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Trace tables and types of error
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A trace table lets you run a program in your head, one line at a time. It is also the best tool for hunting down logic errors.
Using a trace table
Make a column for each variable (and one for output).
Each time a value changes, write the new value on a new row.
total = 0
for i in range(1, 5):
total = total + i
print(total)
Each time a value changes, write the new value on a new row.
total = 0
for i in range(1, 5):
total = total + i
print(total)
Three types of error
Syntax error: the code breaks the rules of the language, so it cannot be translated or run. Example: print("Hi" with a missing bracket.
Runtime error: the program starts but crashes while running. Example: dividing by zero, or using a list index that does not exist.
Logic error: the program runs but gives the wrong result. Example: using < instead of <=.
Runtime error: the program starts but crashes while running. Example: dividing by zero, or using a list index that does not exist.
Logic error: the program runs but gives the wrong result. Example: using < instead of <=.
Finding logic errors
Logic errors give no error message, so you must test.
Predict the correct answer, trace the code, and find the line where the trace stops matching your prediction.
A common cause is an off-by-one error in a loop range.
Predict the correct answer, trace the code, and find the line where the trace stops matching your prediction.
A common cause is an off-by-one error in a loop range.
Trace this code and give the output.
count = 0
for letter in "banana":
if letter == "a":
count = count + 1
print(count)
- The loop visits b, a, n, a, n, a.
- count goes up on each 'a': 1, 2, 3.
Answer: 3
Trace tables track each variable line by line. Syntax errors stop code running, runtime errors crash it while running, and logic errors give wrong results with no message.
The interactive lesson includes the diagrams for this topic.
Check you have got it
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