or Save All button on the toolbar or choosing the File menu and
the Save All option.|
~ Introduction to Programming: C ~ Session 6 - A recap of what we have covered so far |
In This Session...
In this session we will be covering the following topics:-
Last session we covered:-
Introducing scanf to read data from the keyboard into a variable
Conditional Instructions - The if command (incl. about code blocks).
Exercise 16 - (Extra) - Changing our stats program and using it as a separate executable
Remember the steps to create a new project in C++ Builder. These should be followed every time you start a new program.
Also remember that in C++ Builder, each program you write is stored in two files - the Project file which holds details such as version information and any special settings you want to apply to your program (we do not look at this on the beginner's course, but you still need it). The Program or Source file is where you type in the C program to try it out. You need to save both to make your program work properly. If you want to copy a program to a floppy disk, make sure you copy both programs. It is best to call both files the same name (e.g. called it exercise17 for both to create two files - the project file, called either exercise17.mak or exercise17.bpr depending on the version of C++ Builder that you are using, and the source file called exercise17.cpp
In C++ Builder 6, the steps for creating a file are as follows:-
Click on the File menu
Click on the New submenu
Click on the Other submenu. A dialog box pops up.
Double-click on the icon labelled Console Wizard.
Click on the radio button (small circles - clicking makes the circle filled and selects an option) marked Source Type - C and click on the OK button.
If you made any changes to another project, you will be asked whether you wish to save it. Choose the appropriate response. Typically, you would click on No if the project name was Project1 or Project2 etc. - this is the default name given to the automatically created project which appears when you first start C++ Builder.
You should now see some starting C source code which we will not be needing.
To clear the source code, click on the Edit menu and select the Select All option. Press the Delete button on your keyboard to clear the selection.
To save the project, and any changes made to your program, choose the File menu and click on the Save All option or choose the Save All button on the toolbar (hover your cursor over the buttons to see tips on which button does what).
Make sure you store the file on your Z: drive - look in the drop-down list next to the Save In title and click on the bottom-most option, which should be marked with your student ID.
You will be asked to give a name and location for your project (initially called Project1). Give it a name, ending with .mak – e.g. myproj.mak and click on Save to save the project. The C program will automatically be saved with the same name as the project, but with an ending of .cpp (for C Plus Plus). Most C programs end with .C but C++ Compilers invariably recognize C source code, so it is fine to leave this as it is.
Exercise 18 - Create a new project
Create a new project called exercise18 which contains no program in it. Just save it, and save the blank program also as exercise18. Start-up Windows Explorer, and look on the Z: drive. You should see two new files called exercise18.bpr and exercise18.cpp
All of your C programs should have the same basic structure, as shown below. Type out the following program:-
#include <stdio.h>
#include <conio.h>
void main() {
/* program goes here */
getch();
}
Save the program by clicking on the
or Save All button on the toolbar or choosing the File menu and
the Save All option.
Compile and Execute (start) the program by
clicking on the 
button. This is the process that this simple click initiates:-
It goes through a pre-processor stage where it checks out any special compiler commands - usually anything beginning with a # or hash. In this case, it will look for two files called stdio.h and conio.h which give details of commands relating the standard in-out and console in-out respectively. These details are included into the program, as if they were typed where the #include appears.
Syntax checking occurs, where the C Compiler looks through your program and checks to see if the language is in a form that it understands. If it does not, an error message will be displayed at the bottom of the editing window. This is known as a compile-time error as it happens whilst you are trying to compile the program. You can double-click on any of these errors for the program to place the cursor where the error believes the problem to be. The process of correcting these mistakes is known as debugging after the first famous difficult-to-track problem with a computer was found to be with insects (American: bugs) crawling around in the circuitry (or amongst the valves in those days!) - in case you are wondering where such a strange terms comes from!
If the source code is free from bugs, then the compiler turns the source code into object code which is in a form that can be linked in with any other programs - e.g. libraries in C - to form a whole program.
The object code is linked to form an executable program which the computer's CPU or Central Processing Unit can understand. This runs very fast because no translation needs to be done between the C language and the language understood by the computer - that is what the compilation process is all about!
Nothing much happens, other than a cursor flashing in the top-left hand corner of the screen. Press a key to return from the program to view the source code again. Here is what is happening:-
the #include commands tell the computer to look in these files or libraries for any special instructions or functions that have been pre-written to perform specific common tasks.
The main() line tells us that we are creating a new function whose task is to identify what to do when the program starts. All C programs should have a main() function, otherwise the computer will not know where to start executing instructions.
The curly braces { and } show where the program (or list of instructions) starts and ends respectively. Read { as begin and } as end.
The /* and */ start and end a comment. A comment does not do anything other than explain something that is happening in the program. It is a way of remembering important information for next time you need to understand how the program works, or why you have written it in a particular way. Note that a comment can span a number of lines. In this instance, it is being used to show where the main part of your program would ordinarily be located.
The getch() function is held in the conio.h library. It's task is to wait for a key to be pressed on the keyboard. In this case, it is being used to make sure that the black console window does not close before we have a chance to see any results. In this case, there are none. We will pad out the program later so that it does something useful.
printf and character constants
Remember that the printf function is a function that is held in the stdio.h library? It's easiest form is as follows:-
printf("Hello world\n");
Remember that anything between brackets is data to be passed to a function - in this case the printf (means print formatted) function. Also note, that to specify some text that we want to be treated as it looks, rather than as calculations within the program, we surround the text with double-quotes. So in this case we are passing the text Hello world\n to the printf function so that it can be displayed on the black console window.
Note that there are some special command characters that start with a backslash (\) to show certain common actions that should be interpreted. For example:-
\n means new line or move the cursor down one line, and to the bottom-left of the screen. Otherwise known as a carriage return
\a means alert which typically makes the computer make a beep noise
\f means form feed, which is used with printers and some console devices to move to the next page.
\r means carriage return, which is the same as moving the cursor to the left of the line.
\t means horizontal tab - a bit like tab-stops on a typewriter or word-processor. This typically moves the cursor to the next column divisible by 8. Also just called a tab character.
\0 means null - it is used for storing arrays of characters to show where the end of the text is.
\\ means backslash - as the \ character is interpreted along with the following letter, to get an actual backslash, use two in succession. Other characters that use this notation are \! for an exclamation mark and \" for double-quotes.
\" means use a double-quote - to output a quote without ending the string (which is what a double-quote would normally do)
\" means use a single-quote - to output a quote without ending the character constant (which is what a single-quote would normally do)
%% means use a percentage symbol - to output a percentage symbol as it is instead of using the next character to signify a place where data is to be put (e.g. %d and %2.5f etc.)
Type in the following to add to the program above (the new portion is highlighted - remove the comment):-
#include <stdio.h>
#include <conio.h>
void main() {
printf("Hello there\nWelcome!\a");
getch();
}
Notice the semi-colon (;) symbol is used to show where the end of each instruction ends. In this case, the words Hello there will be shown on the black console window, and the cursor moved to the next line down / to the left (\n character constant) followed by the word Welcome! followed by a beep sound from the computer.
Save the program under the same project as
before, and click on the
button to compiler/link/run the program.
FOR YOU: Add another line that displays the message Press A Key followed by a tab character followed by a beep. The cursor will wait to the right of the message for the user to press a key before terminating the program.
A variable is just a box or memory-jogger that stores some data, and is given a name so that you can remember what the data is for. It is essential for performing calculations in a program, or storing the results of user input - e.g. which key they have pressed. The contents are often used by conditional statements such as if and while to decide what to do with the data.
You need to tell the computer what type of information is to be stored in the box so that the computer knows what to do with it. This is called declaring the variable. The most common types are as follows:-
integer - An integer is a whole number (e.g. -2, 0, 2, 10, but not 0.5 or 2.3 or 'X'). Declare using int myVariable;
floating point or real number - This is a number that can contain decimal parts (e.g. -1.5, 0.0, 2.0, 2.3333, 10.12 but not 'X' and preferably not 0 or 2 without the .0 ending). Declare using float myVariable;
character - This stores a single character (e.g. 'A', 'a', '@', '1' but not 'Hello', 2.5 or "Hello\n"). Declare using char myVariable;
Once you have declared your variable, you need to put some data into it. E.g. to declare three integer variables called myNum1, myNum2 and myNum3 and place the values in them:-
#include <stdio.h>
#include <conio.h>
void main() {
printf("Three variables\n");
int myNum1,myNum2,myNum3;
myNum1 = 3; myNum2 = 4; myNum3 = 5;
