Loops and Conditionals

Episode 4 - Loops and Conditionals

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Ultimately, a computer cannot do very much. It can store to memory, fetch from memory, perform a limited set of computations and comparisons, and make decisions. But those simple operations are used to create all the many computer programs we use every day.


We’ll start with decisions. Decisions are made when some condition is either true or false. A variable that can take only the two special Python values True and False is called a Boolean variable. Expressions that evaluate to True or False are Boolean expressions. Comparisons are a common way to form Boolean expressions. Numerical types and strings have comparison operators defined on them. Numbers can be compared as less than, less than or equal, greater than, greater than or equal, equal, and not equal. The corresponding operators are <, <=, >, >=, ==, and !=. Numbers are compared based on standard arithmetical ordering. The same operators can be used with strings, but for strings we use lexical ordering, a standard defined as part of the character set. For instance, capital letters are “less than” lower-case letters, i.e. 'Z'<'a'.

We can use comparisons directly or we can combine them with the Boolean operators and, or, and not. The and and not operators are just what you would expect, but or may be a little unusual. In ordinary language, or is generally exclusive. You can have cake or ice cream but not both. In most programming languages, or is inclusive.

cake_ok or ice_cream_ok # True

There are precedence rules for Boolean operators. The not operator is highest, followed by and, then or.

Comparison operations all have the same precedence, and are evaluated before any Boolean operators are applied.

Like arithmetic expressions, the components of a Boolean expression are evaluated from left to right, accounting for precedence rules. Also like arithmetic operations, parentheses may be used for grouping, either to change the outcome or to improve the clarity for human readers.

How do we use these expressions to make decisions? We combine them with an if statement. We must now start to learn syntax, the correct way to write a statement. The syntax for an if block is

if condition_1:
elif condition_2: #optional
else: #optional

The expression condition_1 must evaluate to True or False. If it is True, the next block of statements is executed. A block is a group of statements that is logically related. In Python, a block must be indented. The number of spaces must be exactly the same for each level of indentation. Four spaces is recommended, though not required (one space is the minimum); please do not use tabs. The colon after the if/elif/else statements is also required. A single statement may follow the colon on the same line, but frequently we place it on a separate line with an indentation, since that is usually easier to read and also makes it quicker to add new statements should we later need to do so.

If condition_1 is False we skip to the next statement. If we want to carry out another test then we use elif (else if). If condition_2 is True we execute the elif block. If it is False we skip that block. If we have code that we wish to execute only if both condition_1 and condition_2 are False, then we use an else. If we do not need this test, we go back to the outer level of code. We are not limited to a single elif, we may use as many as we need; but at most one else is permitted.


Now let us consider repetition. We need to repeat commands in many circumstances. We may need to apply an operation to every item in a list. We may need to process through the lines of a file. All this is called looping.

Python has two forms of loop, the for loop and the while loop. We will first consider the for loop. The syntax is

for item in iterable:

As for the if statement, a colon is required, followed by either a single statement or a code block.

An iteratable is a structure through which our variable can step, one by one, so we iterate through the loop. Very frequently in Python the iterable is a list. The item is called a loop variable and it is a variable that takes on the values contained in the iterator. Since it is a variable the programmer chooses the name.

Recall our groceries list from Episode 2. We can step through the list with

for item in groceries:

We can nest loops within loops.

for item in groceries:
	for letter in item:

Notice how we used the outer loop variable as the iterable in the inner loop. We can do this because item will take on the elements of groceries one by one, and each element of groceries is a string, and an ordered sequence can be an iterable, so it works.

Often we want to iterate through a list of integers. Rather than typing them out, we can use a built-in function in Python called range. Range takes up to three arguments. If only one argument is present, it’s the non-inclusive upper bound and the lower bound is zero.

In Python 2.7, range returns a list. In Python 3 it returns an iterator so we must convert it to a list if we wish to look at its components. In a loop we just use the iterator.


If two arguments are present, the first is the lower bound. It must be smaller than the upper bound.


If three arguments are present, the third is the increment. If an increment is needed then both first and second arguments must be specified even if the lower bound is zero. If the increment is positive, the first argument must be less than the second argument; if it is negative, the first argument must be greater than the second. Range can handle only integers.



for i in range(len(x_list)):

Now let’s look at while loops. A for loop executes a fixed number of iterations, whereas a while loop executes until some condition becomes False. The general form is

while condition:
 	change condition

The condition must be changed or the loop will execute forever (an infinite loop situation). Occasionally this is done intentionally and we use the ability to break out of a loop, which we’ll see later, to stop the execution. More commonly, an infinite loop is an error.

The interpreter evaluates the condition. If it is True, it executes all the statements in the following code block. It then re-evaluates the condition, using updated values. If it is True it continues; if it has become False it skips all the of code block and goes on with the rest of the program.

Study this example:

while i<100:

What happens when you change the condition to i<=100? What would happen if you started with i=101?

Early Termination

If we need to jump out of a loop before it would normally terminate, we can insert a break statement.

while i<20:
	if i==19:


for i in range(20):
	if i==19:

If we wish to skip over the body of the loop from some point and return to the top, we use continue.


while i<20:
 	if i==19:

This episode covers the most fundamental programming constructs, so be sure you understand it before you go on.

Further References

A good introduction to loops and conditionals is available at


VIEW EXERCISE SOLUTION (`Function_Plotter.py`)
    Episode 4, Project 2
    Author:  A. Programmer

import math
import matplotlib.pyplot as plt



for i in range(1,N):

VIEW EXERCISE SOLUTION (`Temperatures.py`)
  Episode 4, Project 1
  Author:  A. Programmer

for i in range(0,61):

for i in range(len(degC)):