Lecture 4: Introduction to Programming

Moreover, we have to give (write) the instruction with a lot of care. The computer understands only the thing we taught it to understand

Creating programs always consists of the following steps:
 

Think! Study and analyze the problem. Collect information. Come up with a possible solution. Don't touch that keyboard yet! Use pen and paper. Write a program. Use whatever editor to enter your program Eliminate the errors from the program. This is called "debugging". There are several types of errors: compile-time errors. Writing errors (For example when we write prinntf instead of printf). Easy to elliminate; the compiler is going to help us by telling us something like "type-mismatch error in line 34" programmatic errors. For example forgetting to initialize (setting to a certain value) our variables. This can also cause so-called "run-time errors", for example "division by zero error". logical errors (for example, we do not know that the Pythagoras rule is a2 + b2 = c2). The program will run without generating errors, but the result will not be what we wanted. Analyze the result. Is this what you wanted? Maybe the program wrote "The square-root of 9 is 4" Clearly not what we wanted. If necessary, go to step 3, 2 or even 1. Spending some more time on point 1 can often save a lot of time in the other steps.

The First Computer Bug  Grace Murray Hopper, working in a temporary World War I building at Harvard University on the Mark II computer, found the first computer bug beaten to death in the jaws of a relay. She glued it into the logbook of the computer and thereafter when the machine stops (frequently) they tell Howard Aiken that they are "debugging" the computer. The very first bug still exists in the National Museum of American History of the Smithsonian Institution. Edison had used the word bug and the concept of debugging previously but this was probably the first verification that the concept applied to computers. (copied from http://www./firstcomputerbug.html)

C was invented in Bell Labs in 1971-1973. It was an evolution of the language B, which in turn was based on BCPL. In 1983 the language was standardized and that became the official version. It is probably the most used programming language in the world.
The evolution of C went hand-in-hand with the evolution of the UNIX operating system which we are going to use in our lectures (in the form of Linux, which is a graphical variant of UNIX). In fact, UNIX itself was written in C.

A program is a sequence of instructions, or statements which inform the computer of a specific task we want it to do.
Most modern program languages are in a very readible format, close to English, making it easy for humans to read and write programs. This in contrast to earlier programming languages, which were closer to things the computer understand. See for example the assembler language (aula 2).

A very simple C program:

#include <stdio.h>

main()
{
  printf("Hello World\n");
}
 

Let's take a look at this program.

• Every line that starts with a hash # are instructions for the compiler or linker rather than command that will be executed at run-time. #include <stdio.h> means that the library stdio ("standard input-output") has to be fetched by the compiler in order to understand what is coming. We will use the command printf which is part of the stdio library.
• After this we can write our own procedures and functions (see the lecture on modular programming).
• "main" is a special function. The first instruction of this function is always the first instruction to be executed. For the first couple of weeks we will only write instructions in this function.
• Therefore, the instruction printf("HellowWorld\n") is executed first and since it is the only instruction this also finishes the program. The instruction writes the text Hello World on the screen and goes to the next line (\n).

Identifiers, as the name already says, are used for identifying things. This can be names of  functions and names of variables and. This we will see in later aulas. Like in most languages, names of identifiers have some restrictions:

• They should start with a letter; "20hours" is not allowed.
• Followed by any combination of letters, digits or the underscore character "_".
• Spaces are not allowed, nor are characters like "(", "{", "[", "%", "#", "?", etc, except "_". The reason why this is so is that these characters are used for other things in C. They are called reserved characters.

{ } [ ] ( ) - = + / ? < > . , ; : ' " ! @ # $ % ^ & * ~ ` \ |

• Identifiers cannot be equal to reserved keywords (and also better to avoid predefined functions) of C, such as words like "int" or "main". Note that identifiers like "int1", "int_" or "Int" are allowed, although it is advised to avoid such confusing names. Note: In many programming environments, we will notice when we are using a reserved keyword because they will change color when we type them in.
• Choose your identifiers well. When a variable is used for storing interest rates, call it, for example "interest" and not "variable1". Although it is not an error to give it the name "variable1", it is much more intelligent to give it a more meaningful name. This helps other people to understand your program (or yourself when you come back to the program after a long time).
• The minimum length of identifiers is 1, the maximum length 255. Make use of this possibility of long names, but also remember that also long names can make the program unreadable. Choose a "golden middle". Which of the following do you think is best:

  r = r + a;
  money = money + interest;
  themoneyintheaccountofpersonwithnameJohnson = themoneyintheaccountofpersonwithnameJohnson + thecurrentinterestrateatthetimeofthiswriting;
• They are case sensitive, "i" is not equal to "I", etc. To make programs more readable, follow a convention all through your program(s). The most often used convention is lowercase for variables and UPPERCASE for CONSTANTS.

• Use meaningful names for variables, procedures and functions.
• Use indentation. Compare the following two programs:

  #include <stdio.h> main() {printf("Hello world!\n")}
and
  #include <stdio.h>

  main()
  {
    printf("Hello world!\n")
  }
Both programs do exactly the same, but the second one is much more readible. The difference is

• Only put one statement per line.
• Use indent. Put (2) extra spaces in the beginning of the line every time we are one level "deeper" in the structures.
• Seperate blocks of text (functions and procedures) with blank lines.
• Avoid the use of "goto" statements. With these statements, the program rapidly starts looking like spaghetti. Whereas in BASIC (Beginner's All-purpose Symbolic Instruction Code) the use of the GOTO statement is nearly unavoidable, in any itself-respecting language, the goto statement should be avoided.
• Comment. Since C is nearly like English, the program itself should be self-explanatory. Still, in places where the idea of the program might not be clear to the programmer, use comments. In C comments are placed after // on a single line, or between /* and */ for multi-line comment.
• Use functions wherever it makes the text more organized. If at many different places the program has to do basically the same thing (for instance reading a line of text form a file), consider putting it in a procedure or function (for example FileReadLn(). This will make the program more readable, more efficient and shorter.