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.
(Chicago_2_Dallas)
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.
(Chicago_2_Dallas_Cost)
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. 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 6. It
will print the exact contents of the string. This is useful for explaining the
values that you are printing.
Press the 
below to see this program run at full speed.
How does this program work? Try pressing the 
button above to
listen to an explanation of the program.
Try editing the program above and running it to answer this question:
-
csp-3-5-1: If the cost per gallon drops to $3.45, can we drive from Chicago to Dallas
for less than $90 in gas?
- Yes
- Yes, the cost would be $89.86 (which you knew by editing the program above and running it)
- No
- Try it -- it's just under $90.
- We should fly instead.
- You might be right, but figure out the cost by editing the above program
-
csp-3-5-2: What would be printed by
- 3.45
- This would be true if it was printing the value of original variable.
- 3.65
- This would be true if it was printing the value of the variable after you changed it to figure out the previous question.
- cost_per_gallon
- Since
cost_per_gallonis in double quotes it is a string, and it will print out those exact characters.
print("cost_per_gallon") if this line was added to the end of the last program?