6.6.1 Built-in and user-devised subprograms

Two Kinds of Subprogram

A subprogram is a named, reusable block of code. There are two sources:

Built-in / library subprograms — provided by the language or its standard libraries; ready to use without writing them yourself.
User-devised subprograms — written by the programmer to solve a specific problem in their own program.

Both are called in the same way — by name, with arguments in parentheses. The distinction matters because built-ins are tested, optimised and documented; user-devised ones must be written, tested and maintained by you.

Built-in Subprograms

import random

# Built-in functions — no import needed
print(len("hello"))          # 5  — length
print(abs(-42))              # 42 — absolute value
print(round(3.7))            # 4  — round to nearest integer
print(round(3.14159, 2))     # 3.14 — round to 2 decimal places
print(int("99"))             # 99  — type conversion
print(str(42))               # "42"
print(max(3, 7, 1))          # 7  — maximum
print(min(3, 7, 1))          # 1  — minimum
print(input("Enter: "))      # reads a line from the user

# Library functions — need import
print(random.randint(1, 6))  # random integer 1 to 6 inclusive
print(random.random())       # random float 0.0 to 1.0

using System;

// Math library methods
Console.WriteLine(Math.Abs(-42));          // 42
Console.WriteLine(Math.Round(3.14159, 2)); // 3.14
Console.WriteLine(Math.Max(3, 7));         // 7
Console.WriteLine(Math.Min(3, 7));         // 3
Console.WriteLine(Math.Pow(2, 8));         // 256

// String methods (built into string type)
string s = "Hello";
Console.WriteLine(s.Length);              // 5
Console.WriteLine(s.ToUpper());           // HELLO

// Random
Random rng = new Random();
Console.WriteLine(rng.Next(1, 7));        // random 1-6

Imports System

Console.WriteLine(Math.Abs(-42))          ' 42
Console.WriteLine(Math.Round(3.14159, 2)) ' 3.14
Console.WriteLine(Math.Max(3, 7))         ' 7
Console.WriteLine(Math.Min(3, 7))         ' 3
Console.WriteLine(Math.Pow(2, 8))         ' 256

Dim s As String = "Hello"
Console.WriteLine(s.Length)              ' 5
Console.WriteLine(s.ToUpper())           ' HELLO

Dim rng As New Random()
Console.WriteLine(rng.Next(1, 7))        ' random 1-6

User-Devised Subprograms

When no built-in solves your specific problem, you write your own. User-devised subprograms use the same call syntax as built-ins — the caller does not need to know how they are implemented.

import random

# User-devised function: roll two dice and return total
def roll_dice():
    """Returns the sum of two random dice rolls."""
    die1 = random.randint(1, 6)
    die2 = random.randint(1, 6)
    return die1 + die2

# User-devised procedure: print a formatted score line
def display_score(name, score, max_score):
    """Prints a formatted score — no return value."""
    percent = round(score / max_score * 100)
    print(name + ": " + str(score) + "/" + str(max_score) + " (" + str(percent) + "%)")

# Using both built-in and user-devised together
total = roll_dice()                         # user-devised
print("Rolled:", total)
display_score("Alice", 78, 100)            # user-devised
display_score("Bob", round(82.7), 100)     # round() is built-in

Random rng = new Random();

int RollDice() {
    return rng.Next(1, 7) + rng.Next(1, 7);
}

void DisplayScore(string name, int score, int maxScore) {
    int percent = (int)Math.Round((double)score / maxScore * 100);
    Console.WriteLine(name + ": " + score + "/" + maxScore + " (" + percent + "%)");
}

int total = RollDice();
Console.WriteLine("Rolled: " + total);
DisplayScore("Alice", 78, 100);
DisplayScore("Bob", (int)Math.Round(82.7), 100);

Dim rng As New Random()

Function RollDice() As Integer
    Return rng.Next(1, 7) + rng.Next(1, 7)
End Function

Sub DisplayScore(name As String, score As Integer, maxScore As Integer)
    Dim percent As Integer = CInt(Math.Round(CDbl(score) / maxScore * 100))
    Console.WriteLine(name & ": " & score & "/" & maxScore & " (" & percent & "%)")
End Sub

Dim total As Integer = RollDice()
Console.WriteLine("Rolled: " & total)
DisplayScore("Alice", 78, 100)
DisplayScore("Bob", CInt(Math.Round(82.7)), 100)

Key Takeaways

Built-in: provided by the language — len(), abs(), round(), max(), min(), input(), print().
Library: imported from a module — random.randint(), math.sqrt().
User-devised: written by the programmer — called identically to built-ins from the caller's perspective.
Use built-ins where they exist; write your own only when no built-in solves the specific problem.