Learn Python from Scratch - Conditional Execution

Ola pythonista's, so this is a follow up of the previous post which was on the introduction of python. Here, I’m going to talk about Conditional execution and iterations in python. Hope you would find it interesting to browse.

There are many conditional constructs in python, and the first one we are going to see is the unidirectional branching statement, the prodigal construct: “if”.

An if statement precedes with a condition that is checked and is recouped to a boolean expression and if it’s results to true then a set of statement are executed otherwise they’re skipped and the instructions after the construct body is executed.

Boolean expressions in python use the following relational operators:

Indentation is an important prospect of python programming language as the language doesn’t use the usual flower braces to represent a construct body as in other high-level languages as C++ or Java and hence an increase indent after conditional or looping statement indicate the body of the construct and the indent is maintained to indicate the scope of the block. To get back to level of the conditional or iteration statement we reduce the indent to indicate the end of the block.

Two-way decisions are possible in python by using if-else construct wherein if the condition evaluates to true a set of statement are executed otherwise the body in the else construct is executed and finally executed the rest of the instructions after the conditional construct. Below is a snippet that depict two-way decision.

//flowchart and snippet

Mutli-branching decision is possible by using an if-elif-else construct, where in if the condition evaluates to true executes a set of statements and skips execution of the rest of the conditional construct otherwise evaluate the next condition that precedes the elif statement and if that evaluates the condition to true then a set of instructions are executed otherwise executes the statements in the else body.

This is the end of conditional construct in python and before we can get to python iterations let’s discuss about how to handle dangerous sections of code that raises an error that can be maskable i.e. for which we can write a piece of code that acts when a set of statements causes a traceback and tries to solve or bring the program out of the troubleshoot problem else the program can terminate if not handled. This is possible in python by use of the try/except structure. You surround a dangerous section of code with try and except. If the code in the try works - the except is skipped. If the code in the try fails - it jumps to the except section.

Now let’s understand python iterations which is last topic of this post. What are iterations?? They’re repetition of a set of statements. They can be categorized into two kinds – definite and indefinite.

While loops are called “indefinite loops” because they keep going until a logical condition becomes false. Sometimes it is a little harder to be sure if a loop of such a kind will terminate or not. A while loop is written in a following manner, its identified by the reserved word while and follows with a condition that evaluates to a Boolean value and only if it’s true that the control flow enter the loop body and hence the while loop is also called entry controlled loop.

For loops are called “definite loops” because they execute the loop body for exact number of times. We can write a loop to run the loop once for each of the items in a set using the Python for construct. Definite loops (for loops) have explicit iteration variables that change each time through a loop.  These iteration variables move through the sequence or set. The iteration variable “iterates” through the sequence (ordered set). The block (body) of code is executed once for each value in the sequence. The iteration variable moves through all of the values in the sequence.

The “is” and “is not” operators in python that are used in logical expression. The “is” operator implies is the same as but have more weight than the logical operator “==”. The “is not” operator logically is opposite of the “is” operator.

So here we are, at the end of the post, let’s summarise the topics we have seen today. (……..) This concludes the post, stay tuned for the next post on the topic of functions and string operations, until then guys enjoy your time browsing through other posts in the blog!!