2.6. Driving from Chicago to Dallas¶
As an example, imagine that you were planning to drive from Chicago to Dallas. If you know how many miles-per-gallon your car gets, and how many miles it is, you can estimate the number of gallons it will take.
Use the CodeLens below to trace through what it would look like for a computer to execute this program:
Press
to go forward executing a single line of the program.Press
to execute all the lines of the program.
Activity: CodeLens 2.6.1 (cspnamenumbers_driving1)
Now, if we know the average cost of a gallon of gas, we can figure out the cost of the trip from Chicago to Dallas in your car.
Warning
Notice that in the code below that we are using camel case variable names like
costPerGallon and costTrip. When using this style, you have to be careful about
capitalization - costperGallon is a different name than costPerGallon. Try changing
just the first copy of costPerGallon below to costpergallon and then run the code
to see what happens.
Activity: CodeLens 2.6.2 (cspnamenumbers_driving2)
What we’re doing above is tracing a program. Normally, we run a program – tell the computer to execute each step of the program as fast as possible. When we do that, we don’t see individual variable values, as we can above. However, we can check values (including the values for variables) by printing them.
The function print can take an input (a variable name inside
of parentheses) whose value will be displayed. The print function can also print a string
(like "Cost to get from Chicago to Dallas") which is a sequence of characters inside a pair of
double quotes as seen in line 8. It will print the exact contents of the string. This is useful
for explaining the values that you are printing.
You can also print multiple things on one line by putting them all inside the parentheses after
print, separated by commas. You can see this on line 12.
Press the 
below to see this program run at full speed.
Try editing the program above and running it to answer this question: