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- Selection: if, elif and else
Selection: if, elif and else
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Selection lets a program make decisions. Python checks conditions from the top and runs the first block that is True.
if, elif, else
Python checks each condition in order and runs only the first block whose condition is True.
else runs if none of them is True.
The colon and the indentation show which lines belong to each block.
mark = 72
if mark >= 70:
print("A")
elif mark >= 50:
print("B")
else:
print("C")
else runs if none of them is True.
The colon and the indentation show which lines belong to each block.
mark = 72
if mark >= 70:
print("A")
elif mark >= 50:
print("B")
else:
print("C")
Combining conditions
Use and, or and not to build conditions.
if age >= 13 and age <= 19: is True only for teenagers.
if day == "Sat" or day == "Sun": is True at weekends.
if age >= 13 and age <= 19: is True only for teenagers.
if day == "Sat" or day == "Sun": is True at weekends.
Nested selection
An if inside another if is nested. The inner if is only checked when the outer condition is True.
Always test the boundaries: what happens at exactly 70 or 50?
Always test the boundaries: what happens at exactly 70 or 50?
What does the grade program print when mark = 50?
- mark >= 70 is False.
- mark >= 50 is True, so print B and skip the rest.
Answer: B
if, elif and else choose which block runs; only the first True branch runs. Conditions use == != < > <= >= with and, or, not. Indentation shows the blocks.
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