- if statement
def f(x):
print("A", end="")
if x == 0:
print("B", end="")
print("C", end="")
print("D")
f(0)
f(1)
A more interesting example:
# These examples define abs(n), which is a nice example here, but it is
# also a builtin function, so you do not need to define it to use it.
def abs1(n):
if n < 0:
n = -n
return n
# again, with same-line indenting
def abs2(n):
if n < 0: n = -n # don't indent this way unless there is a specific reason
return n
# again, with multiple return statements
def abs3(n):
if n < 0:
return -n
return n
# aside: you can do this with boolean arithmetic, but don't!
def abs4(n):
return (n < 0)*(-n) + (n>=0)*(n) # this is awful!
# now show that they all work properly:
print("abs1(5) =", abs1(5), "and abs1(-5) =", abs1(-5))
print("abs2(5) =", abs2(5), "and abs2(-5) =", abs2(-5))
print("abs3(5) =", abs3(5), "and abs3(-5) =", abs3(-5))
print("abs4(5) =", abs4(5), "and abs4(-5) =", abs4(-5))
- if-else statement
def f(x):
print("A", end="")
if x == 0:
print("B", end="")
print("C", end="")
else:
print("D", end="")
if x == 1:
print("E", end="")
else:
print("F", end="")
print("G")
f(0)
f(1)
f(2)
Revisiting abs(n):
def abs5(n):
if n >= 0:
return n
else:
return -n
# or, if you prefer...
def abs6(n):
if n >= 0:
sign = +1
else:
sign = -1
return sign * n
print("abs5(5) =", abs5(5), "and abs5(-5) =", abs5(-5))
print("abs6(5) =", abs6(5), "and abs6(-5) =", abs6(-5))
- if-elif-else statement
def f(x):
print("A", end="")
if x == 0:
print("B", end="")
print("C", end="")
elif x == 1:
print("D", end="")
else:
print("E", end="")
if x == 2:
print("F", end="")
else:
print("G", end="")
print("H")
f(0)
f(1)
f(2)
f(3)
A more interesting example:
def numberOfRoots(a, b, c):
# Returns number of roots (zeros) of y = a*x**2 + b*x + c
d = b**2 - 4*a*c
if d > 0:
return 2
elif d == 0:
return 1
else:
return 0
print("y = 4*x**2 + 5*x + 1 has", numberOfRoots(4,5,1), "root(s).")
print("y = 4*x**2 + 4*x + 1 has", numberOfRoots(4,4,1), "root(s).")
print("y = 4*x**2 + 3*x + 1 has", numberOfRoots(4,3,1), "root(s).")
Another example:
def getGrade(score):
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 70:
grade = "C"
elif score >= 60:
grade = "D"
else:
grade = "F"
return grade
print("103 -->", getGrade(103))
print(" 88 -->", getGrade(88))
print(" 70 -->", getGrade(70))
print(" 61 -->", getGrade(61))
print(" 22 -->", getGrade(22))
- if-else expression
# if-else expression (not an if-else statement!)
def abs7(n):
return n if (n >= 0) else -n
print("abs7(5) =", abs7(5), "and abs7(-5) =", abs7(-5))
- Clarity and style
Note: this section concerns style and not correctness.
All the examples here 'work', but some work more artfully than others.
With that...
- Negated Condition (with "else" clause)
|
# (Possibly) Unclear:
b = True
if not b:
print('no')
else:
print('yes')
|
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# Better:
b = True
if b:
print('yes')
else:
print('no')
|
- Empty "if" Clause
|
# Unclear:
b = False
if b:
pass
else:
print('no')
|
|
# Better:
b = False
if not b:
print('no')
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- Using "if" instead of "and"
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# Messy:
b1 = True
b2 = True
if b1:
if b2:
print('both!')
|
|
# Better:
b1 = True
b2 = True
if b1 and b2:
print('both!')
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- Using another "if" instead of "else"
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# Can lead to bugs:
b = True
if b:
print('yes')
if not b:
print('no')
|
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# Better:
b = True
if b:
print('yes')
else:
print('no')
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Another example:
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# Messy and can lead to bugs:
x = 10
if x < 5:
print('small')
if (x >= 5) and (x < 10):
print('medium')
if (x >= 10) and (x < 15):
print('large')
if x >= 15:
print('extra large')
|
|
# Better:
x = 10
if x < 5:
print('small')
elif x < 10:
print('medium')
elif x < 15:
print('large')
else:
print('extra large')
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Yet Another Example
|
# Messy and can lead to bugs:
c = 'a'
if (c >= 'A') and (c <= 'Z'):
print('Uppercase!')
if (c >= 'a') and (c <= 'z'):
print('lowercase!')
if (c < 'A') or
((c > 'Z') and (c < 'a')) or
(c > 'z'):
print ('not a letter!')
|
|
# Better:
c = 'a'
if (c >= 'A') and (c <= 'Z'):
print('Uppercase!')
elif (c >= 'a') and (c <= 'z'):
print('lowercase!')
else:
print('not a letter!')
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- Using "arithmetic logic" instead of "boolean logic"
|
# Unclear:
x = 42
y = ((x > 0) and 99)
|
|
# Better:
x = 42
if x > 0:
y = 99
|