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lab07-variables
variables

Review

What two operations can the name “+” refer to in python, depending on context?
What two operations can the name “/” refer to in python, depending on context?
What does 7/2 evaluate to?
What does 7.0/2.0 evaluate to?
What does 12/10*3 evaluate to?
What does 12*3/10 evaluate to?
What does 12*3.0/10 evaluate to?
What does 2 + 12*3/10 - 1 evaluate to?
What is python's order-of-operations?
What built-in function converts from integers to floats?
What built-in function converts from floats to integers?
The expression 7 / 2.0 is secretly re-written by python to be .

Last time, we discussed:

Consider the eleven-character string "abcdefghijk". What is the index of the letter that is (most nearly) 2/3 of the way through the string?
Write an expression which computes how far which letter is (closest to) 2/3 of the way through the string "abcdefghijk". (Your answer should involve only 2, 3, "abcdefghijk", *, parentheses, and calling functions.)

Challenge: What is the index of the character that is 2/3 of the way through? Can you even use substring from yesterday, to pull out that one character?

We saw an unwieldy answer to the last one: substring("abcdefhijk", len("abcdefghijk")*2/3-1, len("abcdefghijk")*2/3 ) This expression is hard to read because, in part, the reader has to notice the similarity between the three arguments (e.g. that they each are using the exact same string), and even verify that the similarity is intentional and not ever mis-typed.

We'd like to give a name to sub-parts of this computation.

letters = "abcdefghijk"
substring( letters, len(letters)*2/3 - 1, len(letters)*2/3 )

In fact, we can go further and also give a name to 2/3 of the way through letters:

letters = "abcdefghijk"
partwayThrough = len(letters) * 2/3
substring( letters, partwayThrough-1, partwayThrough )

We say that letters and partwayThrough are variables. Variables can be used to give names to sub-parts of a larger computation.

`=` is pronounced “gets”

We read letters = "abcdefghijk" as “lettters gets "abcdefghijk"”. Note that this is not the same as equality ¹. In particular, = indicates an action: figure out the value of the right-hand-side, and store it in a box with the name on the left-hand-side.

Variables are a named locations which hold a value

Think of a variable as a labeled box: it can hold one value. We'll draw pictures of what's going on inside the comptuer; those pictures are simple now, but will become more involved in the future.

Stepping through an expression-evaluation

When evaluating expressions:

letters = "abcdefghijk"
substring( letters, len(letters)*2/3 - 1, len(letters)*2/3 )

After the first line, python has a box named “letters”. It proceeds step-by-step:

  substring( letters, len(letters)*2/3 - 1, len(letters)*2/3 )
=   substring( letters, len(letters)*2/3 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", len(letters)*2/3 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", len("abcdefghijk")*2/3 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", 11*2/3 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", 22/3 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", 7 - 1, len(letters)*2/3 )
=   substring( "abcdefghijk", 6, len(letters)*2/3 )
=   substring( "abcdefghijk", 6, len("abcdefhijk")*2/3 )
=   substring( "abcdefghijk", 6, 11*2/3 )
=   substring( "abcdefghijk", 6, 22/3 )
=   substring( "abcdefghijk", 6, 7 )
=   "g"

Whew! Notice in particular that python doesn't quite remember the value of letters all the time; instead, whenever it sees that variable, it goes and looks it up in its memory. Nore does python automatically remember the result of len(letters)*2/3; it re-computes it each time it sees that (sub)expression.

Suppose we want to compute the letter which is 'a' shifted back by 32. One solution would be chr( ord('a') - 32 ). But others could be:

startCode = ord('a')
shiftedCode = startCode - 32
chr(shiftedCode)

startLetter = 'a'
startCode = ord(startLetter)
shiftedCode = startCode - 32
shiftedLetter = chr(shiftedCode)
shiftedLetter

TODO:

In the editor-pane (top half), write an equivalent series of expressions to show( makePicture( pickAFile() ) ), using two variables to hold the two sub-results. Use appropriate names. (When you press Load, it should show the file.)
Using exactly two variables, compute the ratio of len("Virginia Polytechnic Institute and State University") to len("Tech") (as a double). Instead, use variables to give names to each of the numerator and denominator, and then Use meaningful variable-names.
Give names to both the names (full name and nickname), as well as the length of each, for a total of four variables. Also, give a (meaningful) name to the final result.
Discuss w/ your partner: We have seen doing this with 0 variables, with 2 variables and with 4 or 5. Which do you prefer, and why?

Note that if we first want to do one thing, and use that result in the next step, and so-on, we can either compose functions, or we can make up name for intermediate results and store those results in those variables.

¹ We'll see soon, how to ask whether two values are equal or not — that's “==” in python :-( . ↩

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©2014, Ian Barland, Radford University
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lab07-variables variables

Review

= is pronounced “gets”

Variables are a named locations which hold a value

Stepping through an expression-evaluation

lab07-variables
variables

`=` is pronounced “gets”