CMPS 12A Introduction to Programming Lab Assignment 7


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The purpose of this assignment is to introduce the C programming language, including standard inputoutput functions. Begin by reading the first six chapters “C Programming Notes” by Steve Summit which
can be found at
Six chapters may sound like a lot, but much of it can be skimmed, since it is largely a review of topics
already covered. You will find that C is very similar to Java when it comes to things like arithmetic
expressions, control structures, and function calls. One area where the two languages differ significantly is
in the way they do input and output. The best place to start is the HelloWorld program:
/* HelloWorld.c */
int main(){
printf(“Hello, World!\n”);
return 0;
Copy this program to a file called HelloWorld.c, then compile it by doing
% gcc –o HelloWorld HelloWorld.c
The Unix command gcc invokes the Gnu C Compiler, which is an open source C compiler freely available
for most platforms. The –o option to gcc tells the compiler what to call the executable object file, in this
case HelloWorld. If you leave out the option –o HelloWorld, the executable will be called a.out.
The last item on the line is the source code file name, i.e. HelloWorld.c. List the contents of your current
directory and you should see a new executable file called HelloWorld. Run it by typing the name of the
file at the command line as follows:
% HelloWorld
Hello, World!
Recall that to run a Java program we needed to type: java program_name (unless we’ve placed the Java
program in an executable jar file, as in a previous lab.) As we see, to run a C program, one types the name
of the executable file. Why this difference? Java programs are executed by a “simulation” of a computer
called the Java Virtual Machine (JVM). In a Unix environment, the JVM is invoked by the command
java. Thus java program_name runs the JVM with one command line argument, i.e. the name of the
.class file containing function main() for the program. This additional layer of software abstraction is
not present for C programs since there is no simulation – you run the executable binary file directly on the
computer hardware.
The first thing to notice in the source file HelloWorld.c is that there is no class keyword. Java is known
as an Object Oriented Programming (OOP) language, meaning that data structures and the procedures that
operate on them are grouped together into one language construct, called the class. Common behavior
amongst classes is specified explicitly through the mechanism of inheritance. The C language however
does not directly support OOP, although OOP can be implemented with some effort. C is known as a
procedural programming language, which means that data structures and functions are separate language
constructs. There are no classes and no inheritance. Most control structures such as loops (while, do-while,
for), branching (if, if-else, switch), and function calls are virtually identical in the two languages.
Comments in C are specified by bracketing them between the symbols /* and */, and may span several
lines. For instance
/* comment */
* comment
* comment
* comment
are all acceptable styles. Note that the single line // comment familiar from Java, is not recognized by
all C compilers, although the gcc compiler does accept this form of comment by default.
Any line beginning with # is a preprocessor directive. These directives, which are literal text substitutions,
are executed during the first phase of compilation. Commands that start with #include are roughly
equivalent to import statements in Java. The line #include inserts the library header file
stdio.h into the program. The commands in stdio.h specify functions for performing input-output
A C program must contain exactly one function called main(), which is the entry point for program
execution. The general skeleton for main() is
int main(){
/* variable declarations */
/* executable statements */
return 0;
Observe that function main() has return type int. A return value of 0 indicates to the caller (namely the
operating system) that execution was nominal and without errors. Typically main will call other functions,
which may in turn call other functions. Function definitions in C look very much like they do in Java.
DataType FunctionName(DataType1 Variable1, DataType2 Variable2, …){
/* local variable declarations */
/* executable statements */
/* return statement (provided return type is not void) */
One important difference however is that local variable declarations must appear at the beginning of the
function body, and may not be interspersed with executable statements, as they can in Java.
The function printf(), which belongs to the library stdio.h, prints formatted text to stdout. As you
know, stdout is a data stream that, by default, is connected to the terminal window. The first argument to
printf() is known as a format string and consists of two types of characters. The first type are simply
characters to be printed. The second type, known as format specifiers, define the way the remaining
arguments will be printed. A format specifier consists of a percent sign followed by a format code. There
must be exactly the same number of format specifiers as there are remaining arguments, and the format
specifiers are matched with the remaining arguments in order. For example
printf(“There are %d days in %s\n”, 30, “April”);
prints the text
There are 30 days in April
Observe that the newline character “\n” is explicitly placed in the output stream. Some common format
specifiers are:
%c character
%d signed decimal integer
%f floating point number (type double or float)
%s string of characters
%e floating point number in scientific notation
%% prints a percent sign
See a good reference on C, such as the URL mentioned above, for other format specifiers. Note that Java
contains a method System.out.printf() that behaves (almost) exactly like C’s printf(). The
following example prints values of some local variables, along with the usual hello message.
/* HelloWorld2.c */
int main(){
int a = 6;
double x = 2.3;
char ch = ‘$’;
char hello_string[] = “Hello, World!”;
printf(“a = %d, x = %f, ch = %c\n”, a, x, ch);
printf(“%s%c”, hello_string, ‘\n’);
return 0;
Notice that a string in C is just a char array, and is initialized in much the same way as a String in Java.
Also notice that when declaring an array in C, the brackets go after the array name, not after the data type.
The output of the above program is:
a = 6, x = 2.300000, ch = $
Hello, World!
The library stdio.h also includes a function called scanf(), a general purpose input function that reads
the from stream stdin normally associated with the keyboard, and stores the information in the variables in
its argument list. It can read all the built-in data types and automatically convert them to the proper internal
format. The first argument to scanf() is a format string consisting of three types of characters: format
specifiers, whitespace characters, and non-whitespace characters. The format specifiers consist of a percent
sign followed by a format code, and tell scanf() what type of data is to be read. For example %s reads a
string while %d reads an integer. Some common format codes are:
%c char
%d int
%f float
%lf double (that’s the letter ‘l’ between % and f, not the number 1)
%s string (i.e. char array)
See a good C reference for other scanf() codes, which are similar but not identical to printf()’s format
codes. Note the URL mentioned at the beginning of this document does not cover scanf(). A short listing
of both printf()’s and scanf()’s format codes can be found at
The format string is read left to right and the characters are matched, in order, with the remaining arguments
to scanf(). A whitespace character (i.e. space, newline, or tab) in the format string causes scanf() to skip
over one or more whitespace characters in the input stream. In other words, one whitespace character in
the format string will cause scanf() to read, but not to store, any number (including zero) of whitespace
characters up to the first non-whitespace character. A non-whitespace character in the format string causes
scanf() to read and discard a single matching character in the input stream. For example, the control string
” %d, %s” causes scanf() to first skip any number of leading whitespace characters, then read one integer,
then read and discard one comma, then skip any number of whitespace characters, then read one string. If
the specified character is not found, scanf() will terminate. All the variables receiving values through
scanf() must be passed by reference, i.e. the address of each variable receiving a value must be placed in
the argument list. In C, the “address-of” operator & gives the address of a variable. Thus if x is a variable,
then the expression &x evaluates to the address in memory referred to by the variable x. The following
program asks the user for three integers, then echoes them back to the screen.
/* BasicIO.c */
int main(void){
int x, y, z;
printf(“Enter three integers separated by”);
printf(” commas, then press return: “);
scanf(” %d, %d, %d”, &x, &y, &z);
printf(“The integers entered were %d, %d, %d\n”, x, y, z);
return 0;
A sample run of this program looks like
Enter three integers separated by commas, then press return: 12, -7, 13
The integers entered were 12, -7, 13
Running again, but this time leaving out the separating commas in the input line gives
Enter three integers separated by commas, then press return: 12 -7 13
The integers entered were 12, 4, -4261060
Since the comma separating the first and second integers was left out scanf() read the first integer, then
expected to read and discard a comma but failed to do so, and then just returned without reading anything
else. The values printed for y and z are the random data stored in uninitialized variables, and will no doubt
be different when you run the program on the same input. Thus we see that scanf() is intended for reading
formatted input. (This is what the “f” it its name stands for). The scanf() function returns a number
equal to the number of variables that were successfully assigned values. That number can be tested and
used within the program, as the following example illustrates.
/* BasicIO2.c */
int main(void){
int n, i;
double x[3];
printf(“Enter three doubles separated by “);
printf(“spaces, then press return: “);
n = scanf(” %lf %lf %lf”, &x[0], &x[1], &x[2]);
printf(“%d numbers were successfully read: “, n);
for(i=0; i at the beginning of your program if you want to use this
function. Important note: see Examples/lab7/Circle.c on the class webpage for instructions on how
to compile such a program.
Your program should behave as follows on sample input:
% Sphere
Enter the radius of the sphere: 9
The volume is 3053.628060 and the surface area is 1017.876020.
% Sphere
Enter the radius of the sphere: 115
The volume is 6370626.303536 and the surface area is 166190.251397.
Submit your program to the assignment name lab7, and perform the usual checks to make sure that it was