Physics 307

A first introduction to C

1. Basics

C is the gold standard of programming languages. It’s quite old, and is used for a bewildering array of things. Linux, Windows, most of your favorite computer games, code running on 60,000-processor supercomputers, and even gcc itself are all written in C. Here’s a very simple C program. (The line numbers aren’t part of the program.)

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#include <stdio.h>

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

Line 1 instructs gcc that it should load the header file for the standard input/output library. This will be the first line of all your programs, since you need this to write text to the screen.

Line 2 is blank. It is a good habit to leave blank lines in your programs to separate different ideas. Just like punctuation is used in English to make it easier to read, punctuation like line-breaks and indentation should be used to make your code easier to read.

Line 3 indicates the beginning of a function called main. All programs must have a main function; the program starts running at the beginning of this function. Soon you will write programs that use other functions besides this one, but don’t worry about that now. Likewise, don’t worry about the keyword int; we’ll learn what it’s for later.

Lines 4 and 6 are a matched pair of curly braces, telling the compiler that everything between them is part of main. Curly braces are used in C to indicate that a large (or small) block of code should be considered as one unit: that it is all part of a function (as it is here), that it is all to be repeated ten times, etc.

Line 5 invokes the printf function to write some text to the screen. The funny \n at the end instructs the computer to go to a new line at the end of the text.

C code consists of statements that are executed one at a time in order (like line 5), interspersed with control commands that vary that order or indicate other things about the code (like line 3). In general, all statements that actually do something, like perform a calculation, create a variable, or display text, end with a semicolon; control commands do not.

Remember that you compile the source code myprogram.c into something you can run by doing gcc myprogram.c -o myprogram at the Linux prompt, and run it by doing ./myprogram.

2. Math and variables

Here’s a slightly more complicated program that introduces how to do math in C:

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#include <stdio.h>

int main()
{
  int x,xsquared;
  printf("Enter a number:\n");
  scanf("%d",&x);
  xsquared = x*x;
  printf("The square of x is %d.\n",xsquared);
}

There are some new things here. Line 5 creates two variables of type int, which store integer values. Their contents are garbage at the moment, whatever occupied that location in memory most recently. This statement merely sets aside enough of the computer’s memory for our variables. C variable names, like xsquared here, can be more than one letter long. You should try to use variable names that give some indication of what the variable represents. Variable names, like everything else in C, are case sensitive: X and x are totally different things to the computer.

Line 6 prompts the user to enter a number, and then line 7 reads in a value from the keyboard and stores it in the variable x using the function scanf. The %d in quotes tells the computer to read in a single integer value (%d means ”an integer”), and the &x tells the computer to store it in the variable x. For now, treat the ampersand as a magic character that must be included when you are using scanf; we’ll learn what it means later.

Line 8 is an assignment statement. While this may look like an equation, it is not quite the same thing. Rather than being a statement of fact that two things are equal, this means ”compute x*x, then set xsquared equal to its value”. x*x = xsquared is not valid, since x*x is not a single variable that can be set equal to anything! On the other hand, x = x+1 is valid C; it simply means to increase the value of x by one.

Line 9 is our old friend printf. As in scanf, the %d is a placeholder for an integer value to be printed, specified after the text in quotes.

There are other sorts of variables besides int that store other sorts of things. Variables of type float store floating-point numbers (decimals), for instance. The ”percent code” used with scanf and printf for float numbers is %f.

Here’s a gotcha: if you are dividing two integers, C will do integer division and drop the remainder, even if the result is later set to a float. This code snippet, for instance, will print ”1” rather than ”1.5” like you might expect:

#include <stdio.h>
int i=2, j=3;
float x;
x = j/i; // 3/2 is 1, not 1.5, if you’re doing integer division!
printf("x is %f\n", x);

Even numeric constants are treated as type int if they don’t have a decimal. Try this:

#include <stdio.h>
printf("3/2 is %f.\n", 3/2); //integer division
printf("3.0/2.0 is %f.\n", 3.0/2.0); //floating-point division

The difference between floating-point division and integer division will matter in your first homework assignment!

3. Loops: repeating things multiple times

Here’s another program that introduces looping structures: commands that tell the computer to repeat chunks of code. Clever use of these is the key to programming: the whole point of using a computer is that it can do repetitive, tiresome mathematical tasks quickly.

The for statement tells the computer to repeat a block of code until something happens. It looks like this:

for(what to do at the start ; under what conditions to keep going ; what to do after each repeat)
{
  [code to be repeated]
}

See if you can guess what this program does before you run it:

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#include <stdio.h>

int main()
{
  int x;
  for(x=0; x<=20; x = x+1)
  {
    printf("%d %d\n", x, x*x);
  }
}

Note that there is no semicolon after the for statement in line 6. This is because it is a control statement itself, rather than an actual instruction to the computer to do something! A common mistake is to put a semicolon after the for statement; this will make the computer repeat the semicolon over and over (i.e. do nothing) rather than what you wanted it to do.

This for statement means ”Before you do this, set x equal to 0. Then, repeat the following block of code so long as x is less than or equal to 20, and after each repetition add 1 to x.” Note that it will run with x ranging from 0 to 19, not 1 to 20!

You may end a loop prematurely using break;. This is useful if you want to do something like “Count up to 1000, but stop prematurely if xyz happens”.

4. Conditional statements

Here’s one more program that illustrates how to use the if statement to make the computer do one thing or another.

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#include <stdio.h>

int main()
{
  int x;
  printf("Enter a number: \n");
  scanf("%d", &x);
  if (x < 0)
  {
    printf("The number is negative.\n");
  }
  if (x == 0) // note the == rather than =
  {
    printf("The number is zero.\n");
  }
  if (x > 0)
  {
    printf("The number is positive.\n");
  }
}

A couple of new things here. The biggest one is the if statement, of course. Note the use of == rather than = to check for equality. Single-equals is the assignment operator that changes the value of a variable; double-equals tests to see if two things are equal. Mixing these up is a common mistake.

Also notice the text beginning with //. This is a comment: text that is completely ignored by the compiler and does nothing, but is included just as a note to anyone reading the code. Use comments liberally; it’s essential that you or someone else reading your code can figure out what it does. At a minimum, any program you submit should have a comment at the top telling who wrote it and what it does.