Basic C

Below shows three files, greeter.h, greeter.c and driver.c that together make up a complete C program. greeter.h defines a function, greet() that takes a message and a name. greeter.c implements this function. Finally, driver.c must include greeter.h to learn the existence of greet().

#pragma once

void greet(char *msg, char *name);

#include "greeter.h"

void greet(char *msg, char *name)
{
  printf("Hi, %s, %s!\n", name, msg);
}

#include "greeter.h"

int main(int arc, char *argv[])
{
  if (argc == 3)
    {
      greet(argv[1], argv[2]);
    }
  else
    {
      puts("Usage: ./driver <msg> <name>")
    }

  return 0;
}

These files can be compiled thus: gcc greeter.c driver.c -o driver. This produces the executable driver. Note how we do not mention the .h file when compiling. That would be redundant since the contents of greeter.h has already been included twice, once in each .c file.

Here is recursive fibonacci:

int fib(int n)
{
  if (n < 2)
    {
      return n;
    }
  else
    {
      return fib(n - 1) + fib(n - 2);
    }
}

And here, written using a while loop:

int fib(int n)
{
  int fib_1 = 0;
  int fib_2 = 1;
  int times = 0;

  while (times < n)
    {
      int tmp = fib_1 + fib_2;
      fib_1 = fib_2;
      fib_2 = tmp;

      times = times + 1;
    }

  return fib_2;
}

Note: bug if n == 0 – exercise!

A do-while loop is a good fit for loops where we know we will go through the loop body at least once. Here is an example of going through a password dialogue. If the correct password is entered, strcmp(password, answer) returns 0 and the loop exits. Otherwise, the password question is repeated.

char *password = "secret";
char *answer = NULL;
do
  {
    ask_question_string("Enter password:", &answer);
  }
while (strcmp(password, answer) != 0);

Concisely, if a do-while loop is do {body} while (guard), we could rewrite the code using a while loop as body; while (guard) {body}. What would the example above look like using a while-loop instead?

Technically, there is nothing you can do with a for loop that you could not do with a while. However, in many (but not all) circumstances, a for loop can be much clearer – by providing a single place (at the start of the loop) for declaring loop variables, loop guards and incrementing the loop variables.

The following snippet uses a for loop to print i = 0, i = 1, etc. up to 1023. Note that the first line has three ;-separated compartments. the first is for declaring one or more loop variables and initialising them; the second is the guard, just like in while or do/while loops; the third is applied at the end of a loop body.

int N = 1024;
for (int i = 0; i < N; ++i)
  {
    printf("i = %d\n", i);
  }
/// Cannot use i variable here!

We could translate the for loop above to an equivalent while loop:

int N = 1024;
int i = 0; 

while (i < N)
  {
    printf("i = %d\n", i);
    ++i;
  }
/// __Can__ use i variable here!

What does the following code print?

int x = 42;
printf("%d ", x++);
printf("%d ", ++x);
printf("%d.", x)

What does the following code print?

int x = 42;
printf("%d ", ++x);
printf("%d ", x++);
printf("%d.", x)

Brain teaser (and sort-of trick question) What does the following code print?

int x = 42;
printf("%d.", x+++x);

The first and second example are quite simple. It is easy for your eyes to transpose the two printf()-lines and confuse one for the other. The final example is interesting because it stresses the parsing situation – does x+++x parse as (x++) + x or as x + (++x) – and does it matter?

Historically, many programmers have enjoyed writing programs which use ++ (and --) when iterating over arrays. Here is a snippet that illustrates this in a very simple way.

char msg[4];
int i = 0;
msg[i++] = 'H';
msg[i++] = 'e';
msg[i++] = 'y';
msg[i++] = '\0';

Clearly, the ability to both look up an index and increment it at the same time is making the code above compact. In the code above, one could argue that adding four separate lines for incrementing i would add “a lot of noise” to the program. However, most-real world examples involve loops where we are mostly changing the index in a single line. For example:

void puts_equivalent(char *str)
{
  int i = 0;
  while (str[i]) putchar(str[i++]);
}

Before we dig in, let us write that code with a proper loop body:

void puts_equivalent(char *str)
{
  int i = 0;
  while (str[i])
    {
      putchar(str[i++]);
    }
}

One reason to dislike this code is because putchar(str[i++]); is a very busy statement – many things going on in a single line:

1. Read the i:th character of str
2. Increment i
3. Print the character we read in 1.

Moving the increment to a line of its own makes the code much clearer:

void puts_equivalent(char *str)
{
  int i = 0;
  while (str[i])
    {
      putchar(str[i]);
      ++i;
    }
}

We can rewrite it thus using a for loop, which could be argued makes the code clearer by collecting the loop variable, guard and change to the loop variable in a coherent space, textually⁶.

void puts_equivalent(char *str)
{
  for (int i = 0; str[i]; ++i)
    {
      putchar(str[i]);
    }
}

Note that in the last two cases, the behaviour is the same if the increment is done using ++i or using i++. This is a good thing for readability and maintainability.

I strongly advocate only using ++ in the prefix position to the extent possible. Many modern programming languages have dropped ++ in favour of +=n which is clearer and more flexible.

The increment operation is often used to do pointer arithmetic. For example, int *ip declares ip to be a pointer to a place in memory where an int is stored⁷.

Remember that ip + 1 returns a pointer to the next integer after the one that ip points to, meaning that pointer arithmetic moves pointers to values of type T in strides of sizeof(T) bytes.

Also, because of C’s preference order rules, *ip++ means read *ip, then increase ip by one, the following code prints 7, 42 and 4711.

int values[] = { 7, 42, 4711 }
int *ip = values;
printf("%d ", *ip++)
printf("%d ", *ip++)
printf("%d ", *ip++)

To increment the value pointed to by some pointer, put the dereference in parentheses: (*ip)++ means increase the value of *ip by one and does not change the value of ip.

Look at the following implementation of strcpy(), the string copying function, that copies a string from b to a. Do you understand how it works?

void strcpy(char *a, char *b)
{
  while (*a++ = *b++) ;
}

Because sometimes strings are not properly null-terminated, we should never use strcpy(). Instead, we should rely on strncpy() that takes an additional argument that upper-bounds the number of characters copied. Here is an implementation of that function using --.

void strncpy(char *a, char *b, int n)
{
  while (n-- > 0 && *a++ = *b++) ;
}

The documentation for strncpy() says (slightly adapted):

If the length of b is less than n, strncpy() writes additional null characters to a to ensure that a total of n bytes are written.

Update strncpy() above to adhere to that specification. Can you do it in a single construct on a single line? (Note – there is nothing better about doing it in a single line, this is just a bit of fun, and also to restrict you from inventing too complex machinery.)

void strncpy(char *a, char *b, int n)
{
  while (n-- > 0 && *a++ = *b++) ;
  while (n-- > 0) *a++ = '\0';
}

Several modern languages abolish ++ and -- because they make code hard to read. Many languages (including C) support more general += and -= operators that can be used to achieve the same effect with similar brevity.

I strongly advocate only using ++ in the prefix position to the extent possible. But even better, use += 1 and -= 1 instead. The ++ and -- are seductive, and it may be hard to stop using them once you have grown accustomed to them.