My program reads the specific line from the file properly, however it reads the whole file form the line I specify onward. I am trying to print just one line at a time. How can I make it just read the one line?
The code:
int main()
{
int lineNumber = 5;
static const char filename[] = "Text.txt";
FILE *file = fopen(filename, "r");
int count = 0;
if ( file != NULL )
{
char line[256]; /* or other suitable maximum line size */
while (fgets(line, sizeof line, file) != NULL) /* read a line */
{
if (count == lineNumber)
{
printf("%s", line);
//in case of a return first close the file with "fclose(file);"
}
else
{
count++;
}
}
fclose(file);
}
}
After you've found the desired line, just use a break to exit the loop:
if (count == lineNumber)
{
printf("%s", line);
break;
}
if (count == lineNumber)
{
printf("%s", line);
//in case of a return first close the file with "fclose(file);"
count++;
}
increment count when you get the line you specify,otherwise count will not proceed to indicate next line. That is why your code printed all lines once you got the line you specified. Because the line number count wont increase once it becomes equal to linenumber. so add count++.
You can even break the loop since you don't require rest of the lines to be read after you get the specified line.
Related
void main(void)
{
FILE* textfile;
char line[1000];
textfile = fopen("omar.txt", "r");
if (textfile == NULL)
return 1;
while (fgets(line, 1000, textfile)) {
printf(line);
}
fclose(textfile);
}
so this code prints the whole content of a text file , what should I do to read the third line in the file for example ?
To read the nth line in a file you can do something like this
int i = 0;
while (fgets(line, 1000, textfile)) {
i++;
if (i == n) {
// do stuff with nth line
break;
}
}
This approach uses a counter to count until the nth iteration is reached. Once it is, you can do what you need to do with the nth line.
Also this may be unrelated but you should never use printf without a format specifier as you have in printf(line);. This can be dangerous and could be used by an attacker to exploit the program. I would recommend that in your case puts(line); is a better alternative.
For example:
int readNthLine(FILE *fi, char *buff, size_t buffsize, size_t line)
{
fseek(fi, 0, SEEK_SET);
{
for(size_t cline = 0; cline < line; cline++)
{
if(!fgets(buff, buffsize, fi)) return -1;
}
}
return 0;
}
This very simple function will work only if the size of the buffer is larger than the length of the longest line in the file.
Of course, you should check the result of any I/O operation.
I'm new to programming in C. And I'm trying to print the first 10 lines of a text file. When I run my program with a text file containing 11 lines of text, only the first line is displayed. I'm not sure why it does that, but I suspect there is something wrong in my while loop. Can someone please help me?
#include <stdio.h>
int main(int argc, char *argv[]){
FILE *myfile;
char content;
int max = 0;
// Open file
myfile = fopen(argv[1], "r");
if (myfile == NULL){
printf("Cannot open file \n");
exit(0);
}
// Read the first 10 lines from file
content = fgetc(myfile);
while (content != EOF){
max++;
if (max > 10)
break;
printf ("%c", content);
content = fgetc(myfile);
}
fclose(myfile);
return 0;
}
You have been already advised to use fgets. However, if your file has lines of unknown length, you may still want to use fgetc. Just make sure you count only newlines, not all characters:
int max = 0;
int content;
while ((content = fgetc(myfile)) != EOF && max < 10){
if (content == '\n') max++;
putchar(content);
}
fgetc() returns the next character in the file, not the next line. You probably want to use fgets() instead, which reads up to the next newline character into a buffer. Your code should probably end up with something like:
// allocate 1K for a buffer to read
char *buff = malloc(1024);
// iterate through file until we are out of data or we read 10 lines
while(fgets(buff, 1024, myfile) != NULL && max++ < 10) {
printf("%s\n", buff);
}
free(buff);
// close your file, finish up...
Read more about fgets() here: https://www.tutorialspoint.com/c_standard_library/c_function_fgets.htm
fgetc function reads the next character not the next ine. for reading the number of lines you should use fgets function. this function reads the full string till the end of the one line and stores it in a string.
your code Shuld be as:-
#include <stdio.h>
int main(int argc, char *argv[])
{
FILE *myfile;
char content[200];
int max = 0;
// Open file
myfile = fopen(argv[1], "r");
if (myfile == NULL)
{
printf("Cannot open file \n");
exit(0);
}
// Read the first 10 lines from file
fgets(content, 200, myfile);
while (content != EOF)
{
max++;
if (max > 10)
break;
printf("%s", content);
fgets(content, 200, myfile);
}
fclose(myfile);
return 0;
}
I'm trying to accomplish the following:
I have a text file with the last printable character "]" in a separate line.
This line is not necessary to be the last line of the file. Some blank lines (line returns) can be there.
The purpose of the project is to insert new text before the line with "]".
The way I try to implement this is to search the file from the end of the file to find the line number with the character "]" (char_line).
Copy line by line from the original file rules1.txt to rules2.txt up to the line char_line. Next step is to append the new text with "]" at the end.
After that, I can delete the original file and rename the new file from rules2.txt to rules1.txt.
The problem I have is that the program finds the line with the character "]" and I can do a printf and see the correct line number.
I am assigning char_line = "%d".
When I'm using if(i < char_line) the file is copied all the way to EOF.
If I assign a numerical value, char_line = 23, the file is copied up to line 22, which is what I want.
This is the part of the code which should find line number for "]", copy line by line rules1.txt to rules2.txt up the line with "]".
#include <stdio.h>
int main(void) {
int end, loop, line;
char str[64];
FILE *file;
FILE *write;
int char_line;
int ret;
file = fopen("rules1.txt", "r");
if (file == NULL) {
printf("Failed to open file\n");
return -1;
}
int ch, line_num = 0;
do {
ch = fgetc(file);
if(ch == '\n')
line_num++;
} while (ch != EOF);
// last line doesn't end with a new line!
// but there has to be a line at least before the last line
if(ch != '\n' && line_num != 0)
line_num++;
fclose(file);
line = line_num-1;
start:
file = fopen("rules1.txt", "r");
if (file == NULL) {
printf("Failed to open file\n");
return -1;
}
for(end = loop = 0;loop<line;++loop){
if(0==fgets(str, sizeof(str), file)){//include '\n'
end = 1;//can't input (EOF)
break;
}
}
if(!end)
if (strncmp ("]", str, 1) == 0){
char_line ="%d";
goto next;
} else if(line >1){
line == line--;//WTF?
fclose(file);
goto start;
} else //What to do here?
next:
file = fopen("rules1.txt", "r");
write = fopen("rules2.txt", "w");
char linec [64]; /* line size */
int i = 0;
while (fgets(linec, sizeof(linec), file)) { /* read line from file */
i++;
if(i < char_line) {
fprintf (write , linec); /* write line to file */
}
}
fclose(file);
fclose(write);
return(0);
}
You have declared int char_line; and later you code
char_line ="%d";
This is nonsense. Since a literal string is some char[] array (better think of it as some constant array), decayed to a pointer, and assigning a pointer to an int does not make sense at all. On many machines (x86-64 notably), a pointer has 64 bits but an int has only 32 bits.
Please, compile your code with all warnings and debug info, so gcc -Wall -Wextra -g with GCC, improve your code to get no warnings, then use the debugger gdb.
Take several days to read some good books on C programming. Be aware and work hard to avoid undefined behavior. Use the debugger to run your program step by step and query its state to understand what is happening.
Read the documentation of every standard function you are using such as fgets.
You probably want a loop that reads every line, and copies sometimes that line to another file.
So what I'm basically doing here is using a command line argument to open a file but only open it 4 lines at a time, then a prompt to print out add'l lines. I can get the file to print out but I cannot figure out how to get it to only print out a few lines at a time. This is where I'm at....thoughts?
#include <stdio.h>
int main(int argc, char *argv[])
{
char line[1000];
FILE *pt;
pt = fopen(argv[1], "r");
if(pt == NULL) return -1;
printf(argv[1], line);
while(fgets(line, 1000, pt) != NULL)
printf("%s", line);
fclose(pt);
return 0;
}
I start from strange line of your code, and then I will try to answer the question.
Statement
printf(argv[1], line);
make me curious - what you what to print, actually?
Here line is not initialized, and argv[1] can hardly be used as format line.
So I suppose it should be just
printf(argv[1]);
or
printf("Filename is %s\n", argv[1]);
As for reading from a file with name provided as argv[1] your code looks able to work, I mean your code read line by line till the end of file and prints these lines at the screen.
If you want to change this logic, e.g. read only 4 first line, add condition with counter, e.g.:
int cnt;
for (cnt = 0; cnt < 4; cnt++) // repeat reading 4 times
{
if (fgets(line, 1000, pt) != NULL)
printf("%s", line);
else
break; // stop when reading fails
}
or (I prefer this version)
int cnt = 0;
while (fgets(line, 1000, pt) != NULL && cnt < 4)
{
printf("%s", line);
cnt++;
}
Such changes allows to stop reading (as well as output), so only 4 or less lines will be shown at console screen.
Finally, for case when you want to show file by groups of 4 (or other constant value), consider the following snippet:
#include <stdio.h>
#define MAX_LINES_TO_PRINT 4
int main(int argc, char *argv[])
{
char line[1000];
FILE *pt;
pt = fopen(argv[1], "r");
if (pt == NULL) return -1;
printf("Filename is %s\n", argv[1]);
int cnt = 0;
while (fgets(line, 1000, pt) != NULL)
{
printf("%s", line);
cnt++;
if (cnt % MAX_LINES_TO_PRINT == 0)
{
int answer;
printf("[%d lines printed] Continue? (Y/N) : ", cnt);
answer = getchar(); // get user's response
while (getchar() != '\n'); // clean input buffer after getchar
if (toupper(answer) == 'N')
{
break; // stop reading the file
}
}
}
fclose(pt);
return 0;
}
Try this program with your file and ask question if something is unclear.
Changing the value in the line #define MAX_LINES_TO_PRINT 4 you can regulate maximum number of lines printed at once (before the next request to continue), e.g. #define MAX_LINES_TO_PRINT 15 make your program printing up to 15 lines.
So both the code to print the lines from the txt file and the code to count the lines in the txt file work fine when the other is commented out but when i try to have both work only the code that comes first works e.g. if i put the code to print out the lines first, the line count is always zero. However if i put the code to count the lines first, the number is correct but the lines from the txt file are not printed :S
#include <stdio.h>
int main(int argc, char const *argv[])
{
const int SIZE = 128;
char line[SIZE];
FILE *srcFile;
int c;
int count = 0; // Line counter (result)
if (argc == 1)
{
printf("No command line arguments given!\n");
return 1;
}
srcFile = fopen(argv[1], "r");
if (srcFile == NULL)
{
perror("\n*** FILE OPEN FAILED ***");
}
else
{
printf("\n*** FILE OPEN SUCCESSFUL ***\n\n");
}
while(fgets(line, SIZE, srcFile) != NULL)
{
printf("%s", line);
}
for (c = fgetc(srcFile); c != EOF; c = fgetc(srcFile))
{
if (c == '\n')
{
count ++;
}
}
if(c != '\n' && count != 0)
{
count ++;
}
printf("The file %s has %d lines\n ", argv[1], count);
fclose(srcFile);
return 0;
}
Here is a quick overview of how working with files is done in most programming languages:
When you open a file in a program you obtain a handle to that file. What the handle representation is depends on the language. In c it is the FILE structure. The handle contains - among other things - a file position indicator. Every read and write to that file through this handle happens at that position. Usually a read/write operation advances this file position indicator. Think about it: how do consecutive reads know to each read where the previous one left? You don't provide an argument to the read function telling it where to read from. You just "say" read. What happens is that each read call reads at the file position indicator and then advances this indicator, thus when the next read happens the handle has an updated file position indicator.
So the solution to your problem is - as mentioned in the comments - to put this position indicator to the beginning of the file. In c this can be done with rewind.
curious how the code fragment would have to look to include the line
count int the same loop as the print lines
Simple. Pseudocode:
line_count = 0
while (read line successful)
{
print line
increment line_count
}
print line_count