small C program doesnt work - c

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

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 6

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 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 freed 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!