Arrays in C

In this section, we will create a small C program that generates 10 random numbers and sorts them. To do that we will use a new variable arrangement called an array.

An array lets you declare and work with a collection of values of the same type. For example, you might want to create a collection of 5 integers. One way to do it would be to declare 5 integers directly:

int a, b, c, d, e;

This is OK, but what if you needed a thousand integers? An easier way is to declare an array of 5 integers.

int a[5];

The 5 separate integers inside this array are accessed by an index. All arrays start at index zero and go to n-1 in C. Thus, int a[5]; contains 5 elements, and the largest valid index is . For example:

int a[5];

a[0] = 12;
a[1] = 9;
a[2] = 14;
a[3] = 5;
a[4] = 1;

One of the nice things about array indexing is that you can use a loop to manipulate the index. For example, the following code intitializes all of the values in the array to 0:

int a[5];
int i;

for (i=0; i<5; i++)
    a[i] = 0;

The following code initializes the values in the array sequentially and then prints them out:

#include <stdio.h> 

int main()
{
    int a[5];
    int i;

    for (i=0; i<5; i++)
        a[i] = i;
    for (i=0; i<5; i++)
        printf("a[%d] = %d\n", i, a[i]);    
}   

Arrays are used all the time in C. To understand a common usage, start an editor and enter the following code:

#include <stdio.h> 
 
#define MAX 10 
 
int a[MAX]; 
int rand_seed=10; 

/* from K&R 
   - returns random number between 0 and 32767.*/ 
int rand()   
{ 
    rand_seed = rand_seed * 1103515245 +12345;    
    return (unsigned int)(rand_seed / 65536) % 32768;  
} 
 
int main()  
{    
    int i,t,x,y; 
  
    /* fill array */    
    for (i=0; i < MAX; i++)    
    {      
        a[i]=rand();      
        printf("%d\n",a[i]);    
    } 
  
    /* more stuff will go here in a minute */ 

    return 0;
} 

This code contains several new concepts. The #define line declares a constant named MAX and sets it to 10. Constant names are traditionally written in all caps to make them obvious in the code. The line int a[MAX]; shows you how to declare an array of integers in C. Note that because of the position of the array's declaration, it is global to the entire program.

The line int rand_seed=10 also declares a global variable, this time named rand_seed, that is initialized to 10 each time the program begins. This value is the starting seed for the random number code that follows. In a real random number generator, the seed should initialize as a random value, such as the system time. Here, the rand function will produce the same values each time you run the program.

The line int rand() is a function declaration. The rand function accepts no parameters and returns an integer value. We will learn more about functions on this page.

The four lines that follow implement the rand function. We will ignore them for now.

The main function is normal. Four local integers are declared, and the array is filled with 10 random values using a for loop. Note that the array a contains 10 individual integers. You point to a specific integer in the array using square brackets. So a[0] refers to the first integer in the array, a[1] refers to the second, and so on. The line starting with /* and ending with */ is called a comment. The compiler completely ignores the line. You can place notes to yourself or other programmers in comments.

Now add the following code in place of the more stuff ... comment:

/* bubble sort the array */    
for (x=0; x < MAX-1; x++)      
    for (y=0; y < MAX-x-1; y++)        
        if (a[y] > a[y+1])        
        {  
            t=a[y];  
            a[y]=a[y+1];  
            a[y+1]=t;        
        } 
/* print sorted array */    
printf("--------------------\n");    
for (i=0; i < MAX; i++)      
printf("%d\n",a[i]); 

This code sorts the random values and prints them in sorted order. Each time you run it you will get the same values. If you would like to change the values that are sorted, change the value of rand_seed each time you run the program.

The only easy way to truly understand what this code is doing is to execute it "by hand". That is, assume MAX is 4 to make it a little more manageable, take out a sheet of paper and pretend like you are the computer. Draw the array on your paper and put 4 random, unsorted values into the array. Execute each line of the sorting section of the code and draw out exactly what happens. You will find that, each time through the inner loop, the larger values in the array are pushed toward the bottom of the array and the smaller values bubble up toward the top.

Exercises

1. In the first piece of code, try changing the for loop that fills the array to a single line of code. Make sure that the result is the same as the original code.
2. Take the bubble sort code out and put it into its own function. (See article 6, if necessary.) The function header will be void bubble_sort(). Then move the variables used by the bubble sort to the function as well and make them local there. Because the array is global, you do not need to pass parameters.
3. Initialize the random number seed to different values.

C Errors to avoid

1. C has no range checking, so if you index past the end of the array, it will not tell you about it. It will eventually crash or give you garbage data.
2. A function call must include "()" even if no parameters are passed. For example, C will accept x=rand;, but the call will not work. The memory address of the rand function will be placed into x instead. You must say x=rand();.

C Details

Variable types

There are 3 standard variable types in C:

Integer: int
Floating point: float
Character: char

An int is a 4-byte integer value. A float is a 4-byte floating point value. A char is a 1-byte single character (like 'a' or '3'). A string is declared as an array of characters.

There are a number of derivative types:

• double (8-byte floating point value)
• short (2-byte integer)
• unsigned short or unsigned int (positive integers - no sign bit)

Operators and Operator Precedence

The operators in C are similar to the operators in most languages:

+ - addition
- - subtraction
/ - division
* - multiplication
% - mod

The / operator performs integer division if both operands are integers and floating point division otherwise. For example:

void main()  
{    
    float a; 
    a=10/3;    
    printf("%f\n",a);  
} 

This code prints out a floating point value since a is declared as type float, but a will be 3.0 because the code performed an integer division.

Operator precedence in C is also similar to that in most other languages. Division and multiplication occur first, then addition and subtraction. The result of the calculation 5+3*4 is 17, not 32, because the * operator has higher precedence than + in C. You can use parentheses to change the normal precedence ordering. (5+3)*4 is 32. The 5+3 is evaluated first because it is in parentheses. See this page for more information on precedence, which becomes somewhat complicated in C once pointers are introduced.

Typecasting

C allows you to perform type conversions on the fly. You do this especially often when using pointers. Typecasting also occurs during the assignment operation for certain types. For example, in the code above, the integer value was automatically converted to a float.

You do typecasting in C by placing the type name in parentheses and putting it in front of the value you want to change. Thus, in the above code, replacing the line a=10/3; with a=(float)10/3; produces 3.33333 as the result because 10 is converted to a floating point value before the division.

Types

You declare named, user-defined types in C with the typedef statement. The following example shows a type that appears often in C code:

#define TRUE  1  
#define FALSE 0 
typedef int boolean; 
 
void main()  
{    
    boolean b; 
    
    b=FALSE;    
    blah blah blah  
} 

This code allows you to declare Boolean types in C programs.

If you do not like the word ``float'' for real numbers, you can say:

typedef float real;

and then later say:

real r1,r2,r3; 

You can place typedef statements anywhere in a C program as long as they come prior to their first use in the code.

Structures

Structures in C allow you to group variable into a package. Here's an example:

struct rec  
{    
    int a,b,c;    
    float d,e,f;  
}; 
 
struct rec r; 

As shown here, whenever you want to declare structures of the type rec, you have to say struct rec. This line is very easy to forget, and you get many compiler errors because you absent-mindedly leave out the struct. You can compress the code into the form:

struct rec  
{    
    int a,b,c;    
    float d,e,f;  
} r; 

where the type declaration for rec and the variable r are declared in the same statement. Or you can create a typedef statement for the structure name. For example, if you do not like saying struct rec r every time you want to declare a record, you can say:

typedef struct rec rec_type; 

and then declare records of type rec_type by saying:

rec_type r; 

You access fields of structure using a period, for example, r.a=5;.

Arrays

You declare arrays by inserting an array size after a normal declaration, as shown below:

int a[10];        /* array of integers */  
char s[100];      /* array of characters
                    (a C string) */  
float f[20];      /* array of reals */  
struct rec r[50]; /* array of records */ 

Incrementing

Long Way     Short Way
i=i+1;       i++;
i=i-1;       i--;
i=i+3;       i += 3;
i=i*j;       i *= j; 

Exercises

1. Try out different pieces of code to investigate typecasting and precedence. Try out int, char, float, and so on.
2. Create an array of records and write some code to sort that array on one integer field.

C error to avoid

1. As described above, using the / operator with two integers will often produce an unexpected result, so think about it whenever you use it.

Functions in C

Most languages allow you to create functions of some sort. Functions let you chop up a long program into named sections so that the sections can be reused throughout the program. Functions accept parameters and return a result. C functions can accept an unlimited number of parameters. In general, C does not care in what order you put your functions in the program, so long as a the function name is known to the compiler before it is called.

We have already talked a little about functions. The rand function seen previously is about as simple as a function can get. It accepts no parameters and returns an integer result:

int rand() 
/* from K&R 
   - produces a random number between 0 and 32767.*/ 
{ 
    rand_seed = rand_seed * 1103515245 +12345; 
    return (unsigned int)(rand_seed / 65536) % 32768; 
} 

The int rand() line declares the function rand to the rest of the program and specifies that rand will accept no parameters and return an integer result. This function has no local variables, but if it needed locals, they would go right below the opening { (C allows you to declare variables after any { - they exist until the program reaches the matching } and then they dissappear. A function's local variables therefore vanish as soon as the matching } is reached in the function. While they exist, local variables live on the system stack.) Note that there is no ; after the () in the first line. If you accidentally put one in, you will get a huge cascade of error messages from the compiler that make no sense. Also note that even though there are no parameters, you must use the (). They tell the compiler that you are declaring a function rather than simply declaring an int.

The return statement is important to any function that returns a result. It specifies the value that the function will return and causes the function to exit immediately. This means that you can place multiple return statements in the function to give it multiple exit points. If you do not place a return statement in a function, the function returns when it reaches } and returns a random value (many compilers will warn you if you fail to return a specific value). In C, a function can return values of any type: int, float, char, struct, etc.

There are several correct ways to call the rand function. For example: x=rand();. The variable x is assigned the value returned by rand in this statement. Note that you must use () in the function call, even though no parameter is passed. Otherwise, x is given the memory address of the rand function, which is generally not what you intended.

You might also call rand this way:

if (rand() > 100) 

Or this way:

rand(); 

In the latter case, the function is called but the value returned by rand is discarded. You may never want to do this with rand, but many functions return some kind of error code through the function name, and if you are not concerned with the error code (for example, because you know that an error is impossible) you can discard it in this way.

Functions can use a void return type if you intend to return nothing. For example:

void print_header()  
{     
    printf("Program Number 1\n");     
    printf("by Marshall Brain\n");     
    printf("Version 1.0, released 12/26/91\n");  
} 

This function returns no value. You can call it with the following statement:

print_header(); 

You must include () in the call. If you do not, the function is not called, even though it will compile correctly on many systems.

C functions can accept parameters of any type. For example:

int fact(int i)  
{    
    int j,k; 
    
    j=1; 
    for (k=2; k<=i; k++) 
        j=j*k;    
    return j;  
} 

returns the factorial of i, which is passed in as an integer parameter. Separate multiple parameters with commas:

int add (int i, int j)  
{    
    return i+j;  
} 

C has evolved over the years. You will sometimes see functions such as add written in the "old style," as shown below:

int add(i,j)  
    int i;  
    int j;  
{    
    return i+j;  
} 

It is important to be able to read code written in the older style. There is no difference in the way it executes; it is just a different notation. You should use the "new style," (known as "ANSI C") with the type declared as part of the parameter list, unless you know you will be shipping the code to someone who has access only to an "old style" (non-ANSI) compiler.

It is now considered good form to use function prototypes for all functions in your program. A prototype declares the function name, its parameters, and its return type to the rest of the programprior to the function's actual declaration. To understand why function prototypes are useful, enter the following code and run it:

#include <stdio.h> 
 
void main()  
{    
    printf("%d\n",add(3));  
} 
 
int add(int i, int j)  
{    
    return i+j;  
} 

This code compiles on many compilers without giving you a warning, even though add expects two parameters but receives only one. It works because many C compilers do not check for parameter matching either in type or count. You can waste an enormous amount of time debugging code in which you are simply passing one too many or too few parameters by mistake. The above code compiles properly, but it produces the wrong answer.

To solve this problem, C lets you place function prototypes at the beginning of (actually, anywhere in) a program. If you do so, C checks the types and counts of all parameter lists. Try compiling the following:

#include <stdio.h> 
 
int add (int,int); /* function prototype for add */ 
 
void main()  
{    
    printf("%d\n",add(3));  
} 
 
int add(int i, int j)  
{    
    return i+j;  
} 

The prototype causes the compiler to flag an error on the printf statement.

Place one prototype for each function at the beginning of your program. They can save you a great deal of debugging time, and they also solve the problem you get when you compile with functions that you use before they are declared. For example, the following code will not compile:

#include <stdio.h> 
 
void main()  
{    
    printf("%d\n",add(3));  
} 
 
float add(int i, int j)  
{    
    return i+j;  
} 

Why, you might ask, will it compile when add returns an int but not when it returns a float? Because older C compilers default to an int return value. Using a prototype will solve this problem. "Old style" (non-ANSI) compilers allow prototypes, but the parameter list for the prototype must be empty. Old style compilers do no error checking on parameter lists.

Exercises

• Go back to the bubble sort example presented eariler and create a function for the bubble sort.
• Go back to earlier programs and create a function to get input from the user rather than taking the input in the main function.

Libraries in C

Libraries are very important in C because the C language supports only the most basic features that it needs. C does not even contain I/O functions to read from the keyboard and write to the screen. Anything that extends beyond the basic language must be written by a programmer. The resulting chunks of code are often placed in libraries to make them easily reusable. We have seen the standard I/O, or stdio, library already: Standard libraries exist for standard I/O, math functions, string handling, time manipulation, and so on. You can use libraries in your own programs to split up your programs into modules. This makes them easier to understand, test, and debug, and also makes it possible to reuse code from other programs that you write.

You can create your own libraries easily. As an example, we will take some code from a previous article in this series and make a library out of two of its functions. Here's the code we will start with:

#include <stdio.h>  
 
#define MAX 10 
 
int a[MAX]; 
int rand_seed=10; 
 
int rand()  
/* from K&R 
   - produces a random number between 0 and 32767.*/  
{    
    rand_seed = rand_seed * 1103515245 +12345;    
    return (unsigned int)(rand_seed / 65536) % 32768;  
} 
 
void main()  
{    
    int i,t,x,y;
  
    /* fill array */    
    for (i=0; i < MAX; i++)    
    {      
        a[i]=rand();      
        printf("%d\n",a[i]);    
    }
  
    /* bubble sort the array */    
    for (x=0; x < MAX-1; x++)      
        for (y=0; y < MAX-x-1; y++)        
            if (a[y] > a[y+1])        
            {  
                t=a[y];  
                a[y]=a[y+1];  
                a[y+1]=t;        
            } 
 
    /* print sorted array */    
    printf("--------------------\n");    
    for (i=0; i < MAX; i++)     
        printf("%d\n",a[i]); 
} 

This code fills an array with random numbers, sorts them using a bubble sort, and then displays the sorted list.

Take the bubble sort code, and use what you learned in the previous article to make a function from it. Since both the array a and the constant MAX are known globally, the function you create needs no parameters, nor does it need to return a result. However, you should use local variables for x, y, and t.

Once you have tested the function to make sure it is working, pass in the number of elements as a parameter rather than using MAX:

#include <stdio.h> 
 
#define MAX 10 
 
int a[MAX]; 
int rand_seed=10; 

/* from K&R 
   - returns random number between 0 and 32767.*/
int rand()   
{    
    rand_seed = rand_seed * 1103515245 +12345;    
    return (unsigned int)(rand_seed / 65536) % 32768;  
}
  
void bubble_sort(int m)  
{    
    int x,y,t; 
     for (x=0; x < m-1; x++)      
        for (y=0; y < m-x-1; y++)        
            if (a[y] > a[y+1])        
            {  
                t=a[y];  
                a[y]=a[y+1];  
                a[y+1]=t;        
            } 
}
  
void main()  
{    
    int i,t,x,y; 
    /* fill array */    
    for (i=0; i < MAX; i++)    
    {      
        a[i]=rand();      
        printf("%d\n",a[i]);    
    } 
    bubble_sort(MAX); 
    /* print sorted array */    
    printf("--------------------\n");    
    for (i=0; i < MAX; i++)      
        printf("%d\n",a[i]); 
} 

You can also generalize the bubble_sort function even more by passing in a as a parameter:

bubble_sort(int m, int a[]) 

This line says, "Accept the integer array a of any size as a parameter." Nothing in the body of the bubble_sort function needs to change. To call bubble_sort change the call to:

bubble_sort(MAX, a); 

Note that &a has not been used in the function call even though the sort will change a. The reason for this will become clear once you understand pointers.

Making a Library

Since the rand and bubble_sort functions in the program above are useful, you will probably want to reuse them in other programs you write. You can put them into a utility library to make their reuse easier.

Every library consists of two parts: a header file and the actual code file. The header file, normally denoted by a .h suffix, contains information about the library that programs using it need to know. In general, the header file contains constants and types, along with prototypes for functions available in the library. Enter the following header file and save it to a file named util.h .

/* util.h */
extern int rand(); 
extern void bubble_sort(int, int []); 

These two lines are function prototypes. The word "extern" in C represents functions that will be linked in later. If you are using an old-style compiler, remove the parameters from the parameter list of bubble_sort.

Enter the following code into a file named util.c.

/* util.c */
#include "util.h" 
 
int rand_seed=10; 
 
/* from K&R
 - produces a random number between 0 and 32767.*/ 
int rand()  
{    
    rand_seed = rand_seed * 1103515245 +12345;    
    return (unsigned int)(rand_seed / 65536) % 32768;  
} 
  
void bubble_sort(int m,int a[])  
{    
    int x,y,t; 
     for (x=0; x < m-1; x++)      
        for (y=0; y < m-x-1; y++) 
            if (a[y] > a[y+1])        
            {  
                t=a[y];  
                a[y]=a[y+1];  
                a[y+1]=t;        
            }  
} 

Note that the file includes its own header file (util.h) and that it uses quotes instead of the symbols < and > , which are used only for system libraries. As you can see, this looks like normal C code. Note that the variable rand_seed, because it is not in the header file, cannot be seen or modified by a program using this library. This is called information hiding. Adding the word static in front of int enforces the hiding completely.

Enter the following main program in a file named main.c.

#include <stdio.h>  
#include "util.h" 
 
#define MAX 10  
 
int a[MAX];  
 
void main()  
{    
    int i,t,x,y; 
    /* fill array */    
    for (i=0; i < MAX; i++)    
    {      
        a[i]=rand();      
        printf("%d\n",a[i]);    
    } 
 
    bubble_sort(MAX,a); 
 
    /* print sorted array */   
    printf("--------------------\n");    
    for (i=0; i < MAX; i++)      
        printf("%d\n",a[i]); 
} 

This code includes the utility library. The main benefit of using a library is that the code in the main program is much shorter.

Compiling and Running with a Library

To compile the library, type the following at the command line (assuming you are using UNIX) (replace gcc with gcc if your system uses cc):

gcc -c -g util.c 

The -c causes the compiler to produce an object file for the library. The object file contains the library's machine code. It cannot be executed until it is linked to a program file that contains a main function. The machine code resides in a separate file named util.o.

To compile the main program, type the following:

gcc -c -g main.c 

This line creates a file named main.o that contains the machine code for the main program. To create the final executable that contains the machine code for the entire program, link the two object files by typing the following:

gcc -o main main.o util.o 

which links main.o and util.o to form an executable named main. To run it, type main.

Makefiles

It can be cumbersome to type all of the gcc lines over and over again, especially if you are making a lot of changes to the code and it has several libraries. The make facility solves this problem. You can use the following makefile to replace the compilation sequence above:

main: main.o util.o  
        gcc -o main main.o util.o 
main.o: main.c util.h 
        gcc -c -g main.c 
util.o: util.c util.h  
        gcc -c -g util.c 

Enter this into a file named makefile, and type make to build the executable. Note that you must precede all gcc lines with a tab. (Eight spaces will not suffice---it must be a tab. All other lines must be flush left.)

This makefile contains two types of lines. The lines appearing flush left are dependency lines. The lines preceded by a tab are executable lines, which can contain any valid UNIX command. A dependency line says that some file is dependent on some other set of files. For example, main.o: main.c util.h says that the file main.o is dependent on the files main.c and util.h. If either of these two files changes, the following executable line(s) should be executed to recreate main.o.

Note that the final executable produced by the whole makefile is main, on line 1 in the makefile. The final result of the makefile should always go on line 1, which in this makefile says that the file main is dependent on main.o and util.o. If either of these changes, execute the line gcc -o main main.o util.o to recreate main.

It is possible to put multiple lines to be executed below a dependency line---they must all start with a tab. A large program may have several libraries and a main program. The makefile automatically recompiles everything that needs to be recompiled because of a change.

If you are not working on a UNIX machine, your compiler almost certainly has functionality equivalent to makefiles. Read the documentation for your compiler to learn how to use it.

Now you understand why you have been including stdio.h in earlier programs. It is simply a standard library that someone created long ago and made available to other programmers to make their lives easier.

Text Files in C

Text files in C are straightforward and easy to understand. All text file functions and types in C come from the stdio library.

When you need text I/O in a C program, and you need only one source for input information and one sink for output information, you can rely on stdin (standard in) and stdout (standard out). You can then use input and output redirection at the command line to move different information streams through the program. There are six different I/O commands in <stdio.h> that you can use with stdin and stdout:

printf		prints formatted output to stdout
scanf		reads formatted input from stdin
puts		prints a string to stdout
gets		reads a string from stdin
putc		prints a character to stdout
getc, getchar	reads a character from stdin

The advantage of stdin and stdout is that they are easy to use. Likewise, the ability to redirect I/O is very powerful. For example, maybe you want to create a program that reads from stdin and counts the number of characters:

#include <stdio.h>  
#include <string.h> 
 
void main()  
{    
    char s[1000];    
    int count=0;     
     while (gets(s))      
        count += strlen(s);    
    printf("%d\n",count);  
} 

Enter this code and run it. It waits for input from stdin, so type a few lines. When you are done, press CTRL-D to signal end-of-file (eof). The gets function reads a line until it detects eof, then returns a 0 so that the while loop ends. When you press CTRL-D, you see a count of the number of characters in stdout (the screen). (use man gets or your compiler's documentation to learn more about the gets function.)

Now, suppose you want to count the characters in a file. If you compiled the program to an executable named xxx, you can type the following:

xxx < filename 

Instead of accepting input from the keyboard, the contents of the file named filename will be used instead. You can achieve the same result using pipes:

cat < filename | xxx

You can also redirect the output to a file:

xxx < filename > out 

This command places the character count produced by the program in a text file named out.

Sometimes, you need to use a text file directly. For example, you might need to open a specific file and read from or write to it. You might want to manage several streams of input or output or create a program like a text editor that can save and recall data or configuration files on command. In that case, use the text file functions in stdio:

fopen	opens a text file
fclose	closes a text file
feof	detects end-of-file marker in a file
fprintf	prints formatted output to a file
fscanf	reads formatted input from a file
fputs	prints a string to a file
fgets	reads a string from a file
fputc	prints a character to a file
fgetc	reads a character from a file

You use fopen to open a file. It opens a file for a specified mode (the three most common are r, w, and a, for read, write, and append). It then returns a file pointer that you use to access the file. For example, suppose you want to open a file and write the numbers 1 to 10 in it. You could use the following code:

#include <stdio.h> 
#define MAX 10 

int main()  
{    
    FILE *f;    
    int x; 
    f=fopen("out","w");
    if (!f)
        return 1;    
    for(x=1; x<=MAX; x++)      
        fprintf(f,"%d\n",x); 
    fclose(f); 
    return 0;
} 

The fopen statement here opens a file named out with the w mode. This is a destructive write mode, which means that if out does not exist it is created, but if it does exist it is destroyed and a new file is created in its place. The fopen command returns a pointer to the file, which is stored in the variable f. This variable is used to refer to the file. If the file cannot be opened for some reason, f will contain NULL.

if (f == 0)

The fprintf statement should look very familiar: It is just like printf but uses the file pointer as its first parameter. The fclose statement closes the file when you are done.

To read a file, open it with r mode. In general, it is not a good idea to use fscanf for reading: Unless the file is perfectly formatted, fscanf will not handle it correctly. Instead, use fgets to read in each line and then parse out the pieces you need.

The following code demonstrates the process of reading a file and dumping its contents to the screen:

#include <stdio.h> 
 
int main()  
{    
    FILE *f;    
    char s[1000]; 
    
    f=fopen("infile","r"); 
    if (!f)
        return 1;   
    while (fgets(s,1000,f)!=NULL)      
        printf("%s",s);    
    fclose(f); 
    return 0; 
} 

The fgets statement returns a NULL value at the end-of-file marker. It reads a line (up to 1,000 characters in this case) and then prints it to stdout. Notice that the printf statement does not include \n in the format string, because fgets adds \n to the end of each line it reads. Thus, you can tell if a line is not complete in the event that it overflows the maximum line length specified in the second parameter to fgets.

C Errors to Avoid

• Do not accidentally type close instead of fclose . The close function exists, so the compiler accepts it. It will even appear to work if the program only opens or closes a few files. However, if the program opens and closes a file in a loop, it will eventually run out of available file handles and/or memory space and crash, because close is not closing the files correctly.