Conditionals

We need to be able to control what parts of a script run based on conditions. For example, if this trial requires a probe to be presented then run this code but if not run that code.

Boolean logic

Python, like most programming languages represents things as being True or False and these correspond to 1 or 0. See what these return (some are obvious, some not):

a = 4
4<5
a>5
a==4
4=4 #error?
4.0==4 #compare a float with an int
a>=3
a=>3
a!=3
True==1
False==0.0

Note

In nearly all programming languages = means ‘make this equal to’ whereas == means ‘test whether this is equal to’. But even experienced programmers will occasionally use the wrong one.

Conveniently, Python does the ‘right thing’ when comparing strings (using the same syntax):

a = "spam"
b='blah'
b > 'aaah'
b<'aaah'
b=='a'
b[2]=='a'
b.endswith('h')
'aaa'.endswith(a) #work out why this is wrong!

As well as those standard comparison operators, Python defines the operation in for testing the contents of any object that Python considers iterable:

'bee' in 'beer'
'bee' in 'hive'
10 in range(5) #maybe find out what range(5) does?!
subj = {'name':'jwp','age':21}
'age' in subj
5 in 10

As with any boolean logic system you can perform AND, OR, NOT operations on these things. These are typed with lower case (and, or, not):

a=='spam'
1==2 or a==4
1==1 and not a<4
subj['name'].startswith('j') and subj['age']<18
not True == False #get your head around this one!
5 not in [1,2,4]

Note

You can also use &, |, ! for and, or, not but few people do. The text version is just more natural to read. You do sometimes see != though.

As soon as Python can determine that the boolean statement is False the rest of the statement is not evaluated. That’s very important for some statements where you first need to check if it’s possible to run the next function, or where evaluating one part will take processing time and isn’t always necessary:

subj['weight']==83 and 'weight' in subj
'weight' in subj and subj['weight']==83 #reorder
keys = ['y']
keys[0]=='y' #check the first key press
keys = []
keys[0]=='y'
len(keys)>1 and keys[0]

if… statements

If statements allow you to say, “if this statement evaluates to be True then run the next lines of code”. To determine what count as the ‘next lines’ you have to indent the code. These examples are getting longer - you might want to switch to using the script editor panel if you’ve been using the shell so far:

if True:
    print('hello')
    print("toast")
if  5==4:
    print('no')

You can optionally define one or more elif statements and an else statement:

if 5==4:
    print('crazy')
elif 3*5>10 and 'a'<'b':
    print('possibly')
else:
    print('catch all')

Try to use an else statement to catch things that you didn’t expect to happen. It can make it easier to find bugs later on:

resp = ['left']
if resp=='left':
    corr=True
elif resp=='right':
    corr=False
else:
    print("Response should be 'left' or 'right' not %s" %(resp))

Would you have spotted what went wrong here without the else statement?

Note

The fact that Python will interpret either ” or ‘ as a string makes it very easy if you want a string to contain one of those characters. e.g. “Won’t hurt” is fine but ‘Won’t hurt’ will cause an error (because the string effectively ends after the n). If you want to be really safe you can start or end a string with triple quotes and then the string can contain either type of quote inside.

Nested if

Statements can nest too. Make sure you understand whether each of the following lines will be run and why:

resps = [0,1,1,0]
if len(resps)>0: #subj responded
    print("mean resp=", sum(resps)/float(len(resps)))
    if resp[0]==0:
        print('first resp correct')
    else:
        print('first resp incorrect')
    print('hello')
print('done')

Float gotchas

Tattoo this somewhere:

NEVER TEST FLOATS FOR EQUALITY

Computers store integers precisely, but floating point numbers are often just approximations. Don’t expect this to work (ever), particularly because sometimes it does!

>>> a = 1
>>> b = 1.0
>>> a == b
True # might be in this particular instance, but often won't be!

>>> print(0.1+0.1+0.1 == 0.3)
False

What? Why?! Most floating point numbers (like 0.1) can’t be perfectly represented in binary numbers. Here are those numbers formatted as strings with 25-decimal places

>>> format(0.1, '.25f')
'0.1000000000000000055511151'
>>> format(0.3, '.25f')
'0.2999999999999999888977698'

Hopefully this will make it clear that the binary representation of 0.3 is not exactly triple the representation of 0.1

So how to test if two non-integers have the same value?

Choose some allowable error that works in your particular case, and test that the absolute value of the difference between the values does not exceed that:

>>> abs(x - y) < 0.00001

Compare the difference to some tiny value that is nonetheless much bigger than the likely rounding error. The abs() function is important: need to look at the absolute value as you don’t know which number will be ‘larger’.


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