Sequence, selection and iteration

Selection chooses which instructions to run by checking a condition. An if handles the first case, elseif checks another case, and else handles anything left over. The Python keywords are if, elif and else:

score = 72
IF score >= 80 THEN
    OUTPUT "High"
ELSE IF score >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Try again"
ENDIF

score = 72
if score >= 80:
    print("High")
elif score >= 50:
    print("Pass")
else:
    print("Try again")

Python uses a colon and indentation to mark each block. The conditions are checked from top to bottom, and only the first matching branch runs.

A switch selects between several exact values of one expression. ERL uses switch, case and default; Python can express the same GCSE logic with if and elif:

switch day:
    case "Sat":
        OUTPUT "Weekend"
    case "Sun":
        OUTPUT "Weekend"
    default:
        OUTPUT "Weekday"
endswitch

day = "Sat"
if day == "Sat":
    print("Weekend")
elif day == "Sun":
    print("Weekend")
else:
    print("Weekday")

Python's match is beyond this specification, so use if/elif/else for exam answers.

A for loop repeats a known number of times. ERL's end value is included: FOR i = 0 TO 9 visits 0, 1, …, 9. Python's range stops before its end value, so range(0, 10) visits the same numbers:

FOR i = 0 TO 9
    OUTPUT i
NEXT i

for i in range(0, 10):
    print(i)

Both print ten values, from 0 to 9.

A while loop checks its condition before each repeat, so it may run zero times. ERL uses WHILE … ENDWHILE; Python uses while with an indented body:

answer = ""
WHILE answer != "yes"
    INPUT answer
ENDWHILE

answer = ""
while answer != "yes":
    answer = input("Type yes: ")

ERL's DO … UNTIL checks after the body, so it always runs at least once. Python can copy that behaviour with while True and a break when the stopping condition is met:

DO
    INPUT answer
UNTIL answer == "yes"

while True:
    answer = input("Type yes: ")
    if answer == "yes":
        break

A trace table records values after each step. For total = 0 followed by for i in range(1, 4): total = total + i, the rows are:

The final output is 6.