Operators and Flow Control

Operators let you combine and compare values; flow control decides which code runs and how often. Together they form the logic of every program.

Operators

Arithmetic

+   add                  5 + 3    → 8
-   subtract             5 - 3    → 2
*   multiply             5 * 3    → 15
/   divide (always float) 7 / 2   → 3.5
//  integer division     7 // 2   → 3     (always rounds down)
%   modulo (remainder)   7 % 2    → 1     (7 = 3×2 + remainder 1)
**  exponent             2 ** 3   → 8     (2 to the power of 3)

Compound assignment

x += 3    →  x = x + 3
x -= 3    →  x = x - 3
x *= 3    →  x = x * 3
x /= 3    →  x = x / 3

Comparison (returns True or False)

==   equal to             5 == 5   → True
!=   not equal to         5 != 3   → True
<    less than            3 < 5    → True
>    greater than         5 > 3    → True
<=   less than or equal   5 <= 5   → True
>=   greater than or equal 5 >= 6  → False

Boolean

and   both True?       True and False  → False
or    at least one?    True or False   → True
not   invert           not True        → False

Precedence (highest to lowest)

**  →  * / // %  →  + -  →  comparison  →  not  →  and  →  or

So 2 + 3 * 4 = 2 + 12 = 14 (not 20).

String / list operators

in        'top' in 'desktop'    → True
+         'hi' + ' ' + 'there'  → 'hi there'   (concatenation)
*         'ha' * 3              → 'hahaha'      (repetition)
[]        lst[0], s[2:5]        (indexing / slicing)

Try it yourself

print(17 // 5)     # expect: ?
print(17 % 5)      # expect: ?
print(2 ** 10)     # expect: ?
print(10 / 3)      # expect: ?
print(10 // 3)     # expect: ?
print(-7 // 2)     # expect: ?  (note: floor = rounds down)
print(8 + 2 * 2)   # expect: ?  (precedence!)
print((8 + 2) * 2) # expect: ?

if / elif / else

score = 75

if score >= 90:
    print("A")       # only if score >= 90
elif score >= 80:
    print("B")       # only if score 80-89
elif score >= 70:
    print("C")       # only if score 70-79  ← this one runs
else:
    print("F")       # all other cases

Important: as soon as one condition is True, the rest are not checked. So score = 95 prints just "A", not "A B C".

Common gotcha: With a chain of overlapping conditions, the first match wins and the rest are skipped.

Loops: for and while

for loop

Use it when you know how many times you want to repeat:

for i in range(5):       # i = 0, 1, 2, 3, 4
    print(i)

for letter in "hi":      # letter = 'h', 'i'
    print(letter)

for item in [10, 20, 30]:  # item = 10, 20, 30
    print(item)

range() variants:

range(5)         # 0, 1, 2, 3, 4
range(2, 5)      # 2, 3, 4
range(0, 10, 2)  # 0, 2, 4, 6, 8       (step of 2)
range(10, 0, -1) # 10, 9, 8, ..., 1    (counting down)

while loop

Use it when you do not know in advance how many times:

x = 0
while x < 5:
    print(x)
    x += 1       # do not forget this, or you get an infinite loop

break and continue

for i in range(10):
    if i == 5:
        break       # STOP the whole loop
    if i == 3:
        continue    # skip 3, move on to 4
    print(i)
# Output: 0, 1, 2, 4   (3 skipped, stops at 5)

Try it yourself

Predict the output before you press Enter:

for i in range(1, 10, 3):
    print(i, end=" ")
# expect: ?

x = 100
while x > 1:
    x = x // 2
    print(x, end=" ")
# expect: ?

Booleans as numbers

In Python, True equals 1 and False equals 0, so they work in arithmetic:

True + 2     # → 3    (True = 1)
False + 2    # → 2    (False = 0)
True * 10    # → 10

This is handy for counting True results, for example sum(x > 0 for x in numbers).