I've got a C program that encounters errors when I enter a while loop.
I initialize a variable (fragmentcount) and write into it using fscanf and assign it a value of 4 (this works)
int fragmentCount;
if ((fscanf(fp, "%i", &fragmentCount)) == 1) {
...
}
However, when I try to access it in a while loop below, fragmentCount = 0
while ((fscanf(fp, "%[#]", discards)) != EOF) {
printf(fragmentCount); // <- pseudocode
}
For a brief experiment, I tried taking away the fscanf as the conditional test for the while loop, and fragmentCount was the correct value (4).
Why is this so? How can I avoid this?
How is discards declared? It is possible that fscanf is reading more data than discards has room for, which might overwrite the value of other variables.
Using the '%[' format without a field width is a bad idea - it leaves your program open to buffer overflow errors.
fscanf reads a value from a file and interprets it according to the format string. The '%i' format string is unknown (perhaps you meant '%d'?) according to http://www.cplusplus.com/reference/clibrary/cstdio/fscanf/, so you are unlikely to read the value you expect.
Apart from file FILE* and the format string, all parameters to fscanf are out parameters, which means the value they contain before the call to fscanf are irrelevant and could be replaced.
Related
I have this .txt file that contains only:
THN1234 54
How can I take only the number 54, to isolate it from the rest and to use it as an integer variable in my program?
If the input is from standard input, then you could use:
int value;
if (scanf("%*s %d", &value) != 1)
…Oops - incorrectly formatted data…
…use value…
The %*s reads but discards optional leading blanks and a sequence of one or more non-blanks (THN1234); the blank skips more optional blanks; the %d reads the integer, leaving a newline behind in the input buffer. If what follows the blank is not convertible to a number, or if you get EOF, you get to detect it in the if condition and report it in the body of the if.
Hmmm…and I see that BLUEPIXY said basically the same (minus the explanation) in their comment, even down to the choice of integer variable name.
Wow. It's been a long time since I have used C. However, I think the answer is similar for C and C++ in this case. You can use strtok_r to split the string into tokens then take the second token and parse it into an int. See http://www.cplusplus.com/reference/clibrary/cstring/strtok/.
You might also want to look at this question as well.
I have a small program I'm writing to practice programming in C.
I want it to use the getchar(); function to get input from the user.
I use the following function to prompt for user input, then loop using getchar() to store input in an array:
The function is passed a pointer referencing a struct's member.
getInput(p->firstName); //The function is passed an argument like this one
void getInput(char * array)
{
int c;
while((c=getchar()) != '\n')
*array++ = c;
*array = '\0'; //Null terminate
}
This function is called multiple times, as it is a part of a function that creates a structure, and populates it's array members.
However when the program executes, The first two calls to it work fine, but any subsequent calls to this function will cause every-other call to getchar() to not wait for keyboard input.
After some debugging I traced the bug to be that getchar(); was for some reason reading in the '\n' character instead of waiting for input, the while loop test fails, and the function returns essentially an empty string.
I have done some research and keep finding to use
while(getchar() != '\n');
at the end of the function in order to properly flush stdin, however, this produces undesirable results, as the program will prompt again for more input after I type ENTER. Pressing ENTER again continues the program, but every-other subsequent calls continue to read in this mysterious '\n' character right off the bat, causing the test to fail, and resulting in empty strings whenever it comes time to print the contents of the the structure.
Could anyone explain to me what is going on here? Why does getchar() keep fetching a '\n' even though I supposedly cleared the input buffer? I have tried just placing a getchar(); statement at the beginning and end of the function, tried 'do while' loops, and taken other jabs at it, but I can't seem to figure this out.
The code you have written has several drawbacks. I'll try to explain them as it is unclear where your code is failing (probably outside the function you posted)
First of all, you don't check for EOF in getchar() result value. getchar(3) doesn't return a char precisely to allow to return al possible char values plus an extra one, EOF, to mark the end of file (this can be generated from a terminal by input of Ctrl-D in unix, or Ctrl-Z on windows machines) That case must be explicitly contempled in your code, as you'll convert the result to a char and will lose the extra information you received from the function. Read getchar(3) man page to solve this issue.
Second, you don't check for input of enough characters to fill all the array and overflow it. To the function you pass only a pointer to the beginning of the array, but nothing indicates how far it extends, so you can be overfilling past the end of its bounds, just overwritting memory that was not reserved for input purposes. This normally results in something called U.B. in the literature (Undefined Behaviour) and is something you must care of. This can be solved by passing a counter of valid positions to fill in the array and decrementing it for each valid position filled. And not allowing more input once the buffer has filled up.
On other side, you have a standar function that does exactly that, fgets(3) just reads one string array from an input file, and stores it on the pointer (and size) you pass to it:
char *fgets(char *buffer, size_t buffer_size, FILE *file_descriptor);
You can use it as in:
char buffer[80], *line;
...
while (line = fgets(buffer, sizeof buffer, stdin)) {
/* process one full line of input, with the final \n included */
....
}
/* on EOF, fgets(3) returns NULL, so we shall be here after reading the
* full input file */
There is more to this code obviously but I am just curious as to what this line of code actually does. I know the while loop and such but am new to the fscanf()
while (fscanf(input_file, "%s", curr_word) == 1)
fscanf() returns the number of input items successfully scanned and stored.
as per the man page
Return Value
These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.
In your case
while (fscanf(input_file, "%s", curr_word) == 1)
fsaacf() will return a value of 1 if it is able to successfully scan a string (as per the %s format specifier) from input_file and put it into curr_word.
fscanf(input_file, "%s", curr_word) reads the input stream input_file and stores the next sequence of non spacing characters into the array pointed to by cuur_word and appends a '\0' byte. As you can see, the size of this array is not passed to fscanf. This is a classical case of potential buffer overflow, a security flaw that can be exploited by a hacker by storing appropriate contents in the input stream.
After gets, the scanf family of library functions is the best source of buffer overflow bugs one can find.
It is very difficult to use fscanf correctly. Most C programmers should avoid it.
Hello i have simply function to read from file
while(fscanf(fp," %255[a-zA-Z]",test) == 1)
{
puste = 1;
push(&drzewo,test);
}
It should read only words which contains only alphabetic characters and that works great. When I have for example a single number in my file my while loop quits; how should I change it?
Of course it stops, since the fscanf() call will fail to do the conversion you're requiring, and thus return 0. What would you expect it to do?
It's often better to read whole lines using fgets(), and then parse them "manually", that way it's easy to just do nothing and read another line if the desired data is not found.
I am having trouble accepting input from a text file. My program is supposed to read in a string specified by the user and the length of that string is determined at runtime. It works fine when the user is running the program (manually inputting the values) but when I run my teacher's text file, it runs into an infinite loop.
For this example, it fails when I am taking in 4 characters and his input in his file is "ABCDy". "ABCD" is what I am supposed to be reading in and 'y' is supposed to be used later to know that I should restart the game. Instead when I used scanf to read in "ABCD", it also reads in the 'y'. Is there a way to get around this using scanf, assuming I won't know how long the string should be until runtime?
Normally, you'd use something like "%4c" or "%4s" to read a maximum of 4 characters (the difference is that "%4c" reads the next 4 characters, regardless, while "%4s" skips leading whitespace and stops at a whitespace if there is one).
To specify the length at run-time, however, you have to get a bit trickier since you can't use a string literal with "4" embedded in it. One alternative is to use sprintf to create the string you'll pass to scanf:
char buffer[128];
sprintf(buffer, "%%%dc", max_length);
scanf(buffer, your_string);
I should probably add: with printf you can specify the width or precision of a field dynamically by putting an asterisk (*) in the format string, and passing a variable in the appropriate position to specify the width/precision:
int width = 10;
int precision = 7;
double value = 12.345678910;
printf("%*.*f", width, precision, value);
Given that printf and scanf format strings are quite similar, one might think the same would work with scanf. Unfortunately, this is not the case--with scanf an asterisk in the conversion specification indicates a value that should be scanned, but not converted. That is to say, something that must be present in the input, but its value won't be placed in any variable.
Try
scanf("%4s", str)
You can also use fread, where you can set a read limit:
char string[5]={0};
if( fread(string,(sizeof string)-1,1,stdin) )
printf("\nfull readed: %s",string);
else
puts("error");
You might consider simply looping over calls to getc().