C multidimensional array size using scanf - c

I'm having some trouble using multidimensional arrays for a program. Essentially, the program uses scanf to read a user ID and a string of chars from a redirected file. the file format is a three digit user ID, a space, and a string of chars representing the answers to multiple choice problems on a test on each line, eg.
111 dabac
102 dcbdc
251 dbbac
The problem I'm running into is that I don't know how many users there are, and I can't read the file data multiple times. I've tried using
for (lineNumber = 0; lineNumber != -1; lineNumber++)
{
int result = scanf("%d ", &data);
if (result == EOF)
break;
for(i = 0; i < numProblems; i++)
{
scanf("%c", &input);
}
to get the number of lines in the file, then set the size of the array. The array is then passed to another function that reads the data, using the same for loop but with
input = arrayName[numProblems][lineNumber];
in the second for loop. The issue I'm running into is that scanf can only read the data from the file once, and I can't store the data in the array until I initialize it, which requires me to know how many users there are.
The way I have it set up, the program can either find the number of lines(users) or store the data in the array (if I set the size to an arbitrary number), but not both.
I have to use scanf because the filename isn't constant (and also this is for a class... the professor requires scanf to be used), and I can't figure out how to get the number of lines in the file and still be able to read the data. If anyone knows of a workaround to either find the number of lines without using scanf, or to read the data twice, I would really appreciate some help. If it would help to post the entire program, I can do that as well.
Thank you,
Erik

There are multiple ways to do this. I would recommend a linked list using structs, and keeping a count of what has been read so far.
If you really want to continue using arrays, allocate the array using malloc first, and then reallocate the array using realloc. You'll have a running count of the size of the array for reallocation.
it should be something like this:
POINTER *array = realloc(orig_array_pt, size);
if (array == NULL)
{
// realloc failed
}
else
{
orig_array_pt = array;
}

As long as the input is an actual file (and not a terminal or some other device), you can use rewind(stdin); to set the FILE pointer back to the beginning of the file...

Luckily scanf() will parse the whole line into multiple variables in one pass. Try something more like
scanf( "%d %s", &uid, answers );
Just count the number of times you're able to successfully parse data out of the stdin and call that your lineNumber count ( if you even need to know the total number of lines ).

Related

Stack Smashing and using malloc

I'm making a program that counts the number of words contained within a file. My code works for certain test cases with files that have less than a certain amount of words/characters...But when I test it with, let's say, a word like:
"loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong", (this is not random--this is an actual test case I'm required to check), it gives me this error:
*** stack smashing detected ***: ./wcount.out terminated
Abort (core dumped)
I know what the error means and that I have to implement some sort of malloc line of code to be able to allocate the right amount of memory, but I can't figure out where in my function to put it or how to do it:
int NumberOfWords(char* argv[1]) {
FILE* inFile = NULL;
char temp_word[20]; <----------------------I think this is the problem
int num_words_in_file;
int words_read = 0;
inFile = fopen(argv[1], "r");
while (!feof(inFile)) {
fscanf(inFile, "%s", temp_word);
words_read++;
}
num_words_in_file = words_read;
printf("There are %d word(s).\n", num_words_in_file - 1);
fclose(inFile);
return num_words_in_file;
}
As you've correctly identified by rendering your source code invalid (future tip: /* put your arrows in comments */), the problem is that temp_word only has enough room for 20 characters (one of which must be a terminal null character).
In addition, you should check the return value of fopen. I'll leave that as an exercise for you. I've answered this question in other questions (such as this one), but I don't think just shoving code into your face will help you.
In this case, I think it may pay to better analyse the problem you have, to see if you actually need to store words to count them. As we define a word (the kind read by scanf("%s", ...) as a sequence of non-whitespace characters followed by a sequence of (zero or more) whitespace characters, we can see that such a counting program as yours needs to follow the following procedure:
Read as much whitespace as possible
Read as much non-whitespace as possible
Increment the "word" counter if all was successful
You don't need to store the non-whitespace any more than you do the whitespace, because once you've read it you'll never revisit it. Thus you could write this as two loops embedded into one: one loop which reads as much whitespace as possible, another which reads non-whitespace, followed by your incrementation and then the outer loop repeats the whole lot... until EOF is reached...
This will be best achieved using the %*s directive, which tells scanf-related functions not to try to store the word. For example:
size_t word_count = 0;
do {
fscanf(inFile, "%*s");
} while (!feof(inFile) && ++word_count);
You are limited by the size of your array. A simple solution would be to increase the size of your array. But you are always susceptible to stack smashing if someone enters a long word.
A word is delimited by spaces.
You can simply store a counter variable initialized to zero, and a variable that records the current char that you are looking at. Every time you read in a character using fgetc(inFile, &temp) that is a space, you increment the counter.
In your current code you simply want to count the words. Therefore you are not interested in the words themselves. You can suppress the assignment with the optional * character:
fscanf(inFile, "%*s");

Trying to read text file data into an array of structs?

Here is what I want to do:
Read my 2 text files into an "array of structs" (this is how it is worded on my assignment).
Dynamically create sufficient memory for each entry read from the file
Here is one of the structs I am working with:
typedef struct {
int eventid;
char eventdate[20];
char venuename[20];
char country[20];
int rockid;
} Venue;
Within my main function I have the array setup to receive the text as:
Venue *(places[20]);
Now comes the more complex part. I need to open the file for reading (I got this to work perfectly) and then dynamically allocate the memory for each entry. I know I need to use malloc to do this but I have never used it before and am sort of at a loss. Here is what I have so far:
void load_data(void)
{
char buffer[20]; //stating that each line can't be longer than 20 chars
int i = 0,len; //declaring 2 int variables
FILE * venuePtr=fopen("venueinfo.txt", "r");
if (venuePtr != NULL)
printf("\n**Venue info file has been opened!**\n");
else{
printf("\nPlease create a file named venueinfo.txt and restart!\n");
} //so far so good...
while (!feof(venuePtr)){ //while we have not found the eof key...
fscanf(venuePtr,"%s",buffer); //we scan each line of text
len = strlen(buffer); //find the length (len) of the string
places[i]=(char*)malloc(len+1); //allocate memory space for the word here
strcpy(places[i],buffer); //copy a word into our array
++i; //finally we move on to the next element in the array
} //end while
The problem resides in the while loop and I have been working this for 2 days straight. I have 5 members in my struct and am thinking that strcpy may not work. This is only part of the problem though I am sure. I just can't wrap my head around reading in everything. The file itself is a super simple txt file and looks like this:
1 Jan10 Citadel Belgium 8
4 May05 Sunrise Belize 6
3 Jun17 Footloose Brazil 4
you are trying to copy string into the venue structure, how do you expect that to work ?
strcpy(venue[i],buffer);
please give an example of your file, you probably need to parse each element and write it to structure members
Stuff you can skip but should eventually do:
use function(void), not just empty() for functions. visual studio, which you are using allows it, but it's bad format.
declaring global variables is likewise bad form. You want to declare them in main and pass them in.
finally, you want to return. return 0 from a successful run of main. If you have a void function, still, return; before the final closing '}' character of the function.
oh and fscan_s isn't portable, it's a Microsoft function.
stuff you actually asked about:
Now, onto your problem. Don't allocate memory and assign it. You have statically allocated memory for your structures, and for your strings, by making them arrays with a given number of characters. If you want to statically allocate memory, you need to use a pointer.
if you scan the first number, the rock id into id, you would assign the first venue's rock id as,
venue[0].rockid = id;
for arrays, you do have to string copy. You already allocated memory for them, so you just have to use strcpy.
but you can't just copy strings into the structure and have it all end up in the right place. You need to get each part and add it seperately
That means you either need to read in each element separately, like "%d%s" to read an int then a string, or whatever, or you need to split up your string after reading in the whole thing. NOTE
That %s won't read the whole line!!! It will stop at the first white-space character (new line, tab, even a space) so if you %s "hi there" you get "hi". You may want to use %[^\n] which while characters don't match \n.
My suggestion is to use fscanf with multiple items, but if you need to split the string up, you want to use sscanf, which lets you scan a string again.
Finally, you don't need to test for feof, and that's actually problematic. It's far better to use while (fscanf(parameters go in here) > 0) since EOF is generally -1 and 0 means no items were scanned. Either way, you are done reading.
I suggest you start small. It looks like you are trying to jump ahead without understanding fundamentals, and C is kinda brutal about needing to know those.
Good luck.
P.S. It's very possible I made a small mistake here just now, writing this, because I'm not a C expert, but I'm sure somebody will come along and help me. Finding mistakes is how we learn, so don't get discouraged. :)

How can I read a specific line from a file, in C?

All right: So I have a file, and I must do things with it. Oversimplifying, the file has this format:
n
first name
second name
...
nth name
random name
do x⁽¹⁾, y⁽¹⁾ and z⁽¹⁾
random name
do x⁽²⁾, y⁽²⁾, z⁽²⁾
...
random name
do x⁽ⁿ⁾, y⁽ⁿ⁾, z⁽ⁿ⁾
So, the actual details are not important.
The problem is: I'll have to declare a variable n, I have an array name[MAX], and I'll fill this array with the names, from name[0] to name[n-1].
Alright, the problem is: How can I get this input, if I don't know previously how many names do I have?
For example, I could do it just fine if that was an user input, from the keyboard: I would do it like this:
int n; char name[MAX];
scanf( "%d", &n);
int i; for (i = 0; i < n; i++)
scanf( "%s", &N[i]);
And I could go on, do the whole code, but you get the point. But, my input now comes from a file. I don't know how can I get the input, all I can do is to fscanf() the whole file, but since I don't know its size (the first number will determine it), I can't do it. As far as I know (please correct me if that's not true, I am very new to this), we can't use the command "for" and get the numbers gradually as if that was coming from the keyboard, right?
So, the only exit I see is to find a way to read a particular line from the file. If I can do this, the rest is easy. The thing is, how can I do that?
I google'd it, I even found some questions in there, though it didn't make any sense at all. Apparently, reading a particular line from a file is really complicated.
This is from a beginner problem set, so I doubt it is something that complicated. I must be missing something very simple, though I just don't know what it is.
So, the question is: How would you do it, for instance?
How to scan the first number n from the file, and then, scan the others 'n' names, assigning each one to an element in an array (first name = name[0], last name = name[n - 1])?
I would suggest looking into End Of File.
while(!eof(fd))
{
...code...
}
Mind you my C knowledge is rusty, but this should get you started.
IIRC eof returns a value (-1) so that's why you need to compare it to something. Here fd being file descriptor of the file you are reading.
Then after parse of text or count of lines you have your 'n'.
EDIT: Since I'm obviously more tired then I thought(didn't notice your 'n' at the top).
Read first line
malloc for 'n' size array
for loop to iterate names.
Here you go.. I leve compiling and debugging as an exercise for the student.
The idea is to slurp the whole file into a single array if you files are always small.
This is so much more efficient than scanf().
char buf[100000], *bp, *N[1000]; // plenty big
memset( buf, '\0', sizeof buf );
if ( fgets( buf, sizeof(buf), fd ) )
{
int n = 0;
char *bp;
if ( buf[(sizeof buf)-2)] != '\0' )
{ // file too long for buffer
printf( stderr, "trouble: file too large: %d\n", (int)(sizeof buf));
exit(EXIT_FAILURE);
}
// now replace each \n with a \0, remembering where each line is.
for ( bp = buf, bp = strchr( bp, '\n' ); bp++ )
N[n++] = bp;
}
If you want to read any size files you need to read the file in chunks, calloc()ing each chunk before a read, and carefully handling of the line fragments left at the end of the current buffer to move them to the next buffer and then properly continuing you reads.
Unless you have a limit on how many lines you can read the N may need to also be set up in chunks, but this time remalloc() might be your friend.
Since the given format seems to imply that the number of names n is given as the first entry in the file, it would be possible to use the style of reading that the OP describes when reading from stdin. Use fscanf to read the first integer from the file (n), then use malloc to allocate the array(s) for the names, then use a for loop up to n to read the names.
However, I am unsure of the meaning of the example data following that with the do x⁽¹⁾, y⁽¹⁾ and z⁽¹⁾ format. Perhaps I am not understanding part of the question. If it means there are potentially more than n names, then you can use realloc to grow the size of the array. One way of growing the array that is not uncommon is to double the length each time.

Why is this C code giving me a bus error?

I have, as usual, been reading quite a few posts on here. I found a particular useful posts on bus errors in general, see here. My problem is that I cannot understand why my particular code is giving me an error.
My code is an attempt to teach myself C. It's a modification of a game I made when I learned Java. The goal in my game is to take a huge 5049 x 1 text file of words. Randomly pick a word, jumble it and try to guess it. I know how to do all of that. So anyway, each line of the text file contains a word like:
5049
must
lean
better
program
now
...
So, I created an string array in C, tried to read this string array and put it into C. I didn't do anything else. Once I get the file into C, the rest should be easy. Weirder yet is that it complies. My problem comes when I run it with ./blah command.
The error I get is simple. It says:
zsh: bus error ./blah
My code is below. I suspect it might have to do with memory or overflowing the buffer, but that's completely unscientific and a gut feeling. So my question is simple, why is this C code giving me this bus error msg?
#include<stdio.h>
#include<stdlib.h>
//Preprocessed Functions
void jumblegame();
void readFile(char* [], int);
int main(int argc, char* argv[])
{
jumblegame();
}
void jumblegame()
{
//Load File
int x = 5049; //Rows
int y = 256; //Colums
char* words[x];
readFile(words,x);
//Define score variables
int totalScore = 0;
int currentScore = 0;
//Repeatedly pick a random work, randomly jumble it, and let the user guess what it is
}
void readFile(char* array[5049], int x)
{
char line[256]; //This is to to grab each string in the file and put it in a line.
FILE *file;
file = fopen("words.txt","r");
//Check to make sure file can open
if(file == NULL)
{
printf("Error: File does not open.");
exit(1);
}
//Otherwise, read file into array
else
{
while(!feof(file))//The file will loop until end of file
{
if((fgets(line,256,file))!= NULL)//If the line isn't empty
{
array[x] = fgets(line,256,file);//store string in line x of array
x++; //Increment to the next line
}
}
}
}
This line has a few problems:
array[x] = fgets(line,256,file);//store string in line x of array
You've already read the line in the condition of the immediately preceding if statement: the current line that you want to operate on is already in the buffer and now you use fgets to get the next line.
You're trying to assign to the same array slot each time: instead you'll want to keep a separate variable for the array index that increments each time through the loop.
Finally, you're trying to copy the strings using =. This will only copy references, it won't make a new copy of the string. So each element of the array will point to the same buffer: line, which will go out of scope and become invalid when your function exits. To populate your array with the strings, you need to make a copy of each one for the array: allocate space for each new string using malloc, then use strncpy to copy each line into your new string. Alternately, if you can use strdup, it will take care of allocating the space for you.
But I suspect that this is the cause of your bus error: you're passing in the array size as x, and in your loop, you're assigning to array[x]. The problem with this is that array[x] doesn't belong to the array, the array only has useable indices of 0 to (x - 1).
You are passing the value 5049 for x. The first time that the line
array[x] = ...
executes, it's accessing an array location that does not exist.
It looks like you are learning C. Great! A skill you need to master early is basic debugger use. In this case, if you compile your program with
gcc -g myprogram.c -o myprogram
and then run it with
gdb ./myprogram
(I am assuming Linux), you will get a stack dump that shows the line where bus error occurred. This should be enough to help you figure out the error yourself, which in the long run is much better than asking others.
There are many other ways a debugger is useful, but this is high on the list. It gives you a window into your running program.
You are storing the lines in the line buffer, which is defined inside the readFile function, and storing pointers to it in the arary. There are two problems with that: you are overwriting the value everytime a new string is read and the buffer is in the stack, and is invalid once the function returns.
You have at least a few problems:
array[x] = fgets(line,256,file)
This stores the address of line into each array element. line in no longer valid when readFile() returns, so you'll have an array of of useless pointers. Even if line had a longer lifetime, it wouldn't be useful to have all your array elements having the same pointer (they'd each just point to whatever happened to be written in the buffer last)
while(!feof(file))
This is an antipattern for reading a file. See http://c-faq.com/stdio/feof.html and "Using feof() incorrectly". This antipattern is likely responsible for your program looping more than you might expect when reading the file.
you allocate the array to hold 5049 pointers, but you simply read however much is in the file - there's no checking for whether or not you read the expected number or to prevent reading too many. You should think about allocating the array dynamically as you read the file or have a mechanism to ensure you read the right amount of data (not too little and not too much) and handle the error when it's not right.
I suspect the problem is with (fgets(line,256,file))!=NULL). A better way to read a file is with fread() (see http://www.cplusplus.com/reference/clibrary/cstdio/fread/). Specify the FILE* (a file stream in C), the size of the buffer, and the buffer. The routine returns the number of bytes read. If the return value is zero, then the EOF has been reached.
char buff [256];
fread (file, sizeof(char), 256, buff);

Provide integer arrays as input from command line

I'm writing a program to access a bin packing algorithm function from a library, I haven't done much with it since college so my C is a bit rusty. The function I'm calling requires I pass in 3 different integer arrays. I'll be calling this from the command line. Should I use argv? Or STDIN? The input arrays could potentially be 50 to 100 elements each. Either way I suppose I will have to write something to parse the strings and get them into arrays, is there an easy way to do that?
For big arrays, I'd rather use standard input, as there are usually operating system limits to how many arguments you can have.
You will also need some kind of input format. Let's say the first number n is the number of elements in the first array, followed by the element values, and so on. Then I'd do something like:
#include <stdlib.h>
#include <stdio.h>
int main()
{
// you need to implement read_number yourself
int n = read_number(stdin);
// allocate array
int *array = (int*) malloc(n*sizeof(int));
// read n numbers into array
for ( int i=0; i < n; ++i )
array[i] = read_number(stdin);
// and so on...
}
You get the general idea. You'd either have to implement read_number yourself or find examples on the net on how to do it. You will need to discern individual numbers somehow, e.g. by parsing each digit up to the next white space character. Then you can separate each digit on stdin by space characters.
For instance, you can use #ypnos suggested scanf solution below.
For that amount of elements you should use stdin. Nobody will type them in by hand anyway and ./program < file is as easy as it gets.
The parsing is no big deal with scanf. Just define that your input should include the number of elements before all the numbers forming an array. Then you can scanf("%d", &elemcount), and then for-loop over elemcount, again using scanf.
The beauty of it is that scanf will deal with all the whitespaces, newlines etc. the user may put in between the numbers of elements and the other numbers.

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