Functions

Like the sausage/hot dog, functions encapsulate a bunch of stuff we do not need to concern ourselves with, or even know about. We are just concerned with the result.

With functions we care about the input and the output. What goes on in the middle we don't need to know. Just like sausages. We care about the result, we don't want to know how they're made and what's inside.
We use functions to do things:

that we do multiple times in a program
that we create once once and do not want to create again
that someone else created for us (bless them).

Costco Hot dogs.

Functions may be:

Created for us and
Created by us

Examples of functions created for us

printf()
print()
sort()
sorted()
cos()
etc. ...

Functions we might create

C to F
mpg to l/100 km
display stuff

Use print from array

C

Convert Temperature.

#include <stdio.h>

// define the C to F function
float c2f(float c) {
    float f = (c * 9/5) + 32;
    return f;
}

// define the display function
void displ(float c) {
    printf("%3.2f degrees C is %3.2f degrees F\n", c, c2f(c));
}

// Display C to F conversions
int main() {
    displ(20);
    displ(30);
    displ(0);
    displ(-40);
}

Running the above gives:

20.00 degrees C is 68.00 degrees F
30.00 degrees C is 86.00 degrees F
0.00 degrees C is 32.00 degrees F
-40.00 degrees C is -40.00 degrees F

Convert Mileage

#include <stdio.h>

// define the to MPG function
float mpg(float lp) {
    // convert l to imp. gal. (.22) km to miles (1.6)
    float m = 1 / (lp/100 * .22 * 1.6);
    return m;
}

int main() {
    printf("mpg for 4 l/100km is %3.2f mpg\n", mpg(4));
}

mpg for 4 l/100km is 71.02 mpg

PHP

Convert Temperature.

#!/usr/bin/env php
<?php

// Define the C to F function
function c2f($c) {
return ($c * 9/5) + 32;
}

// Define the display function
function displ($c) {
echo ("$c degrees C is " . c2f($c) . " degrees F\n");
}

displ(20);
displ(30);
displ(0);
displ(-40);

?>

Running the above gives:

20 degrees C is 68 degrees F
30 degrees C is 86 degrees F
0 degrees C is 32 degrees F
-40 degrees C is -40 degrees F

Convert Mileage.

#!/usr/bin/env php
<?php

// define the to MPG function
function mpg($lp) {
    // convert l to imp. gal. (.22) km to miles (1.6)
    $m = 1 / ($lp/100 * .22 * 1.6);
    return $m;
}

echo ("mpg for 4 l/100km is " . round(mpg(4),2));
?>

Running the above gives:

mpg for 4 l/100km is 71.02

But, running scripts from the command line is not what PHP is about, so let's do a web version.

<html><head><title>Convert lp 100 km to mpg</title></head>
<body>
<h1>Convert litres per 100 km to mpg</h1>
<br>
<form method="POST" action="mpgweb.php">
<b>lp/100km </b><input type="text" name="lp100km">
<input type="submit">
</form>
<br>
<h3>
<?php

// define the to MPG function
function mpg($lp) {
    // convert l to imp. gal. (.22) km to miles (1.6)
    $m = 1 / ($lp/100 * .22 * 1.6);
    return $m;
}

// Is the l per 100 km value set? If so get the value. Otherwise do nothing.
if (isset($_POST['lp100km'])) {
    $lp100 = $_POST['lp100km'];
    // Is it a number? Yes calculate and display otherwise do nothing.
    if (is_numeric($lp100)){
        echo ("Mileage for $lp100 l/100km is " . round(mpg($lp100),2) . " mpg.");
    }
}
?>
</h3></body></html>

Running the above gives something like:

Convert litres per 100 km to mpg

lp/100km

Mileage for 14 l/100km is 20.29 mpg.

http://deid.vicpi.drsol.com/~deid/mpgweb.php

Python

#!/usr/bin/env python3

def c2f(c):
return (c * 9/5) + 32

def displ(c):
print(str(c) + " degrees C is " + str(c2f(c)) + " degrees F");

displ(20)
displ(30)
displ(0)
displ(-40)

Running the above gives:

20 degrees C is 68.0 degrees F
30 degrees C is 86.0 degrees F
0 degrees C is 32.0 degrees F
-40 degrees C is -40.0 degrees F

Convert Mileage.

#!/usr/bin/env python

# define the to MPG function
def mpg(lp):
    # convert l to imp. gal. (.22) km to miles (1.6)
    m = 1 / (lp/100.0 * .22 * 1.6)
    return m

print ("mpg for 4 l/100km is " + str(round(mpg(4),2)))

Running the above gives:

mpg for 4 l/100km is 71.02

Bash

#!/usr/bin/env bash

c2f () {
f=$(( $1 * 90 / 5 / 10 + 32 ))
echo "$1 degrees C is $f degrees F";
}

echo $(c2f 20)
echo $(c2f 30)
echo $(c2f 0)
echo $(c2f -40)

Running the above gives:

20 degrees C is 68 degrees F
30 degrees C is 86 degrees F
0 degrees C is 32 degrees F
-40 degrees C is -40 degrees F

Convert Mileage.

First we need to install bc

apk add bc
bc is a language that supports arbitrary precision numbers with interactive execution of statements.

#!/usr/bin/env bash
# We use bc because bash does not do floating point arithmetic.
mpg () {
    # Pipe the mpg calculation to bc and save the result in m
    # If bc is invoked with the -l option, a math library is preloaded and the
    #     default scale is set to 20.
    m=`echo "1 / ($1 / 100.0 * .22 * 1.6)" | bc -l`;
    echo $m
}

printf "mpg is %.2f \n" $(mpg 4);

Running the above gives:

mpg is 71.02

Homework

Choose your favorite programming language (of the 4 presented here) and modify the mpg program so that it takes input in l/100km and outputs mp(imperial)g.
Write your own function that does something that you find you do frequently (don't think washing dishes or mowing the lawn will work without additional hardware).