Tuesday, August 25, 2009

Python if elif else


In almost every programming language you have some basic commands and functions, basic construction options so to call. "if" is one of those, "the if statement is used to check a condition and if the condition is true, we run a block of statements (called the if-block), else we process another block of statements (called the else-block). The else clause is optional." So we can have a check and if this check is returning a true we can take some action. Lets see in a very basic example how this works, I will show this with a very small python script.

#-----------------------
#!/usr/bin/python

#set some variables
var0 = 2
var1 = 1

if var0 > var1:
print "var0 IS larger than var1"
elif var0 < var1:
print 'var0 IS smaller than var1'
else:
print 'var0 is not larger or smaller than var1, maybe they are the same?'

print 'and we have left the if elif else'
#-----------------------

Sow with this very simple script we can show what action is taken, or what text is printed to the console. You can test this by playing with the values of var0 and var1 and see for yourself what the result is. Basically I do not want to spend to much time on the "if" part as this should be a very basic part of programmers knowledge. The only part that can be tricky is that in some languages elif is written as "els if" or "elsif" or even "if else". In Python it is if, elif and else. Just something you have to know when you start with Python.

So as you can see making decisions with if statements is a very basic way of making decisions. Another thing which is good to know is that you can nest if statements. So if you come into if-block you can create inside this block another if-block to make your decision even more precise. In the script below I first determine if var0 and var1 are equal. If this is not the case we "open" a new if-block to see what is exactly the case. Is var0 larger or smaller than var1. Just play arround with the values of var0 and var1 and you will see what it can do.

#-----------------------
#!/usr/bin/python

#set some variables
var0 = 2
var1 = 2

print 'starting some nesting'

if var0 != var1:
print 'var0 is not the same as var1'
if var0 > var1:
print 'var0 is larger than var1'
elif var0 < var1:
print 'var0 is smaller than var1'
elif var0 == var1:
print 'var0 is the same as var1'

print 'done with the nesting'
#-----------------------

Also something that you might know from other languages is that the content of if-block should be within brackets and that it is constructed something like below:

#-----------------------

if(condition){
action
}
else if (condition II){
action
}
else{
action
}
#-----------------------

In Python this is not used, you might like it or you might not like it that this is not in place however the people who developed the Python language did not see the need of it. I personally think it is a missing part, this is simply because I am within the group of bracket lovers who like to have it nice and tidy inside a couple of brackets. If you are in the same group.... you will get used to it finally.. I did also. Upside, you will never have to count opening and closing brackets again... remember those long nights of debugging a bracket problem?

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