Introduction to C

Comments and questions to John Rowe.

The C programming language

C is a general-purpose programming language which according to one widely quoted but not very meaningful index is the second most popular programming language in the world today. It has a philosophy of simplicity and consistency.

The purpose of C is to unambiguously express an algorithm and its data in a way that is easy for us to understand and modify.

Much more time is spent modifying existing programs than writing new ones.

C's popularity has has spawned several off-shoots which are, with very few exceptions, super-sets of the original language, i.e. any valid C program will also be a valid program in any of those languages. These include C++ , C# (C-sharp) and Objective C, a programming language used on the iPhone and iPad (although Objective C is now being superceded by the Swift programming language). When looking for information on the Web or buying a course book be sure it is about the C programming language, not C++ or one of the other off-shoots.
On the other hand, we can also use a C++ compiler as a C compiler provided we are careful only to use its C features, not any of the C++ extensions. (A compiler is a program that converts the human-readable program we have written into the executable program that will run on our computer.)

• Sumarise the previous two sections.
• Add sufficient comments to the previous two sections that you still understand them when they are minimised.
• From now on you should do this for each session.

Our first C program

In C, as in many languages, the statements that implement our algorithm are grouped into functions each of which which implements a well defined task. (In Logo these are called "procedures", other names can include "routines" or "subroutines".) Traditionally, our first C program looks like this:

/*
 * My first C program. Successfully prints the phrase:
 * "Hello, world"
 */
#include <stdio.h>

int main() {
  printf("Hello, world\n");
  return 0;
}

Though simple, this program illustrates the three components found in any C program: Comments, pre-processor directives and C code proper.

Comments

The first thing we see is a comment, which is there purely for our benefit:

/*
 * My first C program. Successfully prints the phrase:
 * "Hello, world"
 */

Text starting with "/*" and ending with "*/" is ignored by the compiler and is called a comment. It may run over several lines.

You will notice we have made our comment look slightly pretty by starting each line within the comment with a * and making all four stars line up vertically. This is quite a popular style but is not necessary: only the /* */ count. We can use whatever layout we like, for example:

/*
  My first C program. Successfully prints the phrase:
  "Hello, world"
 */

Or:

/*
=========================================================== 
     My first C program. Successfully prints the phrase:
    "Hello, world"
=========================================================== 
*/

Why comments are useful

Comments are there to give us the context of what's going on and to help us understand anything that can't be made clear in the program itself.

Comments at the start of programs and just before functions are extremely useful as it's easy to be slightly unclear or ambiguous about what a function does. Comments inside of functions are less useful.

Single-line comments

C also allows single line comments starting with "//". The "//" and everything after it up to the end of the line is ignored. This is good way of writing short comments but is a bit inconvenient for longer comments as every line must start with // .Thus we could have equally written our initial comment:

//
// My first C program. Successfully prints the phrase:
// "Hello, world"
//

Comments are ignored by the computer and are there to tell us something that is not already clear from the code

1. Write a comment at the start of a program.
2. To remind us of the importance of the initial comment
1. Create a new on-line program in a new window. (Try Right-clicking on this link).
2. You will need to be able to see both windows as once so if the compiler started in a new tab follow these instructions to move the new tab into its own window.
3. At the top of your program, add a simple comment to indicate this is your first set of mini-exercises.
4. Press "Build & Run" - you should see the output "Hello, world" and a message saying that the program exited with status 0.
5. If you program did not run, look back at the previous two sections and work out why.

Pre-processor (#) Directives

 Secondly in our program we see the line:

#include <stdio.h>

Lines starting with a hash # are called pre-processor directives and the system uses them to create an edited version of the C  file to be given to the compiler.

The word "stdio" is short for STanDard Input/Output, i.e. a collection of standardised functions available to us that will read from, and write to, the screen and/or files on our computer. Somewhere in the depths of our computer will be a file called stdio.h  and this command includes that file in our program when it is compiled. This file stdio.h contains all the definitions necessary for us to use these standard input and output functions functions (we will see one of hem in the main program). It's normal to put all of our #include statements at the top of the file.

#include <stdio.h>  allows us to access functions to print out, and read in, data.

We will use this construct quite often as the standard library is divided into convenient groups, each with its own include file. This is also the method by which we can use "third-party" libraries (i.e., software written by other people) for tasks such as graphics, advanced mathematical analysis, etc.
It is also used when our program becomes large enough to require splitting between several files and we need to keep various definitions, etc. consistent between them.

1. Experiment with #include <stdio.h>
2. To practice #include and see what happens when we get it wrong.
1. Deliberately miss-spell "stdio.h" in the line #include <stdio.h> from the start of your program. (For example: "studio.h")
2. Buid & Run. It should fail and a red "error" box appear at the offending line. Also a yellow Triangle of Peril[TM] will appear at the "printf()" line to indicate that the compiler no longer knows exactly what "printf()" is.
3. Completely remove the #include <stdio.h> line from the start of your program.
4. Build and run: it should work but with a warning.
5. Now type the line back in (type not Copy-and-Paste!) and check your program will now build. (Mac users: the # key is Alt-3)

The main() function

So far we have not done very much but finally we come to the actual C code itself. The construct:

The instructions that make up our program are conventionally referred to as "code", as in "code of conduct".
 
Throughout this course we will use the ellipsis ... to indicate omitted code.

int main() {
  // Put your code here.

  return 0; // Indicates program succeeded
}

indicates that we have written a function called main() which will return an integer value to whatever called it. The  main() function has a special place in C in that the program runs by calling and executing main, and hence any functions contained within it, then quitting.

The value returned by main is then passed back to whatever ran the program to indicate whether the program succeeded or failed. The convention is for a program to return zero upon success and a non-zero value on failure.

The actual content of the function lives in between the matched pair of braces {}, or curly brackets.

All executable statements live inside a function and the program starts by executing the contents of the main() function.

The printf() function and character strings

The first statement:

  printf("Hello, world\n");

calls the printf() function which "prints" the output, not to a printer but to the default output device, usually the screen. The "f" at the end of printf stands for "formatted" and we shall see later how to use it to print numbers, etc to the screen as well as plain text.

The printf() function prints output to the screen.

The parentheses, following the word printf enclose the arguments which are passed to the printf function. In this case these is just one, a character string enclosed within double-quotes. (They have to be double quotes, single quotes are used for something else.)

Character strings are enclosed by double-quotes, not single quotes.

1. Make some deliberate mistakes.
2. To make it easier to recognise fix those mistakes when we make them accidentally.
3. When we made the "studio.h" mistake in the previous mini-exercise the code did not build but gave us a red error box. Here we will deliberately make some mistakes to see what happens. This will help us fix the problem when we make these mistakes by accident. 
    
   NB: For each deliberate mistake fix the previous error before making the next one. Don't worry if you will find that the detailed message is sometimes not very clear: what's most useful is that the compiler tells us where the problem is.
   
1. Remove the opening parenthesis (left bracket) in the "printf()" statement so that it now reads:
       printf"Hello, world\n");.
2. Build and Run, look at the position of the error box and the wording of the error and see how you would work out what the problem was when you do it by accident.
   • If you hover your mouse over the error box it will show you the error(s) relating to that point.
3. Fix the previous error and repeat the process in turn for each of the punctuation characters in the statement.
   • What do you think will happen when you remove the comma in "Hello, world\n"? Why?
   • When you remove the final semi-colon ";" at the end of the statement look where the error box appears. Sometimes a mistake on one line shows up as an error on the next one: in general the compiler complains at the first point it can no longer make any sense of the input.

The actual character string itself is fairly straightforward except for the funny '\n' at the end which stands for "new line".

  printf("\"Hello\", he said\n");

We nearly always want a \n at the end of the string but we can put \n etc. anywhere in the string and have as many as we like:

  printf("Menu:\n\nBurger\t$1\nPizza\t$1.50\n");

The output is:

Menu:

Burger  $1
Pizza   $1.50

The backslash is used for characters that we would otherwise not be able to represent, eg \n for new-line, and \t tab..

The printf()statement finishes with a semi-colon (';').

1. printf() and character strings
2. To practice printing text to the screen
1. Insert a new line after the existing printf() statement and add a new printf() statement that prints out the phrase: "My name is: yournamehere\n". Run your code and see what it produces.
   • Type it in by hand, don't use copy and paste. This will help you remember it.
   • When you type the opening "(" the on-line editor will add the closing ")" for you. This is quite helpful although it can occasionally be confusing if you are not expecting it.
     
   • Remember to use the double-quote character, not the single-quote or two single-quotes.
   • Don't forget to put in the semi-colon at the end of your new printf() statement.
2. Now remove the \n after the phrase "Hello, world". What does your output look like now?
3. Put the \n back after "Hello, world" and add one after the colon (:) in the second printf() statement so it now reads "My name is:\n yournamehere\n". What does your output look like now?

"Commenting out" statements

A rather unsophisticated use of the // comment  notation is to temporarily remove a line that we think we might need later. This is often used with diagnostic printf() statements:

  //  printf("Hello, world\n");
 

This is a pretty crude technique but we all use it sometimes! Note that if a statement is over several lines we either need to comment out each line individually. or the use the /* ... */ comment notation.

1. Comment out a print statement
2. To see what happens
1. Comment out one of the printf() statements in the previous exercise by putting // in front of it. You should see the line change colour.
2. Build & run: the line should no longer be printed.
3. Remove the comment // characters, Build & run and check that the text is now printed.
4. Don't use this technique too often!
   

The return statement

The second and final statement is:

  return 0;

The return statement returns from main in both senses of the word: the execution of main immediately stops and the value zero is returned. If we had put any statements after the return statement they would never be reached and, if we were using a nice compiler, it would warn us. Notice again the semi-colon at the end of the statement.

Making our program easier to understand

In the preface to this series we emphasised the importance of avoiding and finding mistakes. Amongst other things this means having code that is really easy to understand.

Consistent layout

Our first example of this is that our program is clearly and consistently laid out. When discussing comments we explained how we adopt a consistent and distinctive style and you will also have seen how in main()we have:

• Left two blank lines before the start of the function.
• Indented everything inside the braces {} to the left by the same amount.

We will return to this in two lessions time but meanwhile be sure to follow these rules.

Arithmetic and variables

The above code has one huge omission: it has no data, no numbers, etc. Now that we have learned the basics of what a simple program looks like it is time to do some useful calculations.

You should at least be aware that computers use "binary", or base-2 arithmetic rather than the "decimal" or base-10 that we are used to, that "binary digits" (zeros or ones) are called "bits" and that an ordered set of eight bits is called a "byte", although you will be relieved to hear that you don't need to be able to do binary arithmetic to program a computer.

As discussed in the preface, algorithms have words and phrases which are used as "place-holders" for the actual numbers:

• The cost_of_the_petrol is the cost_per_litre multiplied by the number_of_litres_sold.
• The cost_of_the_baked_beans is the cost_per_tin multiplied by the number_of_tins_sold.

In computer programming the "place holders" for holding values are known as variables.

The values of variables will typically change as the program progresses, just like the running total at a supermarket checkout.

Integer and floating-point expressions

The above example reminds us that some things (tins of beans, people) are treated as integer units, whereas others (petrol, distance) are allowed to be fractions. In the latter case all measurements are by necessity approximate.

Like most programming languages C reflects this by allowing two categories of expressions and variables: integers and floating-point, i.e. non-integers. (The name "floating-point" comes from the fact that in numbers such as 1.234 ,  12.34 ,  123.4 etc. the decimal point "floats" from left to right.) Floating-point calculations are very important for scientists and engineers so be sure to remember what the term means!

Expressions have both a value and a type.

In general we don't have to worry about this as C "just does the right thing", but we we can occasionally get caught out by things like accidentally including integer division in floating-point calculations which we will deal with in the next lecture.

Floating-point variables are used for things we measure and integer variables are used for things we count. Make a variable an integer only if is it logically impossible for it to have a fractional value.

Floating-point constants

Floating-point constants (9.4 and 11.3) work just as we would expect, scientific notation is available using "En" for "ten to the power n", where n is an integer. ('E' stands for Exponent.)

In the examples to the left the numbers on the same row have the same value.

Some well-known floating-point constants: 2.99792458E8, 6.62606957E-34, -1.60217657E-19, 3.14159265358979.

Floating-point variables

Whenever we take a measurement or use a calculator we should be familiar with the fact that using a finite number of decimal places (or binary places for a computer) limits the accuracy of the calculation. Most computers help deal with this conundrum by offering a choice of two precisions: single precision (four bytes per variable)and double precision (eight bytes per variable). In C these are known, somewhat inconsistently as float and double respectively.

A float is a single-precision, four-byte, floating point variable, a double is a double-precision, eight-byte, floating point variable.

We suggest using doubles and it's vaguely useful to remember they use eight bytes each.

C adopts a "better safe than sorry" approach and by default does most of its arithmetic in double precision anyway.

Floating-point calculations are always approximations to the mathematically correct result.

Declaring variables

Unlike in our turtle example, variables have to be declared before they can be used. This causes the compiler to set aside some memory to store the value of the value of that variable. So for example, if the compiler encounters the declaration double x it may decide to store x from bytes 600-607 inclusive (where byte "1" denotes the first byte in the computer's memory, "2" the second and so on).

Variables must be declared before use.

Variable names

Variables have names which start with a letter (the underscore "_" counts as a letter) followed by zero or more letters or digits. Conventionally only lower-case letters (and digits) are used, with upper-case letters being used for named constants.

The compiler doesn't care what the variables are called; the names are simply there to make things clearer for us, and the people who have to read our code later on.

Variable names start with a letter, then have more letters, underscores ("_") and digits. They should be all lower-case.

int main() {
  double materials, labour, total_cost;

  materials = 9.4;
  labour = 11.3;
  total_cost = materials + labour;

  // Do something else...
  return 0;
}

Step through this code

Seeing what the program does

Clicking on the "Step through this code" link will open a new web page which will simulate what we would do if we were executing the code ourselves armed with a calculator, index cards and a pen and eraser.

1. Clicking the "Start program" button at the top right will cause main() to start and its variables to appear. It will also evaluate the first mathematical expression it comes to (in this case the simple constant 9.4).
   
2. Every time we press "Next step" it will evaluate the next expression (with large expressions being divided into sub-expressions).
   
3. Every time it evaluates an expression it will highlight the appropriate part of the code and print the value of the expression
4. If the sub-expression is an assignment of a value to a variable it will update the value of that variable's index card (with an arrow from the value box to the index card meant to represent the flow of data).
   
5. If the sub-expression is the value of a variable it will draw a reversed arrow from the index card to show where the value has come from.
6. To start again from the beginning just press your browser's "Reload" or "Refresh" button.
7. Finally, if you dislike the human "index-card model" and would like to see a more literal view of how our computers store their data click on the "Show advanced options" button and switch between the two options on the right-hand side (Show index card or Show memory table).
   

• Step through a code.
• To learn how to understand what our program is doing
• Click on the "Step through this code" link above. It will open a new tab or window.
  You will need to be able to see both windows at once so if it is in a new tab follow these instructions to move the new tab into its own window.
• Click on the "Start program" button at the top right of the new window. A table should appear with three variables and their values. Note that the values are (extremely!) random.
• Also note that the program has taken its first "real" step, ie it has started to evaluate the Right Hand Side of "materials = 9.4;". This is obviously quite easy, it's just 9.4! The value of the last evaluated expression appears in a blue box near the top of the page. Notice too that it is preceded by the word "(double)" meaning that the program is treating it as a double-precision floating-point number.
• Click "Next step" again. See how it copies the value of the expression it has just evaluated (9.4) into the value for the variable "materials".
• Click "Next step" twice more and see it do the same for "labour" and "11.3".
• Now ask yourself:
  
• After the program has executed the next statement, what do you expect to be the value of total_cost?
  
• How do you think it will arrive at this?
• Click "Next step" again. See how it finds the value of "materials" by going to the appropriate entry in the table and retrieving the value.
• Carry on clicking next seeing what happens at each stage. Was it what you expected?
• Click on your browser's "Reload" button and step through the program a few more times until you are sure you understand what the program is doing.

Notes

• Most of the complete examples have a "Step through this code" link just as a side-effect of checking that they are correct. We don't expect you to step through every code, just the ones where we ask you to or are marked as "Key code".  But you should know what would happen if you were to. If you're not sure what the code does then step through it to check.
  
• In practice, if we were mentally stepping through our own code we would not go down to the level of doing the maths (although in this case given it is so simple we might). We would just think something like "total_cost is materials plus labour". If we were looking for a mistake in our code and we had written * instead of + we might say "total_cost should be materials plus labour but the code says materials multiplied by labour" which enable us to fix the error.
  

Data is stored in numbered locations

This idea, that the values of variables are stored in numbered locations is extremely important in computer programming. We are introducing it now before we really need it so that you can become familiar with the idea.

Over the next lesson or two make sure you become comfortable with ideas such as "we are storing the value of x on card number 12".

We will develop this idea a little more in the next lesson.

  printf("The cost is: %g\n", total_cost);

  printf("a plus b equals %g\n", a + b);

  printf("%g plus %g equals %g\n", a, b, a + b);

Summary

The text of each key point is a link to the place in the web page.

• Repeat the mega-principle to yourself so many times that you wake up having dreamed about it!