can someone explain me why if i press s button or d button it throws me segmetation fault but with a button and w button it works fine.it is just a function to move player p in a square just like snake.
void move(){
int i;
int j;
char choice;
scanf("%s",&choice);
for (i = 0; i < rows; ++i) {
for (j = 0; j < cols; ++j) {
if(maze[i][j] == 'p' && choice == 'a') {
char tmp = maze[i][j];
maze[i][j] = maze[i][j-1];
maze[i][j-1] = tmp;
}
if(maze[i][j] == 'p' && choice == 'w'){
char tmp = maze[i][j];
maze[i][j] = maze[i-1][j];
maze[i-1][j] = tmp;
}
if(maze[i][j] == 'p' && choice == 'd') {
char tmp = maze[i][j];
maze[i][j] = maze[i][j+1];
maze[i][j+1] = tmp;
}
if(maze[i][j] == 'p' && choice == 's') {
char tmp = maze[i][j];
maze[i][j] = maze[i+1][j];
maze[i+1][j] = tmp;
}
}
}
}
Welcome to Stack Overflow, and welcome to programming!
Per related comments, the segfault is most likely due to overrunning the bounds of your maze[][] array. You ask in a couple of your comments how you can deal with this situation.
In the interest of providing the most constructive advice, instead of giving you code, I'm going to ask a question:
How, in programming, do we cause something to happen or not happen under specific conditions?
So, describe for yourself the conditions under which you should not be making references such as i + 1 or j - 1, and then add corresponding logic to your code.
I also encourage you to ask yourself:
How many times does my move() function alter the maze[][] for a single input?
Write down your answer, and then add output statements, or for example a show() function that prints out the total current state of maze[][], in the inner-most part of your loops in move(). See if your expected answer matches your actual output.
Regarding your description of this program that when you :
"press s button or d button it throws me segmentation fault but with a button and w button it works fine"
... I would say that it only appears to be working when you press the a button or the w button. In fact, whenever you reference outside the bounds of an array (which might happen no matter which of w, a, s, or d is pressed), the behavior is undefined. So, by chance, the way the program gets compiled (for you, at a particular time) might not segfault right away when pressing w. When you access outside the bounds of an array, you might access memory that is okay to access, and it will seem to work; but you might access memory that is off limits to your process, and it will segfault.
Bugs related to undefined behavior can be the most subtle to deal with. Logic to serve as guardrails, and debug output statements to show actual value changes can help quite a bit.
Just off the bat this code is incorrect
char choice;
scanf("%s",&choice);
Choice is one character - how can it hold a string of multiple characters?
This would be better
char choice[101];
scanf("%100s", choice);
Would enable a string of up to 100 characters
The 101 - nothing related to George Orwell - enables the null character to be added
You should also check the return value from scanf. Please read the manual page for scanf
BTW - I have not got to the read the rest of the code
Related
I wrote a program that counts and prints the number of occurrences of elements in a string but it throws a garbage value when i use fgets() but for gets() it's not so.
Here is my code:
#include<stdio.h>
#include<string.h>
#include<ctype.h>
#include<stdlib.h>
int main() {
char c[1005];
fgets(c, 1005, stdin);
int cnt[26] = {0};
for (int i = 0; i < strlen(c); i++) {
cnt[c[i] - 'a']++;
}
for (int i = 0; i < strlen(c); i++) {
if(cnt[c[i]-'a'] != 0) {
printf("%c %d\n", c[i], cnt[c[i] - 'a']);
cnt[c[i] - 'a'] = 0;
}
}
return 0;
}
This is what I get when I use fgets():
baaaabca
b 2
a 5
c 1
32767
--------------------------------
Process exited after 8.61 seconds with return value 0
Press any key to continue . . . _
I fixed it by using gets and got the correct result but i still don't understand why fgets() gives wrong result
Hurray! So, the most important reason your code is failing is that your code does not observe the following inviolable advice:
Always sanitize your inputs
What this means is that if you let the user input anything then he/she/it can break your code. This is a major, common source of problems in all areas of computer science. It is so well known that a NASA engineer has given us the tale of Little Bobby Tables:
Exploits of a Mom #xkcd.com
It is always worth reading the explanation even if you get it already #explainxkcd.com
medium.com wrote an article about “How Little Bobby Tables Ruined the Internet”
Heck, Bobby’s even got his own website — bobby-tables.com
Okay, so, all that stuff is about SQL injection, but the point is, validate your input before blithely using it. There are many, many examples of C programs that fail because they do not carefully manage input. One of the most recent and widely known is the Heartbleed Bug.
For more fun side reading, here is a superlatively-titled list of “The 10 Worst Programming Mistakes In History” #makeuseof.com — a good number of which were caused by failure to process bad input!
Academia, methinks, often fails students by not having an entire course on just input processing. Instead we tend to pretend that the issue will be later understood and handled — code in academia, science, online competition forums, etc, often assumes valid input!
Where your code went wrong
Using gets() is dangerous because it does not stop reading and storing input as long as the user is supplying it. It has created so many software vulnerabilities that the C Standard has (at long last) officially removed it from C. SO actually has an excellent post on it: Why is the gets function so dangerous that it should not be used?
But it does remove the Enter key from the end of the user’s input!
fgets(), in contrast, stops reading input at some point! However, it also lets you know whether you actually got an entire line of of text by not removing that Enter key.
Hence, assuming the user types: b a n a n a Enter
gets() returns the string "banana"
fgets() returns the string "banana\n"
That newline character '\n' (what you get when the user presses the Enter key) messes up your code because your code only accepts (or works correctly given) minuscule alphabet letters!
The Fix
The fix is to reject anything that your algorithm does not like. The easiest way to recognize “good” input is to have a list of it:
// Here is a complete list of VALID INPUTS that we can histogram
//
const char letters[] = "abcdefghijklmnopqrstuvwxyz";
Now we want to create a mapping from each letter in letters[] to an array of integers (its name doesn’t matter, but we’re calling it count[]). Let’s wrap that up in a little function:
// Here is our mapping of letters[] ←→ integers[]
// • supply a valid input → get an integer unique to that specific input
// • supply an invalid input → get an integer shared with ALL invalid input
//
int * histogram(char c) {
static int fooey; // number of invalid inputs
static int count[sizeof(letters)] = {0}; // numbers of each valid input 'a'..'z'
const char * p = strchr(letters, c); // find the valid input, else NULL
if (p) {
int index = p - letters; // 'a'=0, 'b'=1, ... (same order as in letters[])
return &count[index]; // VALID INPUT → the corresponding integer in count[]
}
else return &fooey; // INVALID INPUT → returns a dummy integer
}
For the more astute among you, this is rather verbose: we can totally get rid of those fooey and index variables.
“Okay, okay, that’s some pretty fancy stuff there, mister. I’m a bloomin’ beginner. What about me, huh?”
Easy. Just check that your character is in range:
int * histogram(char c) {
static int fooey = 0;
static int count[26] = {0};
if (('a' <= c) && (c <= 'z')) return &count[c - 'a'];
return &fooey;
}
“But EBCDIC...!”
Fine. The following will work with both EBCDIC and ASCII:
int * histogram(char c) {
static int fooey = 0;
static int count[26] = {0};
if (('a' <= c) && (c <= 'i')) return &count[ 0 + c - 'a'];
if (('j' <= c) && (c <= 'r')) return &count[ 9 + c - 'j'];
if (('s' <= c) && (c <= 'z')) return &count[18 + c - 's'];
return &fooey;
}
You will honestly never have to worry about any other character encoding for the Latin minuscules 'a'..'z'.Prove me wrong.
Back to main()
Before we forget, stick the required magic at the top of your program:
#include <stdio.h>
#include <string.h>
Now we can put our fancy-pants histogram mapping to use, without the possibility of undefined behavior due to bad input.
int main() {
// Ask for and get user input
char s[1005];
printf("s? ");
fgets(s, 1005, stdin);
// Histogram the input
for (int i = 0; i < strlen(s); i++) {
*histogram(s[i]) += 1;
}
// Print out the histogram, not printing zeros
for (int i = 0; i < strlen(letters); i++) {
if (*histogram(letters[i])) {
printf("%c %d\n", letters[i], *histogram(letters[i]));
}
}
return 0;
}
We make sure to read and store no more than 1004 characters (plus the terminating nul), and we prevent unwanted input from indexing outside of our histogram’s count[] array! Win-win!
s? a - ba na na !
a 4
b 1
n 2
But wait, there’s more!
We can totally reuse our histogram. Check out this little function:
// Reset the histogram to all zeros
//
void clear_histogram(void) {
for (const char * p = letters; *p; p++)
*histogram(*p) = 0;
}
All this stuff is not obvious. User input is hard. But you will find that it doesn’t have to be impossibly difficult genius-level stuff. It should be entertaining!
Other ways you could handle input is to transform things into acceptable values. For example you can use tolower() to convert any majuscule letters to your histogram’s input set.
s? ba na NA!
a 3
b 1
n 2
But I digress again...
Hang in there!
I am probably not going to get help here because my question is far from being specific (I don't even know what exactly wrong with it) but, according to my professor's tests, there is something wrong with it (wrong in terms of correctness - it doesn't provide correct number of direct and indirect matches) (I have no access to his tests). As far as I have been testing, it passes all of my tests. However, there are over a couple hundred million possible outcomes (I think) and I can't test them all because I don't know how to do automated testing...
Here is my code that performs the "logic" part of the game called mastermind, which is compares a string of randomly generated letter (8 max) with user input string (a guess). I wanted to see if anyone has encountered this game in the past and knows the logic of how it supposed to compare two strings and generate the correct number of exact and inexact guesses.
// userInput->position - a length of a string(max 8)
// userInput->code - randomly generated code
// userInput->arr - user input string
void checkForExactMatch(Data* userInput) {
int i;
for (i = 0; i < userInput->position; i++) {
if (userInput->code[i] == userInput->arr[i]) {
userInput->exactMatch++;
userInput->arr[i] = 'a';
}
else
checkForInExactMatch(userInput, i);
}
}
void checkForInExactMatch(Data* userInput, int i) {
int j;
for (j = 0; j < userInput->position; j++) {
if (userInput->arr[j] == userInput->code[i]) {
userInput->arr[j] = 'a';
userInput->inExactMatch++;
break;
}
}
}
Looking over your code there were a couple of observations to be made. First in your for checkForExactMatch() the call to checkForInExactMatch is inside your for loop. So on the first mismatch you call checkForInExactMatch and when you return from checkForInExactMatch -- you call it again on the next iteration unless your first mismatch just happens to be on the final character.
To address that issue, you should fully determine whether you have an exact match or not, completing the for loop before checkForInExactMatch is called.
In your checkForInExactMatch, you have to decide whether a single common-character or some minimum length substring constitutes an inexact match.
It sounds like you have things worked out, and good job for pushing through to a solution. Depending on how you approached it, keeping a simple flag in checkForExactMatch()such as int matched = 1; and then loop turning your test around
for (i = 0; i < userInput->position; i++)
if (userInput->code[i] != userInput->arr[i]) {
matched = 0;
break;
}
Then it's just a simple test of
if (matched) {
userInput->exactMatch++;
userInput->arr[i] = 'a';
}
else
checkForInExactMatch(userInput, i);
So long as what you have done accomplishes something similar, you are fine. Let me know if you have further questions.
I am making a little game using getch and print, as a bit of a test for myself, and surprisingly it actually works quite well but i'm having a bit of an issue. I am printing all of the tiles using a loop and printf, and of course as the loop has to process a bit, everytime the character moves, it re-prints everything, which causes a bit of a stutter, due to the loop printing. What can I possibly do to combat this?
This is how the tiles are printed
void Game() {
int X = 0;
int Y = 0;
int PrintWall = 0;
do {
for (int i = 0; i < 80; i++) {
X = i;
PrintWall = 0;
for (int j = 0; j < 12; j++) {
if (X == WallX[j]) {
if (Y == WallY[j]) {
PrintWall = 1;
}
}
}
if (X == Player.XCoor && Y == NegativeToPositive(Player.YCoor)) {
printf("#");
}
else if (PrintWall == 1){
printf("#");
}
else {
printf(".");
}
}
Y++;
} while (Y != 22);
}
You may want to try building the full screen of data into a buffer first and THEN print it. Start with an array of characters as long as you need it to be:
char buffer[SIZE_X * SIZE_Y];
Then go through your loop above, but instead of using printf(), set the character at that location in the buffer to what you want it to be:
buffer[(SIZE_X * Y) + X] = /* '#', '#', or '.' as appropriate */
Then, once you are through the loop, you will print the whole buffer to the screen:
printf(buffer);
Just make sure you don't accidentally overwrite your null terminator with another character or you may get more text than you bargained for.
That is just a limitation of printing the whole "board" on every change. The only way to fix it is to only print what changes. Depending on your OS etc you may be able to print individual characters or lines instead of the whole board.
You could look at a library like curses to give character level control (there may be something better - don't know, I don't do this often enough to know ;-)
As other answers have said, printing in bigger pieces (like a whole buffer at a time) is better than printf for each individual character but you will still probably have the same issue but it is certainly worth a try.
Consider separating initialization and the game loop. Draw the game board as init and update everything in your do-while-loop.
You should limit the number of moving walls.
If a wall moves, you delete/replace the wall mark # at the old position, write out the mark on your interface at the new place and then change the mark's data in the arrays.
After that, you just check, if the new player position hits an obstacle. If no, delete/replace the current mark of the player with the default character and write the new position.
If the two players hit each other, you would need your old condition.
My code is causing a segmentation fault when accessing an array element even though that element was already accessed without a problem.
int charToInt(char a)
{
int b;
if(isdigit(a))
{
b = a - '0' - 1;
}
if(isalpha(a))
{
b = a - 65;
}
return b;
}
int validPosition(char **array, int r, int c, char* position, int slots)
{
int i,k;
if(strlen(position) == 5)
{
if(!isalpha(position[0]) || !isdigit(position[1]) || position[2]!=' ' || (position[3]!='N' && position[3]!='E' && position[3]!='W' && position[3]!='S')) //lathos gramma
{
printf("\n%s", "Invalid answear.This is an example of a valid answear: A5 N");
return 2;
}
if( charToInt(position[0]) > r - 1 || charToInt(position[1]) > c - 1 )//ama vgainei eksw apo ta oria
{
printf("\n%s", "The position you choosed is out of the bountries...");
return 2;
}
printf("\n%s%c%s","position[3] is: ",position[3], " but it doesn't work >_<"); // position[3] is N
if(position[3] == 'N') //the problem is here <~~~~~~~~~~~~~~~~~~~<
{
printf("\n%s", "come on");
if(charToInt(position[0]) + slots < r)
{
for(i=charToInt(position[0])-1; i<charToInt(position[0])+slots; i++)
{
if(array[i-1][charToInt(position[1])-1] != '.')
{
printf("\n%s", "The position you choosed is not valid because there is oneother ship there");
return 2;
}
}
}
else
{
printf("\n%s", "The ship is going out of the bountries...");
return 2;
}
}
}
}
When position holds the string "A9 N", the printf correctly outputs 'N' for position[3]. For some reason when it tries to do if(position[3] == 'N'), however, a segmentation fault occurs.
Example program run:
Example of positioning: G3 E
Aircraft carrier (5 places), Give location and direction: A9 N
1
position[3] is: N but it doesn't work >_<
Well, based on your updates, it seems you have a variety of problems. For future reference, actually adding in the (possibly simplified) code showing how you were calling the function in question is better than trying to describe it using prose in a comment. There will be less guesswork for the people trying to help you.
If I'm reading your comment correctly, the code that calls validPosition looks something like this:
// "r and c are 9 and 9 in the specific example(rows columns)."
int rows = 9;
int columns = 9;
// "slots=5."
int slots = 5;
// "array is a 2d array and it contains characters(created with malloc)."
char **array = malloc(rows * columns * sizeof(char));
// "i created char position[10] in the function that called this function"
char position[10];
// "and with fgets(position, 10, stdin); i putted A9 N inside it."
fgets(position, 10, stdin);
validPosition(array, rows, columns, position, slots);
The first problem is your description of the allocation of array (I apologize if I misunderstood your comment and this isn't actually what you are doing). It should look similar to the code below for a dynamically sized two-dimensional array used with two subscripting operations (array[index1][index2], as it is in validPosition). Pointers-to-pointers (char **array) act differently than fixed sized multi-dimensional arrays (array[SIZE1][SIZE2]) when you access them that way.
// each entry in array should be a pointer to an array of char
char **array = malloc(rows * sizeof(char*));
for(i = 0; i < rows; i++)
array[i] = malloc(columns * sizeof(char));
You also need to be careful about using position after the fgets call. You should check the return value to make sure it isn't NULL (indicating an EOF or error condition). The string may not be \0-terminated in this case. In fact, all the elements may still be uninitialized (assuming you didn't initialized them before the call). This can lead to undefined behavior.
The next issue is that validPosition does not return a value on every code path. One example is if strlen(position) != 5. The other is if you enter the for loop and array[i-1][charToInt(position[1])-1] != '.' is never true (that is, the ship placement is deemed valid).
As strange as it is for an English speaker to say this to a Greek author, lets ignore internationalization and focus only on the default C local. The checks on position[0] should therefore be sufficient, though you might consider allowing your users to use lowercase letters as well. When converting position[1] from 1-based to 0-based, however, you do not account for the case when it is '0', which will result in charToInt returning -1. Furthermore, you're erroneously doing the subtraction again in the second array subscript of array[i-1][charToInt(position[1])-1].
Similarly, as pointed out by Jite and BLUEPIXY, you are doing two extra subtractions on the result of charToInt(position[0]): one in the for loop initializer (i=charToInt(position[0])-1) and one in the first array subscript of array[i-1][charToInt(position[1])-1].
Once you fix that, you might find that you are sometimes incorrectly telling the user that their selection is invalid. This is because you are checking charToInt(position[0]) + slots < r instead of <= r.
As I mentioned in my comment, one of the accesses to array is very probably the culprit behind your segmentation violation, not position[3] == 'N'. The reason you don't see the output of printf("\n%s", "come on"); is that your stdout appears to be line-buffered and there's no end of line to flush it. It is generally automatically flushed on normal program termination, however you're seg-faulting so that doesn't happen.
Finally, these are only the semantic errors I noticed. Stylistically, the code could also stand to be improved. For instance, it seems you're going to be implementing else if(position[3] == 'E', else if(position[3] == 'W', and else if(position[3] == 'S' clauses with similar logic to your if(position[3] == 'N' clause. This increases the likelihood you'll introduce an error by incorrectly copying-and-pasting and also increases your work later when you need to make a change in four places instead of one.
Since the terminology 'Segmentation Fault' I believe you are on Linux machine.
Use gdb to find the cause of error. Here are the steps.
Compile with additional -g flag (ex. gcc -g my_prog.c)
Run debugger: gdb a.out
Use 'list' command to find the line for break point (eg. first line of your function)
Set breakpoint on that line with: b 25 (if 25 is that line)
Run program with 'run' command
Use command 'next' to execute next line of code
Now the execution will pause on that line, you can examine memory, print variable contents
and stuff. But generally you want to determine on which line the execution fails and what was in which variable.
With a little playing with memory, you will easily find where the problem is. Personally, my code wont work with gdb support.
Perhaps segmentation fault at array[i-1][charToInt(position[1])-1]
i:charToInt(position[0])-1 : charToInt('A') - 1 : -1 <- Array out-of-bounds
I'm actually writing about the same program as before, but I feel like I've made significant progress since the last time. I have a new question however; I have a function designed to store the frequencies of letters contained within the message inside an array so I can do some comparison checks later. When I ran a test segment through the function by outputting all of my array entries to see what their values are, it seems to be storing some absurd numbers. Here's the function of issue:
void calcFreq ( float found[] )
{
char infname[15], alpha[27];
char ch;
float count = 0;
FILE *fin;
int i = 0;
while (i < 26) {
alpha[i] = 'A' + i++;
}
printf("Please input the name of the file you wish to scan:\n");
scanf("%s", infname);
fin = fopen ( infname, "r");
while ( !feof(fin) ) {
fscanf(fin, "%c", &ch);
if ( isalpha(ch) ) {
count += 1;
i = 0;
if ( islower(ch) ) { ch = toupper(ch); }
while ( i < 26 ) {
if ( ch == alpha[i] ) {
found[i]++;
i = 30;
}
i++;
}
}
}
fclose(fin);
i = 0;
while ( i < 26 ) {
found[i] = found[i] / count;
printf("%f\n", found[i]);
i++;
}
}
At like... found[5], I get this hugely absurd number stored in there. Is there anything you can see that I'm just overlooking? Also, some array values are 0 and I'm pretty certain that every character of the alphabet is being used at least once in the text files I'm using.
I feel like a moron - this program should be easy, but I keep overlooking simple mistakes that cost me a lot of time >.> Thank you so much for your help.
EDIT So... I set the entries to 0 of the frequency array and it seems to turn out okay - in a Linux environment. When I try to use an IDE from a Windows environment, the program does nothing and Windows crashes. What the heck?
Here are a few pointers besides the most important one of initializing found[], which was mentioned in other comments.
the alpha[] array complicates things, and you don't need it. See below for a modified file-read-loop that doesn't need the alpha[] array to count the letters in the file.
And strictly speaking, the expression you're using to initialize the alpha[] array:
alpha[i] = 'A' + i++;
has undefined behavior because you modify i as well as use it as an index in two different parts of the expression. The good news is that since you don't need alpha[] you can get rid of its initialization entirely.
The way you're checking for EOF is incorrect - it'll result in you acting on the last character in the file twice (since the fscanf() call that results in an EOF will not change the value of ch). feof() won't return true until after the read that occurs at the end of the file. Change your ch variable to an int type, and modify the loop that reads the file to something like:
// assumes that `ch` is declared as `int`
while ( (ch = fgetc(fin)) != EOF ) {
if ( isalpha(ch) ) {
count += 1;
ch = toupper(ch);
// the following line is technically non-portable,
// but works for ASCII targets.
// I assume this will work for you because the way you
// initialized the `alpha[]` array assumed that `A`..`Z`
// were consecutive.
int index = ch - 'A';
found[index] += 1;
}
}
alpha[i] = 'A' + i++;
This is undefined behavior in C. Anything can happen when you do this, including crashes. Read this link.
Generally I would advise you to replace your while loops with for loops, when the maximum number of iterations is already known. This makes the code easier to read and possibly faster as well.
Is there a reason you are using float for counter variables? That doesn't make sense.
'i = 30;' What is this supposed to mean? If your intention was to end the loop, use a break statement instead of some mysterious magic number. If your intention was something else, then your code isn't doing what you think it does.
You should include some error handling if the file was not found. fin = fopen(..) and then if(fin == NULL) handle errors. I would say this is the most likely cause of the crash.
Check the definition of found[] in the caller function. You're probably running out of bounds.