Computer Science Basics — Fundamentals
This section goes deeper into the basics from the Overview — not new topics, but the details and reasoning you need to actually use them confidently, not just recognize them.
Variables: What's Actually Happening
When you write age = 16, Python does two things: creates an integer object holding the value 16, and makes the name age point to it. This matters because of how assignment behaves with existing variables:
python
a = 10
b = a # b now points to the same value as a
a = 20 # a now points to a NEW value; b is unaffected
print(a) # 20
print(b) # 10 — b didn't change just because a did
This trips up beginners constantly: b = a copies the value at that moment, not a permanent link between the two variables.
Type Conversion — Moving Between Types Deliberately
Python won't automatically combine incompatible types — this is why print("Age: " + age) fails if age is a number, but print("Age:", age) works fine (the comma-separated form converts automatically for display; direct + concatenation does not).
python
age = 16
print("Age: " + str(age)) # convert int -> str explicitly, works
# print("Age: " + age) # this line would crash — TypeError
age_text = "16"
age_number = int(age_text) # convert str -> int
print(age_number + 1) # 17
price_text = "99.99"
price = float(price_text) # convert str -> float
print(price * 2) # 199.98
Rule worth memorizing: + between a string and a number always fails. Convert one side deliberately with str(), int(), or float() before combining them.
Operators, All Four Categories
python
# Arithmetic
print(7 + 3, 7 - 3, 7 * 3, 7 / 3, 7 // 3, 7 % 3, 7 ** 2)
# 10 4 21 2.333... 2 1 49
# Comparison — always produce True/False
print(5 > 3, 5 == 5, 5 != 3, 5 <= 4)
# True True True False
# Logical — combine True/False values
age = 16
has_id = True
print(age >= 18 and has_id) # False (age condition fails)
print(age >= 18 or has_id) # True (has_id alone makes it True)
print(not has_id) # False
# Assignment shortcuts
score = 10
score += 5 # same as: score = score + 5 -> 15
score -= 2 # -> 13
score *= 2 # -> 26
Common mistake: confusing = (assignment — "make this equal to") with == (comparison — "is this equal to"). if score = 10: is a syntax error in Python precisely to catch this mistake early; the correct form is if score == 10:.
Nested Conditions and Loops
Once a single if or for isn't enough, you nest them — one inside another. This is where indentation (which you've been using without much thought so far) actually starts to matter for logic, not just style:
python
# Nested if: check age, then check a second condition only if the first passes
age = 20
has_license = True
if age >= 18:
if has_license:
print("Can drive")
else:
print("Can't drive yet -- no license")
else:
print("Too young to drive")
# Nested loop: a multiplication grid
for i in range(1, 4):
for j in range(1, 4):
print(f"{i} x {j} = {i*j}", end=" ")
print() # new line after each row
# Output:
# 1 x 1 = 1 1 x 2 = 2 1 x 3 = 3
# 2 x 1 = 2 2 x 2 = 4 2 x 3 = 6
# 3 x 1 = 3 3 x 2 = 6 3 x 3 = 9
The inner loop completes fully for every single pass of the outer loop — that's why the grid fills row by row.
break, continue, and pass
Three keywords that change a loop's normal flow, each doing something different:
python
# break: exit the loop immediately, entirely
for i in range(1, 10):
if i == 5:
break
print(i, end=" ")
# Output: 1 2 3 4 (stops before 5, loop ends completely)
# continue: skip just this iteration, keep looping
for i in range(1, 6):
if i == 3:
continue
print(i, end=" ")
# Output: 1 2 4 5 (3 is skipped, but the loop continues)
# pass: does nothing -- a placeholder when syntax requires a
# statement but you don't have one yet
for i in range(5):
if i == 2:
pass # "handle this later" -- doesn't stop or skip anything
print(i)
# Output: 0 1 2 3 4 (pass didn't change the loop's behavior at all)
break and continue are genuinely different — break ends the loop, continue just skips ahead to the next round of it. Confusing the two is a common source of logic bugs.
String Methods You'll Use Constantly
python
name = " Priya Sharma "
print(name.strip()) # "Priya Sharma" -- removes leading/trailing spaces
print(name.strip().upper()) # "PRIYA SHARMA"
print(name.strip().split()) # ['Priya', 'Sharma'] -- splits into a list of words
print("-".join(["a", "b", "c"])) # "a-b-c" -- opposite of split
text = "Computer Science"
print(text.replace("Science", "Basics")) # "Computer Basics"
print(text.find("Science")) # 9 -- index where it starts
print("science" in text.lower()) # True -- membership check
Next Steps
Move to Intermediate for tuples, dictionaries, sets, exception handling, and a proper introduction to recursion and simple functions with classes — the tools that let you organize larger programs instead of writing everything as one script.
