Programs can get very big as more and more lines of instructions are added. Also, in some cases you may need to re-use bits of code over and over again. This is where subroutines and functions come in.
A subroutine is a set of instructions that does a specific task, for example, you could have a subroutine that calculates the average of an array of numbers and displays the result. A subroutine will always have a name, and sometimes provided with a list of values (known as parameters) that it can use, in this case an array and the number of items in the array (called num). A subroutine never returns a value, so we use the word void to say 'no value is returned'. We will use a counted loop, called a for loop, which will count through the array from 1 to num and use a variable called sum to store the sum of the values, and another variable called avg to store the average. The Console.Writeline command will display the result on the Output window.
E.g.
public void average (Int16 numbers[],Int16 num) { Int16 sum, n; /* Define variables here */ float avg; sum = 0; /* Set sum of numbers to zero */ for (n=1; n<=num; n++) { /* Count from 1 to num, and increase count by 1 */ sum = sum + numbers[n]; } avg = sum / num; Console.Writeline( "The average is" + avg.ToString()); /* Display string 'The average is ' and convert avg to string and append it. */ }To use this subroutine, you can call it from the main program or even other subroutines, using its name and pass it the variables or values that it needs.
e.g.
Int16 numbers[5+1]; /* Define array of five integers, including one for index 0 */
numbers[1] = 456; numbers[2] = 172; numbers[3] = 78; numbers[4] = 102; numbers[5] = 321; average( numbers[], 5);
and it should display the following result:
The average is 225.8
Functions
A function is very similar to a subroutine, it is still a set of instructions that does a specific task, but this time it will return a value, the type of value is specified by the variable type e.g. int, char, float, long etc. To return a value, you use the return statement and give it a value. Then the function also can be used in expressions, so if you have a complex calculation to perform regularly, you can create a function and and use it in expressions and assignments.
e.g.
float average (Int16 numbers[], Int16 num) /* Function is called average and returns a floating point number */
{
Int16 sum, n; /* Define variables here */
float avg;
sum = 0; /* Set sum of numbers to zero */
for (n=1; n<=num; n++) { /* Count from 1 to num */
sum = sum + numbers[n];
}
avg = sum / num;
return (avg);
}
Now you can use it in expressions or assignments from the main program, subroutines or even other functions.
e.g.
float MyAverage = average(numbers[], 5); Console.Writeline("The average is " + MyAverage.ToString());
Many programming languages have built in libraries of functions which you can use in your programs. There are functions that do complex arithmatic such as square root, sines, cosines, tangents, minimum, maximum values. There are functions that are performed on strings such as length of a string, substrings, concatenate strings and so on.
C has a number of built in functions you can use. By including the Math class, you have access to a range of useful functions for your programs.
| Function | Description | Example |
| Abs | Absolute value | r = Math.Abs(n) |
| Exp | Exponential | r = Math.Exp(n) |
| Log | Natural logarithm | r = Math.Log(n) |
| Log10 | Base 10 logarithm | r = Math.Log10(n) |
| Max | Max value from 2 possible values | r = Math.Max(n, p) |
| Min | Min value from 2 possible values | r = Math.Min(n, p) |
| Pow | Raise number to power of value | r = Math.Pow(n, p) |
| Round | Rounds a value to nearest integer | r = Math.Round(n) |
| Sqrt | Square root | r = Math.Sqrt(n) |
| Sign | Return integer indicating sign of value | r = Math.Sign(n) |
NB: These use 'double' variables or values.
See this basic information on using these functions when working with triangles.
| Function | Description | Example |
| acos | Arcosine | r = Math.Acos( n); |
| asin | Arcsine | r = Math.Asin(n) |
| atan | Arctangent | r = Math.Atan(n) |
| atan2 | Arctangent of x/y | r = Math.Atan2(x, y) |
| cos | Cosine | r = Math.Cos(n) |
| sin | Sine | r = Math.Sin(n) |
| tan | Tangent | r = Math.Tan(n) |
| cosh | Cosine Hyperbolic | r = Math.Cosh(n) |
| sinh | Sine Hyperbolic | r = Math.Sinh(n) |
| tanh | Tangent Hyperbolic | r = Math.Tanh(n) |
Programs can do a lot with characters, strings and numbers.
| Function | Description | Example |
| ToString | convert value to a string | a = c.ToString() |
| Convert.ToChar | convert number to a ascii character | c = Convert. ToChar(n) |
| ToLower | convert to a lower case | a = s.ToLower() |
| ToUpper | covert to upper case | a = s.ToUpper() |
| Function | Description | Example |
| Clone | Clone a string as a reference | dest = s.Clone() |
| Length | Length of a string | n = str.Length() |
| IndexOf | Find character in string | n = str.IndexOf(c) |
| Concat | Concatenate/combine strings | s = string.Concat(c1, c2) |
| Compare | Compare strings | Compare(str1, str2) |
| Function | Description | Example |
| ToSingle | Expression to floating point number | f = Convert.ToSingle(s) |
| ToDouble | Expression to double precision number | d = Convert.ToDouble(s) |
| ToInt16 | Expression to 16 bit integer | i = Convert.ToInt16(s) |
| ToInt32 | Expression to integer | i = Convert.ToInt32(s) |
| ToInt64 | Expression to long | l = Convert.ToInt64(s) |
| ToBoolean | Expression to boolean | b = Convert.ToBoolean(s) |
| ToDateTime | Expression to date time | d = Convert.ToDateTime(s) |