Reading a File with fgets

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From the course by Duke University

Interacting with the System and Managing Memory

14 ratings

Duke University

Interacting with the System and Managing Memory

14 ratings

Course 4 of 4 in the Specialization Introduction to Programming in C

The final course in the specialization Introduction to Programming in C will teach you powerful new programming techniques for interacting with the user and the system and dynamically allocating memory. You will learn more sophisticated uses for pointers, such as strings and multidimensional arrays, as well as how to write programs that read and write files and take input from the user. Learning about dynamic memory allocation will allow your programs to perform complex tasks that will be applied in the final part of the specialization project: a Monte Carlo simulation for calculating poker hand probabilities.

From the lesson

Module 1: Interacting with the user and system

So far, our programs have had a rather limited interaction with the user or rest of the system, printing some results to standard output (typically to the terminal). Now that we have learned about topics such as strings and arrays, we are ready to learn how to write a program that takes input from the user, takes arguments on the command line, accesses files, and does many other things we typically think of real programs as doing.

Reading a File with fgetc4:26

Reading a File with fgets5:01

Meet the Instructors

Andrew D. Hilton
Associate Professor of the Practice
Electrical and Computer Engineering
Anne Bracy
Senior Lecturer
Computer Science, Cornell University
Genevieve M. Lipp
Adjunct Professor
Electrical and Computer Engineering/Mechanical Engineering

In this video, we have an example of reading from a file with fgets,

which reads lines of a specified length from a file.

In this example, we will convert each line of text to an integer with the function atoi,

add each integer to the sum and print the result.

For the purposes of this example,

the array that we're going to read

our characters into is going to have a relatively small size.

We define line size here as a constant five,

make the array that big and also tell

fgets that we want to read into an array of size five.

We're going to do this for the purposes of example so we can

see what happens when fgets cannot read all of the characters on the line.

As before, we've omitted some error checking,

assuming that we would print an error message and exit under these two conditions.

We begin with our execution arrow just inside main where argc has

the value 2 and argv is a pointer to the array of command line arguments.

We assume that the name of the program is sum numbers and

that argv at one is the name of the input file,

nums.txt, which has the numbers in it.

We see if argc is not equal to 2.

It is 2, so we don't do any of that error code.

We're going to open this file for reading as we've seen before.

The file has numbers in it,

and the current position the file is at the start.

F is not null,

so we don't do anything special there.

We make a box for total which is zero,

then we create the five boxes for line,

each of which will hold a character.

Now, we're ready to call fgets.

It's going to read into line,

filling in the boxes with the information it reads from the file.

We tell it that there are at most five boxes that it can read into,

which really means four characters plus a null terminator character,

which it has to put at the end.

The last argument tells it to read from F. As we do this,

notice that the cursor in the file showing the current position is at the start,

and it's going to read across the line as we fill in the boxes with the text.

So we just read 123 and a new line,

as well as a null terminator into line,

which filled the five boxes.

And now the current position is in the file is at the start of the next line.

Remember that the function strchr takes a string and a character and

returns a pointer to the first occurrence of that character if it is found in the string.

If it is not found, it returns null.

So this is going to check if the line contains a new line,

letting us know if fgets was not able to read the entire line.

In this case, it returns a pointer to the fourth box,

which is not null,

so line contains a new line,

and everything is good.

We're now going to convert the text 1, 2,

3 to the integer 123,

and add it to total which is zero.

Zero plus 123 is 123,

so we change total to 123,

and we return to the top of the while.

Now, we're going to call fgets another time.

It's going to read this next line into the variable line.

Notice that it wrote one for new line,

then the null terminator here and left

the last box unchanged because it had already read the entire line.

We again do strchr of line and the new line character.

We find that it is not null,

then we add 14 to 123,

and we get 137.

We update total to 137 and we go around the loop again.

Notice that the number on the next line,

negative 56789 is too big to fit into the variable line.

It has more than the four characters we're able to read.

So fgets is going to read as much as it can and then stop.

It's going to read negative 567,

and then it's going to stop because we told it that

the array has at most five characters,

and it needs the last one for the null terminator.

The position in the file is between seven and eight.

If we were to read again,

we would read eight and nine whether we use fgets

or fgetc or any other function that reads from a file.

However, for this code,

it's not set up to be able to handle that,

so we have error checking for not reading the whole line.

If we were to make line size bigger,

we would make this all succeed.

Here, we do the check for the new line character,

strchr returns null, and will do the error handling.

We print, "Line is too long," and we exit failure.

Notice that we haven't closed the file,

since we'll learn how to do that shortly.

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