0) {
        printf("%s", over80);
    }
    else {
        printf("No input line was over 80 characters long");
    }

    return 0;
}

int getline(char current[]) {
    int i = 0, c;
    while (((c = getchar()) != EOF) && c != '\n') {
        current[i] = c;
        ++i;
    }

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

    current[i] = '\0';

    return i;
}

void copy(char from[], char to[]) {
    int i = 0;
    while ((to[i] = from[i]) != '\0') {
        ++i;
    }
}

Thank you very much for your help !

Answer 1

max can be not initialized if no long line is found. Using it in if (max > 0) is then undefined behavior.

Answer 2

This line:

  while (len = (getline(current)) > 0) {

assigns the value of (getline(current)) > 0) to len , which is not what you want (len will be 0 or 1 afterwards.

EDIT: Just saw AusCBloke's comment, you should also check for both len > max and len > MINLINE or you'll just get the latest line longer than 80 chars, not the longest overall line.

You should also initialize max to 0, so it should be

  max = 0;
  while ((len = getline(current)) > 0) {
    if ((len > MINLINE) && (len > max)) {

Other minor errors/tips:

• The built in functions strcpy and strncpy do what your copy function does, there's no need to reinvent the wheel.
• In your getline function, use MAXLINE to prevent buffer overflows.

Answer 3

Assuming that this is a homework, here's a hint: this piece of code looks very suspicious:

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

Since i represents a position and is never assigned a character, you are effectively checking if the position is equal to the ASCII code of '\\n' .

Answer 4

Your copy method doesn't null terminate the string:

void copy(char from[], char to[]) {
    int i = 0;
    while ((to[i] = from[i]) != '\0') {
        ++i;
    }
    to[i] = '\0'
}

which probably explains the weird characters being printed.

You could use the builtin strcpy() to make life easier.

Answer 5

If you supply input data that has lines with more than 1000 characters you will overflow your fixed size buffers. By feeding in such input I was able to achieve the following output:

╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠ ╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠ ╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠

Answer 6

I can't test your code right now, but it may be caused by character arrays not being cleaned. Try memset-ing the char arrays to 0.

Answer 7

There are a number of problems with your code. Mostly they are due to wheel-reinvention.

int getline(char current[]);

You don't need to define your own, getline() , there is already one in stdio.h .

void copy(char from[], char to[]);

There are also a number of functions for copying strings in string.h .

It's also a good idea to initialise all 0f your variables, like this:

    int len = 0; // current input line length

...this can prevent problems later, like comparisons to max when you haven't initialised it.

If you initialise max like this...

    int max = MINLINE; // the length of a longest line that's over 80 characters

...then it's easier to do the length comparison later on.

    char* current = NULL;
    size_t allocated = 0;

If current is NULL , then getline() will allocate a buffer for storing the line, which should be free d by the user program. getline() also takes a pointer to a size_t , which contains the amount of bytes needed to store the line.

    while (len = (getline(current)) > 0) {

Should be replaced by the following...

    while ((len = getline(&current, &allocated, stdin)) > 0) {

...which updates and compares len to 0 .

Now, instead of...

        if (len > MINLINE) {

...you need to compare with the last longest line, which we initialised earlier...

        if (len > max) {

...and then you're good to update max as you were...

            max = len;

Where you called your copy() use strncpy() , which will prevent you writing over 1,000 characters into the allocated buffer:

            strncpy(over80, current, MAXLINE);

Because we initialised max , you'll need to change your check at the end from if (max > 0) to if (max > MINLINE) .

One more tip, changing the following line...

        printf("No input line was over 80 characters long");

...to...

        printf("No input line was over %d characters long", MINLINE);

...will mean that you only have to change the #define at the top of the file to increase or decrease the minimum length.

Don't forget to...

    free(current);

...to prevent memory leaks!