3. Form and Function

Here we will be looking at functions and control flow statements that can be used to simplify common instructions and separate usage and implementation. Important concepts for any programming project.

3.1. What is your quest?

Say we wanted to do a calculation. Say we wanted to do a calculation often. It would be useful (increasingly so with added complexity) to separate the calculation into a function.

Functions are sets of instructions that are executed whenever the function is called by name and can optionally take in data and even provide an output.

Imagine we had a reason to count how many times the letter “a” is in a given string and return the result. Instead of writing the instructions required to perform this task each time it is needed we could write a function that does it for us. For example in python we would do the following:

>>> def count_a(text):
...     count = 0
...     for letter in text:
...         if letter == "a":
...             count += 1
...     return count
>>> count_a("A test sample")
1

The above code presents a host of new programming features that will be covered in this section.

Firstly we defined a function called count_a and told it to take one parameter called text. This parameter is our input that we will count.

In programming we have the concept of code blocks, these are a section of code that belongs to some other code. Some languages use braces such as {} to say that anything in between those symbols is a code block. However Python uses white space indentation do denote a code block. This means that anything on the same indentation level is part of that code block and further indentations denote nested code blocks. After defining our count_a function we need to give it a block of code that defines what that function will do when it is called latter. Code blocks are used whenever we need to define what a specific thing should do when it is called; the code of a function, what to do in a loop, code to execute if a condition is met, etc.

The next line should be rather familiar by now. We are simply creating a integer variable to store the count, which starts at zero.

Here is where things get interesting, control flow. We need to check over each letter in the text variable and see if it is the letter we are looking for. To accomplish this we have employed a for loop. For loops are very common in programming however some languages have different types of for loops. In python the for loop is more of a foreach loop, unfortunately it is not named as such. Basically it takes any iterable value (more on that later but for now its anything that can be looped over) and gives you each piece of that value until there are no more pieces left. Here we have given the for loop the text value and told it to call each piece of it letter.

Next we introduce another new type of control flow called the if statement. An if statement evaluates a boolean expression and then allows you to execute instructions if it is true. The boolean expression we are using here is letter == "a". Meaning if the piece of the text parameter we are looking at currently is the same as a string containing the letter a then the boolean expression evaluates to true. The if statement, now having an expression that equals true then executes its code. In this case that is to add 1 to the count variable.

When adding one to the count variable we are using the in place addition operator += because we want to store the result in the same place. we could instead use the following:

count = count + 1

this would do the calculation of whatever is stored in the count variable plus one and then store it into the count variable. This does the exact same thing but is a little longer. However we could not just say count + 1 and thats it because that is an expression and that expression will return the sum of count plus one but has no idea that it needs to be stored.

After the if statement the for loop will return to the top and get the next letter from text and do it all again until we have gotten to the end of the input.

Once all letters have been checked the count variable will now be storing the exact amount of the letter a that are contained within the text that this function is given. But its of no use to just count the letters and then forget about it so we need to return the results using, you guessed it, return count. By now it should be rather obvious that this will simply return whatever is stored in the variable count back out to whatever has called this function.

Finally we see our brand new function in action by calling count_a("A test sample") return the result 1. In our function we only checked for the letter a but we must remember that this is not the same as A as strings are case sensitive. If we wanted to check for both lowercase and uppercase a we could change the boolean expression for the if statement to a little to the following:

letter == "a" or letter == "A"

Then the if statement has two boolean expressions that make up the larger boolean expression. Firstly it checks for the lowercase a then if that is false it will continue to the uppercase A check and if that is also false then the entire boolean expression will be false and not execute the if statements instructions. In python you can also use the and keyword if both of these boolean expressions needed to be true in order to proceed. In this case however the and keyword would do no good because a letter cannot be lowercase and uppercase at the same time so we use or.

3.2. Go with the flow man

In programming we often use control flow statements to alter the way our code performs.

3.2.1. If your happy and you know it

For example if we want code to do one thing or another depending on a variable we use an if statement.

The if statement evaluates a boolean expression and executes the body of the if statement if that boolean is positive. If however it is negative the code can either; look for an else if and evaluate that expression next, look for an else statement and do that instead of any condition, or finally continue normal execution of the code.

In python we can give the following example that takes in a message code and then prints the corresponding message:

>>> def message(code):
...     if code == 1:
...         print("Hello world!")
...     elif code == 2:
...         print("Goodbye cruel world!")
...     else:
...         print("I don't even know what to say...")
...     return True
>>> message(1)
Hello world!
True
>>> message(2)
Goodbye cruel world!
True
>>> message(3)
I don't even know what to say...
True

Each part of an if statement is executed one after another until a positive boolean expression is found. An else block is optional as is an else if (called elif in python) however if an else is used it must be last in the chain as it acts as a sort of “catch all” in that if none of the if statements are executed then the else surely will be.

3.2.2. Do a barrel roll!

Doing something over and over again until a particular time is done using loops which are most commonly; for, foreach, and while. In some cases only one of the first two loops are available. In python there is a loop that is started using the keyword for however it behaves like a foreach loop.

There are some minor but important differences between each loop type.

3.2.2.1. For

Usually takes 3 statements; variable to count with, how to increment the variable, and the condition in which to stop the for loop. In c/c++ a for loop would look something like this:

for (int count; count >= 10; count++){
    dosomething();
}

This would, in order:

• create an integer variable called count.
• ensure the loop will stop when the number stored in count is greater then or equal to the number 10
• instruct the for loop to increase the number stored in count by 1 each loop.
• call the function dosomething with no arguments.

The body of the loop is the function call dosomething();. The body of a loop gets called until its end condition is met and the loop as played itself out. Alternatively the body of a loop can tell the loop itself to break and thus the loop will stop and return to executing the code outside of the loop.

3.2.2.2. Foreach

A foreach loop typically wakes two statements. A iterable object and a name to use for each element from that object.

Firstly, an iterable is anything that has multiple elements to be retrieved or iterated over. A list of names is iterable making a for loop the best way to perform some action that uses each name on the list.

The following python code (python uses the for keyword even though it is really behaves like a foreach) will iterate over a list of friends names and then call the print function with the current name until all names have been iterated over.

>>> friends = ["nekroze", "lyshkah"]
>>> for name in friends:
...     print(name)
nekroze
lyshkah

The foreach loop is a little bit of a newer concept then the for loop. Many programmers would use a for loop that had an end condition be the length of a list and just use the counter as an index to the list. This was doing essentially the same thing as the foreach loop does but is much more complicated. Python gets away with having only a foreach loop (using the for keyword however) because you can still get the original for loop functionality by generating an iterable range for example, but many languages have both.

3.2.2.3. While

The while loop takes only a condition and will keep looping until that condition is met. A while loop is much like a for except it is up to the programmer to create the counter variable and implement how it is increased.

>>> loops = 0
>>> while loops < 10:
...     loops += 1
>>> loops
10

An important thing to note, especially with while loops, is that the condition can be any expression that can equate to a boolean. This can even be a function so long as it returns something that can be considered a boolean.