CPSC120B - Day 4 Notes

The Expanding Demand for Coding Skills

The Expanding Demand for Coding Skills

Reading Questions

Quiz

Variable Reassignment

Once a variable has been given a value, it’s value can be changed
That’s why they are called variables
```
x = 1
x = 2
print(x)
```
Recall that the assignment operator, =, does not have the same meaning that it has in mathematics.
The above two equations would be impossible in mathematics
In a program they are possible because only line is executed at a time
When a variable is assigned to antoher variable, the value is copied
```
x = 1
y = x
x = 2
print(y)
```
So the variable y in the above example does not change it’s value when the value of x is changed
And don’t forget order matters
```
x = 1
print(y)
y = x
```
The above program produces an error because the variable y does not have a value when the print statement is executed

Variable Updating

Can also use a variable’s current value in computing a new value for it
```
x = 1
x = x + 2
print(x)
```
This looks weird becasue in mathmatics this would be impossible
But remember the assignment operator, =, is not equality
This works because the assignment operator has the lowest precedence
So on the second line in the above example, x is added to 2, and becasue x already has a value, 3, the assigment changes the value of x
Beware of updating a variable that has not been defined yet
```
y = y + 3
print(y)
```
Will produce an error because y has no value to add to 3
This can be used to reduce the number of variables a program needs

Take this version of the paper folding program from lab

zero_fold_thickness = 0.004
one_fold_thickness = zero_fold_thickness * 2
two_fold_thickness = one_fold_thickness * 2
three_fold_thickness = two_fold_thickness * 2
four_fold_thickness = three_fold_thickness * 2
five_fold_thickness = four_fold_thickness * 2
six_fold_thickness = five_fold_thickness * 2
seven_fold_thickness = six_fold_thickness * 2
eight_fold_thickness = seven_fold_thickness * 2
nine_fold_thickness = eight_fold_thickness * 2
ten_fold_thickness = nine_fold_thickness * 2
print("A piece of paper folded in half 10 times would be", ten_fold_thickness, "inches thick.")

By using variable updating, this can be simplified

paper_thickness = 0.004
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
paper_thickness = paper_thickness * 2
print("A piece of paper folded in half 10 times would be", paper_thickness, "inches thick.")

Of course, this could also be much simplified by using an exponent, but not allo programs will have that advantage

Lab

lab4