How to read a data point from a text file in C - c

I have a text file full of points of the following format on different lines
LONG,LONG
i can successfully read each line and print it out, but I how can I parse the string in C such that I get each long of each point on its own?
Thanks!

if you have the line already, it's easiest to use sscanf() to do this:
long a, b;
if(sscanf(line, "%ld,%ld", &a, &b) == 2)
{
/* Successfully parsed two long integers, now store them somewhere I guess. */
}
Note that it's a good idea to check the return value of sscanf(), this protects you from wrongly accepting illegal data and getting undefined results.
You can do it in multiple steps too if you need more control, as #dasblinkenlights suggested. You can use strtol() to parse the first number from the start of the line, then if that succeeds look for the comma, and then parse the second number. It can be faster than sscanf(), but I wouldn't expect too much for something this simple.

There are many solutions to this.
One is to read the line, read the first long with strtol find the position of the comma that follows with strchr, and read the second number from there.
Another solution would be to read the line, and pass it to sscanf function with the format that accepts two comma-separated LONGs.

Use the string variant of scanf() if you say you've already got the line:
char* line;
long long1;
long long2;
sscanf(line, "%ld,%ld", &long1, &long2);
Indeed as #unwind suggests in his +1 answer, it's a very good idea to check the return value of scant(), which is the number of successfully read values.

Related

take a specific number from a txt file in c program

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.

fprintf prints a new line at the beginning of the file

I'm using a fprintf function to print to a new file
I'm using the following command to write multiple times:
fprintf(fp, "%-25s %d %.2f %d",temp->data.name, temp->data.day, temp->data.temp, temp->data.speed);
The problem is that sometimes the file gets an extra new line as the first character.
Could this be lelftovers from some buffer, I don't really know...
typedef struct Data {
char name[26];
int day;
int speed;
float temp;
} Data ;
#spatz you were right, I'm kind of new to the string format thing and I was told to make one for a fscanf where I was to expect an undetermined amount of space between the bits of data, here is what I came up with, I'm pretty sure its the source of the problem:
check=fscanf(fp1, "%20c%*[^0-9]%d%*[^0-9]%f%*[^0-9]%d%*[^\n]%*c", name, &day, &temp, &speed);
only the first line get read normally and everything afterwards reads the new line of the previous line.
Can someone please show me the proper way to write this thing?
Rather than calling fscanf() over and over, and hoping that the newlines match up how you want, use fgets() to get one line at a time, parse it using fscanf(), and do error handling on a line-by-line basis. This will be less error-prone, and it sounds like it will clear up your problem with no extra effort.
Your problem is that name starts with a newline, and that newline ends up in the file.
In order to properly parse the file I would have to know its format, but for now I assume it's <string> <int> <int> <float> where the number of spaces between each element may vary.
The format string I would start with is simply "%s%d%d%f", and let fscanf() deal with the whitespace. With this format string I was able to properly parse lines like
foo 3 4 7
If this does not satisfy you feel free to elaborate on the format of the file you are parsing and I'll try to come up with solutions.

Reading and comparing numbers from txt file C

I am new to C programming, so I am having difficulties with the problem below.
I have a text file inp.txt which contains information like the following:
400;499;FIRST;
500;599;SECOND;
670;679;THIRD;
I need to type a number and my program needs to compare it with numbers from the inp.txt file.
For example, if I type 450, it's between 400 and 499, so I need write to the word FIRST to the file out.txt
I have no idea how to convert a character array to an int.
I think you'll want these general steps in your program (but I'll leave it to you to figure out how you want to do it exactly)
Load each of the ranges and the text "FIRST", "SECOND", etc. from the file inp.txt, into an array, or several arrays, or similar. As I said in the comment above, fscanf might be handy. This page describes how to use it - the page is about C++, but using it in C should be the same http://www.cplusplus.com/reference/clibrary/cstdio/fscanf/. Roughly speaking, the idea is that you give fscanf a format specifier for what you want to extract from a line in a file, and it puts the bits it finds into the variables you specify)
Prompt the user to enter a number.
Look through the array(s) to work out which range the number fits into, and therefore which text to output
Edit: I'll put some more detail in, as asker requested. This is still a kind of skeleton to give you some ideas.
Use the fopen function, something like this (declare a pointer FILE* input_file):
input_file = fopen("c:\\test\\inp.txt", "r") /* "r" opens inp.txt for reading */
Then, it's good to check that the file was successfully opened, by checking if input_file == NULL.
Then use fscanf to read details from one line of the file. Loop through the lines of the file until you've read the whole thing. You give fscanf pointers to the variables you want it to put the information from each line of the file into. (It's a bit like a printf formatting specifier in reverse).
So, you could declare int range_start, range_end, and char range_name[20]. (To make things simple, let's assume that all the words are at most 20 characters long. This might not be a good plan in the long-run though).
while (!feof(input_file)) { /* check for end-of-file */
if(fscanf(input_file, "%d;%d;%s", &range_start, &range_end, range_name) != 3) {
break; /* Something weird happened on this line, so let's give up */
else {
printf("I got the following numbers: %d, %d, %s\n", range_start, range_end, range_name);
}
}
Hopefully that gives you a few ideas. I've tried running this code and it did seem to work. However, worth saying that fscanf has some drawbacks (see e.g. http://mrx.net/c/readfunctions.html), so another approach is to use fgets to get each line (the advantage of fgets is that you get to specify a maximum number of characters to read, so there's no danger of overrunning a string buffer length) and then sscanf to read from the string into your integer variables. I haven't tried this way though.

Reading in data from a string in C (like scanf)

I've read in and stored a data file that I am processing into an array of char arrays, one char array for each line in the file and I now want to process the individual lines. I'm not however sure how to do this.
I read each line in like so:
/* Read the whole file into an array */
char read_lines[FILE_LENGTH][FILE_WIDTH];
for(i=0;i<FILE_LENGTH;i++) {
fscanf(data_file, "%[^\n]", read_lines[i]);
fscanf(data_file, "%[\n]", dump);
}
I need to read the data in each line which is formatted as %d\t%d\t%d\t%d\t%d and I'm not really sure how to read a specific variable into a scanf function. I know that fscanf() reads from a file and scanf() reads from user input, is there a function that reads from a variable?
I come from a python background and in python, I would just use the following code:
read_lines = open('file.txt').readlines()
for line in lines:
i = lines.index(line)
first[i], second[i], third[i], forth[i], fifth[i] = line.split('\t')
I really cannot see how to do the equivalent in C. I've done a fair bit of research but I couldn't find anything useful. Any help would be appreciated!
Thanks!
Perhaps check out sscanf. It is just like it's cousin scanf and fscanf but takes a string instead. Here is a snip from the above link.
The sscanf function accepts a string from which to read input, then, in a manner similar to printf and related functions, it accepts a template string and a series of related arguments. It tries to match the template string to the string from which it is reading input, using conversion specifier like those of printf.
You can use the strtok function [read the manpage] to split a string
e.g. http://www.gnu.org/s/libc/manual/html_node/Finding-Tokens-in-a-String.html

Using scanf to read in certain amount of characters in C?

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().

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