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The following discussion of a subscription is based on the Python Language Reference -- 6.3.2. Subscriptions .

Description

A subscription selects an item of a sequence ( string , tuple or list ) or mapping ( dictionary ) object, as in the following :

primary "[" expression_list "]"

I discussed strings earlier. I will discuss lists in this and the next module. I will discuss tuples and mappings later.

The primary

According to the Python Language Reference -- 6.3.2. Subscriptions

"The primary must evaluate to an object that supports subscription, e.g. a list or dictionary. User-defined objects can support subscription by defining a __getitem__() method."

For example, in the earlier module titled Itse1359-1070-Strings Part 2 . the primary was a reference to an object of type str where the reference was stored in a variable named aStr , as shown in Listing 1 . A diagram showing the variable and the object was shown in Figure 1 in that module.

Listing 1 . A string as a primary.
aStr = "This is a string" print(aStr[0]) #print T print(aStr[3]) #print s

A clarification

This terminology can be a little confusing. Perhaps the following will help to clarify things.

An ordinal number is an adjective that describes the numerical position of an object, e.g., first,second, third, etc.

Indexing is the process of assigning an ordinal number to each data item contained in some sort of a container.

The index is the value of an ordinal number that has been assigned to an item.

A subscription is the process of selecting an item of a sequence ( string , tuple or list ) or mapping ( dictionary ) object.

The expression_list

The earlier description included the term expression_list .

The Python Language Reference -- 6.3.2. Subscriptions states:

"If the primary is a sequence, the expression_list must evaluate to an integer or a slice..."

An example of an integer expression_list

In the string example in Listing 1 , the integer was 0 in one case and 3 in the other case.

Negative integers

If the (index) value is negative, the length of the sequence is added to it to obtain the actual index used to access the sequence.

Example of a negative index

For example, aSequence[-1] selects the last item of the sequence.

This is illustrated in the script shown in Listing 2 , which prints the last character in a string.

Listing 2 . Example of a negative index.
aStr = "xyz" print(aStr[-1]) # the output is z

The length of the string is 3.

The sum of the length and the negative index is 2.

The character in the string at an index value of 2 is "z".

More negative-integer rules

The value resulting from adding the length of the sequence to the specified index value must be a non-negative integer less than the number of items in thesequence.

Then, the subscription selects the item whose index is that value (counting from zero) .

Good and bad negative integers

This is illustrated by the script in Listing 3 , which shows both valid and invalid negative index values. (The index value of -4 violates the above rule, thus producing an IndexError.)

Listing 3 . Good and bad negative integers.
aString = "xyz" print(aString[-1]) print(aString[-2]) print(aString[-3]) print(aString[-4]) #The output from this script is shown below.z yx Traceback (most recent call last):File "1359-1080-03.py", line 9, in<module>print(aString[-4])IndexError: string index out of range

Questions & Answers

find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
I know this work
salma
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
hmm well what is the answer
Abhi
how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
ninjadapaul
20/(×-6^2)
Salomon
okay, so you have 6 raised to the power of 2. what is that part of your answer
ninjadapaul
I don't understand what the A with approx sign and the boxed x mean
ninjadapaul
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
Salomon
I'm not sure why it wrote it the other way
Salomon
I got X =-6
Salomon
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
ninjadapaul
oops. ignore that.
ninjadapaul
so you not have an equal sign anywhere in the original equation?
ninjadapaul
hmm
Abhi
is it a question of log
Abhi
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I rally confuse this number And equations too I need exactly help
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salma
Commplementary angles
Idrissa Reply
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Sherica
im all ears I need to learn
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Tamia
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Uday
hi
salma
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
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Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
Kim
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Al
y=10×
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rolling four fair dice and getting an even number an all four dice
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Kristine 2*2*2=8
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Differences Between Laspeyres and Paasche Indices
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No. 7x -4y is simplified from 4x + (3y + 3x) -7y
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Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
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what's the easiest and fastest way to the synthesize AgNP?
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Cied
types of nano material
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I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
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Yasmin
what is the function of carbon nanotubes?
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I'm interested in nanotube
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what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
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how did you get the value of 2000N.What calculations are needed to arrive at it
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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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