How it works
Algorithms combine ordered steps, decisions, and repetition to solve problems.
Trace each branch condition and each loop update before deciding what is displayed or returned.
Common error
A loop may run zero times, one time, or more times depending on the initial values.
Worked example
Trace pseudocode using AP CSP assignment and condition rules.
Data / pseudocode
x <- 2
REPEAT 3 TIMES {
x <- x + 2
IF (x > 5) { DISPLAY(x) }
}The successive x values are 4, 6, and 8. Only 6 and 8 satisfy the condition, so those values are displayed in that order. The update occurs before each test inside the loop.
Exercises
3.1.1
Trace: total <- 0; FOR EACH n IN [2, 5, 4] { IF (n > 3) { total <- total + n } }; DISPLAY(total).
Show solution
2 is skipped. 5 changes total to 5, and 4 changes it to 9. The displayed value is 9.
3.1.2
Write pseudocode that displays the even integers from 2 through 8 inclusive.
Show solution
Solution pseudocode
n <- 2
REPEAT 4 TIMES {
DISPLAY(n)
n <- n + 2
}
The output is 2, 4, 6, 8.