I'm writing a program in C designed to allow a user to input the name of a file so that the program can read the contents of that file. The input must be in the format "/i:", which means that I have to strip off the first three characters to obtain the file name.
I'm storing the file name in an array of characters with a preset length (because I don't know how long of a file name the user will input). The commands I'm using are:
achBaseOutput[100];
FILE * InputFile
//Instruct users on input
fgets(achBaseOutput, 100, stdin);
//Strips first three characters, scoots file name to start at achBaseOutput[0]
InputFile = fopen(achBaseOutput, "r");
When I execute the code, I get an error box that says:
Debug Assertion Failed!
File: fgets.c
Line 57
Expression: (str != NULL)
Unfortunately, the input format of the file name isn't up to me, so I can't change the /i: formatting for the input.
How can I format the input string so that fopen accepts it as a valid argument?
First you must define the array properly, with a type
char achBaseOutput[100];
The library function fgets retains any newline you typed (or was in the file) at the end of the line.
It can be removed like this, (needs #include <string.h>):
achBaseOutput [ strcspn(achBaseOutput, "\r\n") ] = 0; // remove trailing newline etc
You can ignore, rather than remove, the first 3 characters by opening the file like this:
InputFile = fopen(achBaseOutput + 3, "r");
Related
This is the code.
FILE* fPtr;
FILE* fTemp;
char path[100];
char buffer[BUFFER_SIZE];
char newline[BUFFER_SIZE];
int line, count;
printf("Enter path of source file: ");
scanf("%s", path);
printf("Enter line number to replace: ");
scanf("%d", &line);
/* Remove extra new line character from stdin */
fflush(stdin);
printf("Replace '%d' line with: ", line);
scanf("%s", &newline);
/* Open all required files */
fPtr = fopen(path, "r");
fTemp = fopen("replace.tmp", "w");
/* fopen() return NULL if unable to open file in given mode. */
if (!fPtr)
{
/* Unable to open file hence exit */
printf("\nUnable to open file.\n");
printf("Please check whether file exists and you have read/write privilege.\n");
exit(EXIT_SUCCESS);
}
/*
* Read line from source file and write to destination
* file after replacing given line.
*/
count = 0;
while ((fgets(buffer, BUFFER_SIZE, fPtr)) != 0)
{
count++;
/* If current line is line to replace */
if (count == line)
fputs(newline, fTemp);
else
fputs(buffer, fTemp);
}
/* Close all files to release resource */
fclose(fPtr);
fclose(fTemp);
/* Delete original source file */
remove(path);
/* Rename temporary file as original file */
rename("replace.tmp", path);
printf("\nSuccessfully replaced '%d' line with '%s'.", line, newline);
return 0;
I wanted to replace a line supposedly the content of the text file is this
> Andy,06/05/2000,US,0654852,254845,313132
> Fan,865644,4654654,654654,465456
> Ben,04/01/1995,SG,0674874,213454,132158
Supposedly I wanted to change the of Fan so I run the code above, it gave me this. I do not want this to happen.
> Andy,06/05/2000,US,0654852,254845,313132
> Fanny,865644,4654654,654654,465456Ben,04/01/1995,SG,0674874,213454,132158
And if I want to change the name of Andy it gave me this
Landy,06/05/2000,US,0654852,254845,313132Fanny,865644,4654654,654654,465456Ben,04/01/1995,SG,0674874,213454,13215
Why it does that?
How do I delete specific line and replace it ?
Assume that the replacement line has a different size than the original one. You cannot do that in standard C11 (check n1570) without copying the file to a new place (because you cannot overwrite a sequence of bytes in the middle of a file by another sequence of different length).
Read carefully the documentation of <stdio.h>
Lines are just a convention in C: they are ending by some end-of-line character (\n). A file could have a single line and contain a megabyte.
So you could use getline to read lines. Or use fgets. In both cases you should check for failure. With fgets what would happen if the line is bigger than the buffer? With getline what would happen with a file containing a single line of a gigabyte which does not fit into memory?
Be aware that stdout is buffered (and the buffer size could vary from one run to the next one and could be different if you use command pipelines). See setvbuf and fflush. In practice, take the habit of ending your printf format control string with \n and/or explicitly calling fflush
Many open source programs doing what you want already exist. GNU ed comes to mind. Consider studying its source code for inspiration.
Please read how to debug small programs. If you use a recent GCC compiler with some GDB debugger, compile with all warnings and debug info, so gcc -Wall -Wextra -g then use gdb to understand the behavior of your program. Specify on paper the input file syntax using EBNF and read more about parsing techniques, including recursive descent parsing.
Notice that:
fflush(stdin);
is undefined behavior. You should fflush output streams only.
PS. You could later read about databases then consider using sqlite.
fgets will read from the file up to and including the newline character at the end of the line. The scanf call you use to get the replacement string does not, so when you write out newline it does not contain a newline character.
Solutions include explicitly adding the newline (possibly with fputc('\n', fTemp);, or using fgets(newline, BUFFER_SIZE, stdin); instead of the scanf to read your input string.
Yes I want to use
fgets(new,line,buffer_sizze,stdin);
but it just won't ask for input unless I put it inside of main().
When I put it inside of a function that I created, it won't ask for input from the user which is why I used scanf.
Is there a way to put it \n without asking the user to type \n.
Or any solution to why it's not getting input when I used fgets.
For the people that has the same problem as me.
Fgets not asking for any input.
Try use getchar().
That solved my problem.
For unknown reason.
I'm working on a program where the user inputs the file path, then an additional string with the file name is concatenated to that. I will be using this for multiple files in the same directory.
I am using printf statement is just to see if the operation works, however when the output is displayed the filepath is printed twice, then the filename added at the end. For example
Input filepath is C:\Documents\
the output comes out C:\Documents\C:\Documents\HR_1.txt
How can this be corrected?
The relevant code is below
int main()
{
char folder[50]="";
printf("Please type file location\n");
printf("An example of file location is C:\\Documents\\projects\\[Folder]\\");
printf("\n");
scanf("%s",folder);
printf(folder);
/*Clearing Heart rate file names, opening file*/
FILE*HR1=NULL;
printf(strcat(folder,"HR_1.txt"));
}
}
You have the output of two calls to printf mashed together.
The first parameter to printf should always be a string literal, not a variable. That prevents unintended escape sequences from being interpreted and allows you to put newlines in your formatting.
Because your two calls to printf, one before appending and one after, don't include a newline, they appear on the same line.
So change this:
printf(folder);
...
printf(strcat(folder,"HR_1.txt"));
To:
printf("%s\n", folder);
...
printf("%s\n", strcat(folder,"HR_1.txt"));
Output:
C:\Documents\
C:\Documents\HR_1.txt
I'm trying to read text file with C. Text file is a simple language file which works in embeded device and EACH LINE of file has a ENUM on code side. Here is a simple part of my file :
SAMPLE FROM TEXT FILE :
OPERATION SUCCESS!
OPERATION FAILED!\nRETRY COUNT : %d
ENUM :
typedef enum
{
...
MESSAGE_VALID_OP,
MESSAGE_INVALID_OP_WITH_RETRY_COUNT
...
}
Load Strings :
typedef struct
{
char *str;
} Message;
int iTotalMessageCount = 1012;
void vLoadLanguageStrings()
{
FILE *xStringList;
char * tmp_line_message[256];
size_t len = 0;
ssize_t read;
int message_index = 0;
xStringList = fopen("/home/change/strings.bin", "r");
if (xStringList == NULL)
exit(EXIT_FAILURE);
mMessages = (Message *) malloc(iTotalMessageCount * sizeof(Message));
while ((read = fgets(tmp_line_message, 256, xStringList)) != -1 && message_index < iTotalMessageCount)
{
mMessages[message_index].str = (char *) malloc(strlen(tmp_line_message));
memcpy(mMessages[message_index].str, tmp_line_message, strlen(tmp_line_message) -1);
message_index++;
}
fclose(xStringList);
}
As you se in the Sample from text file i have to use \n Feed Line character on some of my lines. After all, i read file successfuly. But if i try to call my text which has feed line \n, feed line character just printed on device screen as \ & n characters.
I already try with getline(...) method. How can i handle \n character without raising the complexity and read file line by line.
As you se in the Sample from text file i have to use \n Feed Line
character on some of my lines.
No, I don't see that. Or at least, I don't see you doing that. The two-character sequence \n is significant primarily to the C compiler; it has no inherent special significance in data files, whether those files are consumed by a C program or not.
Indeed, if the system recognizes line feeds as line terminators, then by definition, it is impossible to embed a literal line feed in a physical line. What it looks like you are trying to do is to encode line feeds as the "\n" character sequence. That's fine, but it's quite a different thing from embedding a line feed character itself.
But after all, i read file successfuly.
But if i try to call my text which has feed line \n, feed line
character just printed on device screen as \ & n characters.
Of course. Those are the characters you read in (not a line feed), so if you write them back out then you reproduce them. If you are encoding line feeds via that character sequence, then your program must decode that sequence if you want it to output literal line feeds in its place.
I already try with getline(...) method. How can i handle \n character
without raising the complexity and read file line by line.
You need to process each line read to decode the \n sequences in it. I would write a function for that. Any way around, however, your program will be more complex, because the current version simply doesn't do all the things it needs to do.
This function print the length of words with '*' called histogram.How can I save results into text file? I tried but the program does not save the results.(no errors)
void histogram(FILE *myinput)
{
FILE *ptr;
printf("\nsaving results...\n");
ptr=fopen("results1.txt","wt");
int j, n = 1, i = 0;
size_t ln;
char arr[100][10];
while(n > 0)
{
n = fscanf(myinput, "%s",arr[i]);
i++;
}
n = i;
for(i = 0; i < n - 1; i++)
{
ln=strlen(arr[i]);
fprintf(ptr,"%s \t",arr[i]);
for(j=0;j<ln;j++)
fprintf(ptr, "*");
fprintf(ptr, "\n");
}
fclose(myinput);
fclose(ptr);
}
I see two ways to take care of this issue:
Open a file in the program and write to it.
If running with command line, change the output location for standard out
$> ./histogram > outfile.txt
Using the '>' will change where standard out will write to. The issue with '>' is that it will truncate a file and then write to the file. This means that if there was any data in that file before, it is gone. Only the new data written by the program will be there.
If you need to keep the data in the file, you can change the standard out to append the file with '>>' as in the following example:
$> ./histogram >> outfile.txt
Also, there does not have to be a space between '>' and the file name. I just do that for preference. It could look like this:
$> ./histogram >outfile.txt
If your writing to a file will be a one time thing, changing standard out is probably be best way to go. If you are going to do it every time, then add it to the code.
You will need to open another FILE. You can do this in the function or pass it in like you did the file being read from.
Use 'fprintf' to write to the file:
int fprintf(FILE *restrict stream, const char *restrict format, ...);
Your program may have these lines added to write to a file:
FILE *myoutput = fopen("output.txt", "w"); // or "a" if you want to append
fprintf(myoutput, "%s \t",arr[i]);
Answer Complete
There may be some other issues as well that I will discuss now.
Your histogram function does not have a return identifier. C will set it to 'int' automatically and then say that you do not have a return value for the function. From what you have provided, I would add the 'void' before the function name.
void histogram {
The size of arr's second set of arrays may be to small. One can assume that the file you are reading from does not exceed 10 characters per token, to include the null terminator [\0] at the end of the string. This would mean that there could be at most 9 characters in a string. Else you are going to overflow the location and potentially mess your data up.
Edit
The above was written before a change to the provided code that now includes a second file and fprintf statements.
I will point to the line that opens the out file:
ptr=fopen("results1.txt","wt");
I am wondering if you mean to put "w+" where the second character is a plus symbol. According to the man page there are six possibilities:
The argument mode points to a string beginning with one of the
following sequences (possibly followed by additional characters, as
described below):
r Open text file for reading. The stream is positioned at the
beginning of the file.
r+ Open for reading and writing. The stream is positioned at the
beginning of the file.
w Truncate file to zero length or create text file for writing.
The stream is positioned at the beginning of the file.
w+ Open for reading and writing. The file is created if it does
not exist, otherwise it is truncated. The stream is
positioned at the beginning of the file.
a Open for appending (writing at end of file). The file is
created if it does not exist. The stream is positioned at the
end of the file.
a+ Open for reading and appending (writing at end of file). The
file is created if it does not exist. The initial file
position for reading is at the beginning of the file, but
output is always appended to the end of the file.
As such, it appears you are attempting to open the file for reading and writing.
A friend of mine needs to use MATLAB for one of his classes, so he called me up (a Computer Science Major) and asked if I could teach him C. I am familiar with C++, so I am also familiar with the general syntax, but had to read up on the IO library for C.
I was creating some simple IO programs to show my friend, but my third program is causing me trouble. When I run the program on my machine using Eclipse (with the CDT) Eclipse's console produces a glitchy output where instead of prompting me for the data, it gets the input and then prints it all at once with FAILURE.
The program is supposed to get a filename from user, create the file, and write to it until the user enters a blank line.
When I compile/run it on my machine via console (g++ files2.c) I am prompted for the data properly, but FAILURE shows up, and there is no output file.
I think the error lies with how I am using the char arrays, since using scanf to get the filename will create a functional file (probably since it ignores whitespace), but not enter the while loop.
#include <stdio.h>
#define name_length 20
#define line_size 80
int main() {
FILE * write_file; // pointer to file you will write to
char filename[name_length]; // variable to hold the name of file
char string_buffer[line_size]; // buffer to hold your text
printf("Filename: "); // prompt for filename
fgets(filename, name_length, stdin); // get filename from user
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
write_file = fopen(filename, "w"); // create/overwrite file user named
if (!write_file) {printf("FAILURE");} // failed to create FILE *
// inform user how to exit
printf("To exit, enter a blank line (no spaces)\n");
// while getting input, print to file
while (fgets(string_buffer, line_size, stdin) != NULL) {
fputs(string_buffer, write_file);
if (string_buffer[0] == '\n') {break;}
}
fclose(write_file);
return 0;
}
How should I go about fixing the program? I have found next to nothing on user-terminated input being written to file.
Now if you will excuse me, I have a couple of files to delete off of my University's UNIX server, and I cannot specify them by name since they were created with convoluted filenames...
EDIT------
Like I said, I was able to use
scanf("%s", filename);
to get a working filename (without the newline char). But regardless of if I use scanf or fgets for my while loop, if I use them in conjunction with scanf for the filename, I am not able to write anything to file, as it does not enter the while loop.
How should I restructure my writing to file and my while loop?
Your check for the newline is wrong; you're looking at the last character in filename but it may be before that if the user enters a filename that's shorter than the maximum. You're then trying to open a file that has a newline in it's name.
These lines seem to be incorrect:
if (filename[name_length-1] == '\n') // if last char in stream is newline,
{filename[name_length-1] = '\0';} // remove it
You verify the name_length - 1 character,, which is 19 in your case without any regard of the introduced filename's length. So if your file name's length is less then 18 you won't replace the '\n' character at the end of your string. Obviously the file name can't contain '\n' character.
You need to get the size of you file name first with strlen() as an example.
if (filename[strlen(filename) - 1] == '\n')
{
filename[strlen(filename) - 1] = '\0';
}
(Don't forget to include the string.h header)
I hope I was able to help with my weak english.