ex1

Exercise 1: C variables and functions

Like many programming languages, C enables the programmer to include variables, which can represent arbitrary values, and functions, which encapsulate some specific task. The C programming language is specific about the data type of each variable including the return type of a function and the types of a function's arguments. The data type of a variable must be declared before using the variable. Optionally, the value of the variable may be set when the variable is declared. For example, both of the following are valid C:

int a;
a = 5;

int a = 5;

If desired, multiple variables may be declared on a single line:

int a, b, c;

Just as the type of a variable must be declared, so must the type of the return value and parameters of a function be declared:

int
my_func(int a, int b, int c) 
{
  return a + b*c;
}

Data types

C includes a number of built-in data types. The exact definition of each data type may depend on the architecture of the computer and implementation of the compiler; feel free to read the ISO/IEC C standards if you are curious. Take care when choosing a data type that it will behave as you expect. With this caveat in mind, the following table gives some rough rules for common data types:

Data type	description
int	Signed 16-bit integer (or greater), which can hold values between -215+1 and 215-1 (-32767 and 32767)
char	An integer type that can contain the basic character set; typically 8 bits or larger
float	A floating point number. Typically 32 bits.
double	A floating point number, able to represent at least as many value as float. Typically 64 bits.
void	An "empty" data type

A variable of one data type may be converted to another data type by casting, for example as follows:

int a = 1;
float b;
b = (float) a;

Loops and Conditionals

C enables the programmer to include multiple types of loops, including the for-loop, the while-loop, and the do...while-loop.

for-loop

For-loops may be used to iterate over a series of values. The for-loop has the following syntax:

for (initialization; condition; increment) {
   statements;
}

Here, initialization, condition, and increment should be replaced with relevant values. For example, we could compute the sum of the first 100 integers:

int i;
int n = 100; // The number of times to go through the loop
int sum = 0;
for (i=0;i<n;i++)
{
  sum += i;
}
printf("%d\n", sum);

while-loop

The while-loop continues to run as long as a specified condition is true. In contrast to the for-loop, it is more commonly used when the number of iterations is not known in advance. The while-loop has the following syntax:

while (condition)
{
  statements;
}

For example, we could use a while loop to compute the first power of two that is greater than 1000:

#include <math.h>
#include <stdio.h>

int
main (void)
{
  int exponent = 0;
  while (pow(2,exponent) <= 1000)
  {
    exponent++;
  }
  printf("%d\n", exponent);
}

do-while loop

The do-while is a variation on the while-loop in which the code inside the loop is always executed at least once.

An example program

Consider the following the program, which calculates 1/n! for a specified value of n, and prints the value to standard output.

#include <stdio.h>

// Calculate the factorial of an integer.
int
factorial (int n)
{
  result = 1;
  for i
}