Complete beginner with Elif [Python]

Question

I tried to do the first assignment from http://www.cplusplus.com/forum/articles/12974/

g = int(raw_input('Enter the grade you scored: '))
if g >= 90 & g <= 100:
 print 'Your grade is: A'
elif g >= 80 & g < 90:
 print 'Your grade is: B'
elif g >= 70 & g < 80: 
 print 'Your grade is: C'
elif g >= 60 & g < 70:
 print 'Your grade is: D'
elif g >= 50 & g < 60:
 print 'Your grade is: E' 
elif g >= 0 & g <= 49:
 print 'Your grade is: F'
else:
 print 'You can only enter an integer within the range of 0-100.'

The problem is that whenever I run this program, any number I input that is greater than 0 will get:

Your grade is: A

Any help greatly appreciated. Thanks!

Answer 1

Bi Rico's answer is simple and correct. To explain the situation further:

• The & operator computes the bitwise AND of two integers. For example, 5 & 3 == 1.
• The precedence of & is above the comparison operators (such as < and >=). So a < b & c < d actually means a < (b & c) < d.
• Python allows chained comparisons. For example, a < b == c >= d translates into a < b and b == c and c >= d.

Putting these facts together, this is what happens when you run your program:

• Assume that g is assigned an integer value between 0 and 100, inclusive.
• So the if-test g >= 90 & g <= 100 means (g >= (90 & g)) and ((90 & g) <= 100).
• Bitwise AND is always smaller than or equal to both arguments (i.e. (a & b <= a) and (a & b <= b)).
• Thus 90 & g <= g is always true. Likewise, 90 & g <= 100 is always true, because 90 <= 100.
• Therefore the first if-test is always true, so the body will execute and the elif/else clauses will never execute.

Answer 2

The problem in your code is that you're using & when you want to use and, try this:

if g >= 90 and g <= 100:
 print 'Your grade is: A'
...

• Antonio: Well that was easy. Thank you!

Answer 3

The code you had their was spot on but syntax is very important in Python. You want to use and where you have & like this:

g = int(raw_input('Enter the grade you scored: '))
if g >= 90 and g <= 100:
 print 'Your grade is: A'
elif g >= 80 and g < 90:
 print 'Your grade is: B'
elif g >= 70 and g < 80: 
 print 'Your grade is: C'
elif g >= 60 and g < 70:
 print 'Your grade is: D'
elif g >= 50 and g < 60:
 print 'Your grade is: E' 
elif g >= 0 and g <= 49:
 print 'Your grade is: F'
else:
 print 'You can only enter an integer within the range of 0-100.'

This will display what you need the correct way.

Answer 4

Using the correct keyword is always important as certain things can cause errors. Use and, not &.