Intermediate C Programming

Reading and Writing Files 151

1. The starting address of an array of characters to store the data.

2. The number of characters to read.

3. A FILE * to read from.

If the second argument is n, the function reads as many as n - 1 characters from the

ﬁle. The function then adds the ending character, ’\0’, automatically. The function may

read fewer characters if (i) a new line character occurs before reading n - 1 characters

or (ii) the ﬁle has reached its end. Please note that fgets does not stop when it reads a

space. It can read multiple words in the same line even though these words are separated

by one or more spaces. For fscanf(...’’%s’’ ...), the size between % and s is optional.

For fgets, the size is a required argument. If fgets succeeds in reading anything from the

ﬁle, it returns the value of the ﬁrst argument, i.e., the starting address to store the data. If

fgets fails to read anything from the ﬁle, it returns NULL.

The following program reads a ﬁle line by line and counts the number of lines. We assume

that the maximum length of each line is 80 characters, i.e., at least one ’\n’ occurs within

every 80 characters.

// fgets .c1

#in clude < stdio .h >2

#in clude < string .h >3

#in clude < stdlib .h >4

#d ef in e MAX _LINE _ LENGT H 815

// assume that the maximum length of each line is a l ready know6

int main ( i n t argc , char * argv [])7

FILE * fptr ;9

int numLine = 0; // must initializ e to zero10

char oneLine [ MAX_ LINE_L ENGTH ];11

i f ( argc < 2)12

// must check argc before using argv [1]13

{14

printf (" Need to provide the file ’s name .\ n") ;15

return EXIT _FAILUR E ;16

}17

fptr = fopen ( argv [1] , " r") ;18

i f ( fptr == NULL )19

{20

printf (" fopen fail .\ n") ;21

// do not call fclose ( fptr ) here22

return EXIT _FAILUR E ;23

}24

printf (" The name of the file is %s .\ n ", argv [1]) ;25

while ( fgets ( oneLine , MAX_LINE_LENGTH , fptr ) != NULL )26

{27

numLine ++;28

}29

fclose ( fptr );30

printf (" The file has %d lines .\ n" , numLine );31

return EXIT _SUCCES S ;32

}33

152 Intermediate C Programming

When the program cannot read from the ﬁle any more, fgets returns NULL. This means

that the end of the ﬁle has been reached. The C library has a function called getline and

it can be used to a line of arbitrary size.

Previous Chapter

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11. Programming Problems Using File (1/2)

Table of Contents for Intermediate C Programming

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Intermediate C Programming