I am using fscanf to read in the date and then fgets to read the note.
However after the first iteration, fscanf returns a value of -1.
I used GDB to debug the program step by step. It works fine until the first use of fgets. When I try print out the line read by fgets on the first iteration, it gives me this:
(gdb) print line
$6 = "\rtest\r18/04/2010\rtest2\r03/05/2010\rtest3\r05/08/2009\rtest4\r\n\000\000\000\000q\352\261\a\370\366\377\267.N=\366\000\000\000\000\003\000\000\000\370xC\000\000\000\000\000\000\000\000\000\001\000\000\000\227\b\000\000\070\367\377\267H\364\377\267\362\202\004\bdoD\000\354\201\004\b\001\000\000\000\304oC\000p\363\377\277\260zC\000D\363\377\277\n!B\000\064\363\377\277\354\201\004\b(\363\377\277TzC\000\000\000\000\000\070\367\377\267\001\000\000\000\000\000\000\000\001\000\000\000\370xC\000\001\000\000\000\000\000\312\000\000\000\000\000\377\260\360\000\001\000\000\000\277\000\000\000\364\317\000\000\344\261\\\000\000\000\000\000p\363\377\277|\233\004\b\350\362\377\277 \204\004\b\005\000\000\000|\233\004\b\030\363\377\277"
It looks like fgets reads the remaining entries and then stores them all in a single string.
I am not sure why it is doing this.
Here is the main code:
int main(int argc, char* argv[]) {
FILE* file;
int numEntries, i = 0;
int index = atoi(argv[1]);
char line[SIZE];
JournalEntry *entry;
/*argument provided is the entry user wants to be displayed*/
if (argc > 2) {
perror("Error: Too many arguments provided");
}
file = fopen("journalentries.txt", "r");
if (file == NULL) {
perror("Error in opening file");
}
if (fscanf(file, "%d", &numEntries) != 1) {
perror("Unable to read number of entries");
}
entry = (JournalEntry*)malloc(numEntries * sizeof(JournalEntry));
if (entry == NULL) {
perror("Malloc failed");
}
for (i = 0; i < numEntries; i++) {
if (fscanf(file, "%d/%d/%d", &entry[i].day, &entry[i].month, &entry[i].year) != 3) {
perror("Unable to read date of entry");
}
if (fgets(line, sizeof(line), file) == NULL) {
perror("Unable to read text of entry");
}
}
printf("%d-%02d-%02d %s: ", entry[index].year, entry[index].month, entry[index].day, entry[index].text);
if(ferror(file)) {
perror("Error with file");
}
fclose(file);
free(entry);
return 0;
}
The file that I have to read:
The very first line contains the number of entries to be read
4
12/04/2010
test
18/04/2010
test2
03/05/2010
test3
05/08/2009
test4
The struct JournalEntry located in the header file:
typedef struct {
int day;
int month;
int year;
char text[250];
} JournalEntry;
It looks like fgets reads the remaining entries and then stores them all in a single string.
Yes, '\r' is not line terminator. So when fscanf stops parsing at the first invalid character, and leaves them in the buffer, then fgets will read them until end of line. And since there are no valid line terminators in the file, that is until end of file.
You should probably fix the file to have valid (Unix?) line endings, for example with suitable text editor which can do it. But that is another question, which has been asked before (like here), and depends on details not included in your question.
Additionally, you need dual check for fscanf return value. Use perror only if return value is -1, otherwise error message will not be related to the error at all. If return value is >=0 but different from what you wanted, then print custom error message "invalid input syntax" or whatever (and possibly use fgets to read rest of the line out of the buffer).
Also, to reliably mix scanf and fgets, I you need to add space in the fscanf format string, so it will read up any whitespace at the end of the line (also at the start of next line and any empty lines, so be careful if that matters), like this:
int items_read = scanf("%d ", &intvalue);
As stated in another answer, it's probably best to read lines with fgets only, then parse them with sscanf line-by-line.
Don't mix fscanf() and fgets(), since the former might leave stuff in the stream's buffer.
For a line-oriented format, read only full lines using fgets(), then use e.g. sscanf() to parse what you've read.
The string you see when running GDB really ends at the first null character:
"\rtest\r18/04/2010\rtest2\r03/05/2010\rtest3\r05/08/2009\rtest4\r\n\000"
The other data after is ignored (when using ordinary str-functions);
Related
Today I decided to learn to code for the first time in my life. I decided to learn C. I have created a small program that checks a txt file for a specific value. If it finds that value then it will tell you that that specific value has been found.
What I would like to do is that I can put multiple files go through this program. I want this program to be able to scan all files in a folder for a specific string and display what files contain that string (basically a file index)
I just started today and I'm 15 years old so I don't know if my assumptions are correct on how this can be done and I'm sorry if it may sound stupid but I have been thinking of maybe creating a thread for every directory I put into this program and each thread individually runs that code on the single file and then it displays all the directories in which the string can be found.
I have been looking into threading but I don't quite understand it. Here's the working code for one file at a time. Does anyone know how to make this work as I want it?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
//searches for this string in a txt file
char searchforthis[200];
//file name to display at output
char ch, file_name[200];
FILE *fp;
//Asks for full directory of txt file (example: C:\users\...) and reads that file.
//fp is content of file
printf("Enter name of a file you wish to check:\n");
gets(file_name);
fp = fopen(file_name, "r"); // read mode
//If there's no data inside the file it displays following error message
if (fp == NULL)
{
perror("Error while opening the file.\n");
exit(EXIT_FAILURE);
}
//asks for string (what has to be searched)
printf("Enter what you want to search: \n");
scanf("%s", searchforthis);
char* p;
// Find first occurrence of searchforthis in fp
p = strstr(searchforthis, fp);
// Prints the result
if (p) {
printf("This Value was found in following file:\n%s", file_name);
} else
printf("This Value has not been found.\n");
fclose(fp);
return 0;
}
This line,
p = strstr(searchforthis, fp);
is wrong. strstr() is defined as, char *strstr(const char *haystack, const char *needle), no file pointers in it.
Forget about gets(), its prone to overflow, reference, Why is the gets function so dangerous that it should not be used?.
Your scanf("%s",...) is equally dangerous to using gets() as you don't limit the character to be read. Instead, you could re-format it as,
scanf("%199s", searchforthis); /* 199 characters + \0 to mark the end of the string */
Also check the return value of scanf() , in case an input error occurs, final code should look like this,
if (scanf("%199s", searchforthis) != 1)
{
exit(EXIT_FAILURE);
}
It is even better, if you use fgets() for this, though keep in mind that fgets() will also save the newline character in the buffer, you are going to have to strip it manually.
To actually perform checks on the file, you have to read the file line by line, by using a function like, fgets() or fscanf(), or POSIX getline() and then use strstr() on each line to determine if you have a match or not, something like this should work,
char *p;
char buff[500];
int flag = 0, lines = 1;
while (fgets(buff, sizeof(buff), fp) != NULL)
{
size_t len = strlen(buff); /* get the length of the string */
if (len > 0 && buff[len - 1] == '\n') /* check if the last character is the newline character */
{
buff[len - 1] = '\0'; /* place \0 in the place of \n */
}
p = strstr(buff, searchforthis);
if (p != NULL)
{
/* match - set flag to 1 */
flag = 1;
break;
}
}
if (flag == 0)
{
printf("This Value has not been found.\n");
}
else
{
printf("This Value was found in following file:\n%s", file_name);
}
flag is used to determine whether or not searchforthis exists in the file.
Side note, if the line contains more than 499 characters, you will need a larger buffer, or a different function, consider getline() for that case, or even a custom one reading character by character.
If you want to do this for multiple files, you have to place the whole process in a loop. For example,
for (int i = 0; i < 5; i++) /* this will execute 5 times */
{
printf("Enter name of a file you wish to check:\n");
...
}
Im very new to this language, can you help me:
Instead of making the user input col, row, and direction(scanf). I want to extract the data from file(format below)
From the file format i do not want to extract the first line(5,6), i only want to extract the remaining lines.
Below is a code of how to extract data from a file(using command line arguments), but this code extract the first line also, and only prints the lines.I do not want to print the line but to extract the data from a file instead of making the user input it.
File format:
colrow direction(starting from 2nd line)
5,6
A0 H
D0 V
C1 V
A4 H
F0 v
code of scanf
yourcolumn = getchar();
col = charToNum(yourcolumn); //function to input column
printf("enter row");
scanf("%d",&row);
printf("h: horizontally or v: vertically?\n");
scanf(" %c",&direction);
Code for extracting data from file:
#include <stdio.h>
int main(int argc, char* argv[])
{
char const* const fileName = argv[1]; /* should check that argc > 1 */
FILE* file = fopen(fileName, "r"); /* should check the result */
char line[256];
while (fgets(line, sizeof(line), file)) {
/* note that fgets don't strip the terminating \n, checking its
presence would allow to handle lines longer that sizeof(line) */
printf("%s", line);
}
/* may check feof here to make a difference between eof and io failure -- network
timeout for instance */
fclose(file);
return 0;
}
Since you are reading line-by-line, I suggest you restructure you file reading to match your logic
while (EOF != fscanf(file, "%[^\n]\n", line)) {
printf("> %s\n", line);
}
Is a way that one can read every line, one at a time. You can lookup the caveats of using fscanf and how to adjust the code to safely read without overflowing your line buffer.
Then, if you want to skip the first line, your code could look like this
if (EOF != fscanf(file, "%[^\n]\n", line)) {
// skip the first line
}
while (EOF != fscanf(file, "%[^\n]\n", line)) {
printf("> %s\n", line);
}
And your processing logic will look a lot like your mental process.
Yes, you could use a line counter, and only process if the counter is high enough; but, it is generally better to avoid introducing variables, if you can live without them. This is because an extra added variable doesn't make the code too hard to reason about; but, after you've repeated that "extra variable" rationale five or six times, the code quickly turns into something that's harder to maintain and harder to reason about. By the time you hit twenty or more extra variables, the odds of maintaining the code quickly without breaking it are lower.
Read the first line also with fgets() into a string and then scan the string for row, direction.
char line[256];
if (fgets(line, sizeof(line), file)) {
if (sscanf(line, "%d %c", &row, &direction) != 2) {
printf("Invalid first line '%s'\n", line);
} else {
while (fgets(line, sizeof(line), file)) {
printf("%s", line);
}
}
}
I am trying to read lines from a list to my structs, and it is almost working. I am not really sure what the problem is, but the last line of the text file wont show up when I call for the structs and I do not think the words are placed right...
void loadFile(char fileName[], Song *arr, int nrOf) {
FILE *input = fopen(fileName, "r");
if (input == NULL) {
printf("Error, the file could not load!");
} else {
fscanf(input, "%d", &nrOf);
fscanf(input, "%*[^\n]\n", NULL);
for (int i = 0; i < nrOf; i++) {
fgets(arr[i].song, sizeof(arr[i].song), input);
fgets(arr[i].artist, sizeof(arr[i].artist), input);
fgets(arr[i].year, sizeof(arr[i].year), input);
}
for (int i = 0; i < nrOf; i++) {
printf("%s", arr[i].song);
printf("%s", arr[i].artist);
printf("%s", arr[i].year);
}
rewind(input);
printf("The file is now ready.\n");
}
fclose(input);
}
The text file starts with a number on the first line to keep track of how many songs there are in the list. I therefore tried with this:
fscanf(input, "%d", &nrOf);
fscanf(input, "%*[^\n]\n", NULL);
to be able to skip the first line after nrOf got the number.
EDIT:
Here is the struct:
typedef struct Song {
char song[20];
char artist[20];
char year[5];
} Song;
Here is the text file:
4
Mr Tambourine Man
Bob Dylan
1965
Dead Ringer for Love
Meat Loaf
1981
Euphoria
Loreen
2012
Love Me Now
John Legend
2016
And the struct is dynamic allocated:
Song *arr;
arr = malloc(sizeof(Song));
there are a combination of reasons why the last line(s) do not print
The main reason is the last line(s) were never read
Should not call fclose() in any execution path where the file failed to open
there is no need to call rewind() when the next statement is fclose()
Since the calls to printf() for the fields in the Song array are output, one right after another, this will result in a long long single line output to the terminal, Hopefully the terminal is set to automatically scroll after so many columns of output, but that cannot be depended upon.
When outputting an error message, it is best to output it to stderr, not stdout. The function: perror() does that AND also outputs the reason the OS thinks the error occurred. (it does this by referencing errno to select which error message to output.)
the following is the key problem:
if the input file contains one song info per line then the field year will either contain a trailing newline or the newline will not have been read. If the newline was not read, then the next call to fgets() which was trying to input the song title will only receive a newline then all following fields (of all songs) will be progressively further off.
Suggest after reading a song fields, use a loop to clear out any remaining characters in the input line, similar to:
int ch;
while( (ch = getchar( input )) && EOF != ch && '\n' != ch );
I know this is a dumb question, but how would I load data from a multiline text file?
while (!feof(in)) {
fscanf(in,"%s %s %s \n",string1,string2,string3);
}
^^This is how I load data from a single line, and it works fine. I just have no clue how to load the same data from the second and third lines.
Again, I realize this is probably a dumb question.
Edit: Problem not solved. I have no idea how to read text from a file that's not on the first line. How would I do this? Sorry for the stupid question.
Try something like:
/edited/
char line[512]; // or however large you think these lines will be
in = fopen ("multilinefile.txt", "rt"); /* open the file for reading */
/* "rt" means open the file for reading text */
int cur_line = 0;
while(fgets(line, 512, in) != NULL) {
if (cur_line == 2) { // 3rd line
/* get a line, up to 512 chars from in. done if NULL */
sscanf (line, "%s %s %s \n",string1,string2,string3);
// now you should store or manipulate those strings
break;
}
cur_line++;
}
fclose(in); /* close the file */
or maybe even...
char line[512];
in = fopen ("multilinefile.txt", "rt"); /* open the file for reading */
fgets(line, 512, in); // throw out line one
fgets(line, 512, in); // on line 2
sscanf (line, "%s %s %s \n",string1,string2,string3); // line 2 is loaded into 'line'
// do stuff with line 2
fgets(line, 512, in); // on line 3
sscanf (line, "%s %s %s \n",string1,string2,string3); // line 3 is loaded into 'line'
// do stuff with line 3
fclose(in); // close file
Putting \n in a scanf format string has no different effect from a space. You should use fgets to get the line, then sscanf on the string itself.
This also allows for easier error recovery. If it were just a matter of matching the newline, you could use "%*[ \t]%*1[\n]" instead of " \n" at the end of the string. You should probably use %*[ \t] in place of all your spaces in that case, and check the return value from fscanf. Using fscanf directly on input is very difficult to get right (what happens if there are four words on a line? what happens if there are only two?) and I would recommend the fgets/sscanf solution.
Also, as Delan Azabani mentioned... it's not clear from this fragment whether you're not already doing so, but you have to either define space [e.g. in a large array or some dynamic structure with malloc] to store the entire dataset, or do all your processing inside the loop.
You should also be specifying how much space is available for each string in the format specifier. %s by itself in scanf is always a bug and may be a security vulnerability.
First off, you don't use feof() like that...it shows a probable Pascal background, either in your past or in your teacher's past.
For reading lines, you are best off using either POSIX 2008 (Linux) getline() or standard C fgets(). Either way, you try reading the line with the function, and stop when it indicates EOF:
while (fgets(buffer, sizeof(buffer), fp) != 0)
{
...use the line of data in buffer...
}
char *bufptr = 0;
size_t buflen = 0;
while (getline(&bufptr, &buflen, fp) != -1)
{
...use the line of data in bufptr...
}
free(bufptr);
To read multiple lines, you need to decide whether you need previous lines available as well. If not, a single string (character array) will do. If you need the previous lines, then you need to read into an array, possibly an array of dynamically allocated pointers.
Every time you call fscanf, it reads more values. The problem you have right now is that you're re-reading each line into the same variables, so in the end, the three variables have the last line's values. Try creating an array or other structure that can hold all the values you need.
The best way to do this is to use a two dimensional array and and just write each line into each element of the array. Here is an example reading from a .txt file of the poem Ozymandias:
int main() {
char line[15][255];
FILE * fpointer = fopen("ozymandias.txt", "rt");
for (int a = 0; a < 15; a++) {
fgets(line[a], 255, fpointer);
}
for (int b = 0; b < 15; b++) {
printf("%s", line[b]);
}
return 0;
This produces the poem output. Notice that the poem is 14 lines long, it is more difficult to print out a file whose length you do not know because reading a blank line will produce the output "x�oA". Another issue is if you check if the next line is null by writing
while (fgets(....) != NULL)) {
each line will be skipped. You could try going back a line each time to solve this but i think this solution is fine for all intents.
I have an even EASIER solution with no confusing snippets of puzzling methods (no offense to the above stated) here it is:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
string line;//read the line
ifstream myfile ("MainMenu.txt"); // make sure to put this inside the project folder with all your .h and .cpp files
if (myfile.is_open())
{
while ( myfile.good() )
{
getline (myfile,line);
cout << line << endl;
}
myfile.close();
}
else cout << "Unable to open file";
return 0;
}
Happy coding
How do I read input one string at a time to call another function in C. I thought this would work, but my output hangs:
#define BUFFMT "%255"
#define LINE_LEN 256
#define START_COUNT 1
// filename is declared in the main file elsewhere. I know the file opens since I tried an //old method I use to read one line at time using fgets, but I didn't know how to do one //string at a time. Thanks.
FILE *OpenFile(const char *fileName)
{
FILE *fptr;
if ((fptr = fopen(fileName, "r")) == NULL) {
fprintf(stderr, "Error opening file %s, exiting...", fileName);
exit(EXIT_FAILURE);
}
return fptr;
}
LIST *CreateList(FILE *fp)
{
char buf[LINE_LEN];
while (scanf(BUFFMT"s", buf) != EOF) {
printf("%s: \n", buf);
}
}
scanf() is going to read from the terminal, so it's going to hang waiting for you to type in your input. Use fscanf(fp, BUFFMT"s", buf) instead.
Try this instead of your scanf:
fgets (buf, sizeof (buf), fp)
Have you tried using fgets()
fgets()
fgets() reads up to size-1 characters
from stream and stores them in buffer.
fgets() stores the null character
('\0') after the last character read
into the buffer and returns 'buffer'
if everything works fine, or NULL on
error or end of file.