reading a text in a file

**abood1190** · 10-12-2011

this is the code but i have some questions on it !!?

Code:

#include <stdio.h>#include <string.h>
#define LINE_SIZE 80 /* # characters per line. */
#define SCREEN_LINES 24 /* # lines per screen. */
/* This function displays the contents of a text file
* one screenful at a time.
* It returns 0 if successful and -1 otherwise.
*/
int DisplayFile(char FileName[])
{
FILE *fp;
char LineBuffer[LINE_SIZE], *ptr;
int LineCount = 0;
fp = fopen(FileName, "r");
if (fp == NULL) {
printf("File '%s' not found.\n", FileName);
return -1;
}
/* Display the text one screen at a time */
do {
/* Read and display a single line */
ptr = fgets(LineBuffer, LINE_SIZE, fp);
if (ptr != NULL) {
printf("%s", LineBuffer); /* Display the line */
LineCount++;
}
/* Have we displayed a screenful yet (or finished) ? */
if (LineCount == (SCREEN_LINES - 1) || ptr == NULL) {
printf("\n\t\tPress RETURN to continue");
/* Wait for user to press ENTER before continuing */
while (getchar() != '\n')
; /* do nothing */
LineCount = 0; /* reset counter */
}
} while (ptr != NULL);
return 0;
}
int main(void)
{
int result;
result = DisplayFile("sample.txt");
return result;
}

i dont get this part ???????????

Code:

do {/* Read and display a single line */
ptr = fgets(LineBuffer, LINE_SIZE, fp);
if (ptr != NULL) {
printf("%s", LineBuffer); /* Display the line */
LineCount++;
}

**Adak** · 10-12-2011

fgets() puts input from the stream pointed to by ptr, into the char array linebuffer[], up to the size of the variable in the middle of the call to fgets(). Input terminates when a newline char is found, so it's pulling in one line of text, with each call to fgets().

When it reaches the end of the file, it will return NULL to the ptr.

This is a great way to get sting input from the user, also. Just replace the fp in there, with stdin.

Try it, but unlike scanf():

1) fgets adds the newline to the string, and the end of string marker char: '\0', which you will not see if you print out the string (you might notice the cursor moving as the newline is printed, though). So if you enter a name: Bob, and compare it with a list of names, it won't compare equal to "Bob".

fgets() has entered "Bob" as "Bob\n". To change Bob\n back to Bob, you need to:

Code:

if(linebuffer[len-1]=='\n')
   linebuffer[len-1]='\0';  //overwriting the newline fgets added

Now Bob will compare equal to the Bob from the user, using the standard strcmp(string1, string2) method. Strings can't be compared with an == sign, in C.

2) fgets is size safe. The user can't overflow the buffer and hijack your program, by buffer overrun with fgets(). If the user tries, the extra input will be truncated (cut off).

**Babkockdood** · 10-12-2011

Code:

do {/* Read and display a single line */
ptr = fgets(LineBuffer, LINE_SIZE, fp);
if (ptr != NULL) {
printf("%s", LineBuffer); /* Display the line */
LineCount++;
}

This code is essentially saying:

read a line from file fp, store it in LineBuffer, with a size of LINE_SIZE (80)
if successful, print LineBuffer to stdout, and increment LineCount

Read the man page for fgets. man page fgets section 3

**abood1190** · 10-15-2011

Thank you Adak and Babkockdood. That makes a lot of sense ^_^

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