---
title: "Control Flow: Conditions and Loops"
date: 2026-01-03T16:06:29Z
modified: 2026-01-03T16:06:31Z
permalink: "https://www.micheledpierri.com/python/control-flow/"
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status: publish
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featured_image_alt: Children dressed in lab coats are studying a python
timestamp: 2026-01-03T16:06:31Z
tags: []
---
## Lesson Objectives
By the end of this lesson, you will be able to:
- Write conditional logic using `if`, `elif`, and `else`
- Iterate over ranges of values using `for` loops
- Control execution with `while` loops
- Understand why control flow is fundamental for data processing and modeling
This lesson marks a transition:
**Python stops being declarative and starts being algorithmic.**
---
## 1️⃣What Is Control Flow
Control flow defines how code executes:
- which instructions run
- when they run
- how many times they run
Without control flow, programs would execute linearly and be almost useless for real data.
---
## 
2️⃣Conditional Statements (if, elif, else)
### Basic Structure
age = 20
if age >= 18:
print("Adult")
else:
print("Minor")
```
age = 20
if age >= 18:
print("Adult")
else:
print("Minor")
```
How it works:
- the condition after `if` is evaluated
- if `True`, the indented block runs
- otherwise, the `else` block runs
### Multiple Conditions (elif)
score = 75
if score >= 90:
print("Excellent")
elif score >= 60:
print("Passed")
else:
print("Failed")
```
score = 75
if score >= 90:
print("Excellent")
elif score >= 60:
print("Passed")
else:
print("Failed")
```
Only one branch is executed.
---
## 3️⃣ Boolean Expressions
Conditions rely on boolean expressions.
#### Common Operators
== equal
!= not equal
> greater than
< less than
>= greater or equal
<= less or equal
```
== equal
!= not equal
> greater than
< less than
>= greater or equal
<= less or equal
```
Example:
x = 10
print(x > 5) # True
```
x = 10
print(x > 5) # True
```
These expressions are everywhere in data filtering and decision rules.
---
## 4️⃣Indentation Is Not Optional
Python uses indentation, not braces.
if x > 0:
print("Positive")
print("Still inside if")
```
if x > 0:
print("Positive")
print("Still inside if")
```
Incorrect indentation causes errors or logical bugs.
Rule:
> Indentation defines program structure.
---
## 
5️⃣for Loops
for loops are used to iterate over sequences.
### Basic Example
for i in range(5):
print(i)
```
for i in range(5):
print(i)
```
Output:
0
1
2
3
4
```
0
1
2
3
4
```
range(5) generates numbers from 0 to 4.
### Custom Ranges
for i in range(1, 6):
print(i)
```
for i in range(1, 6):
print(i)
```
Output:
1
2
3
4
5
```
1
2
3
4
5
```
This pattern is extremely common in simulations and analyses.
---
## 6️⃣while Loops
while loops run as long as a condition is True.
x = 0
while x < 5:
print(x)
x += 1
```
x = 0
while x < 5:
print(x)
x += 1
```
Important:
- you must update the condition
- otherwise, you create infinite loops
---
## 7️⃣ Choosing Between for and while
General rule:
- use for when you know how many iterations you need
- use while when the stopping condition is dynamic
In data analysis:
- for → iterating over datasets
- while → iterative algorithms and convergence
---
## 8️⃣Control Flow in Data Analysis
Control flow enables:
- filtering observations
- conditional transformations
- iterative model fitting
- simulations and bootstrapping
Almost every analytical pipeline depends on it.
---
## FAQ — Frequently Asked Questions
**Q: Why does Python not use {} like other languages?**
A: Python prioritizes readability and enforces indentation-based structure.
**Q: Can I nest if statements?**
A: Yes, but excessive nesting reduces readability.
**Q: Is for slower than vectorized operations?**
A: Often yes. Loops are foundational, but later we’ll optimize.
**Q: What happens if a condition is always True?**
A: In a while loop, the program never stops.
---
## Exercises
### Exercise 1
Write an **if** statement that checks if **x** is positive.
### Exercise 2
Write an **if/else** that checks if a number is even or odd.
### Exercise 3
Use **elif** to classify a score (fail/pass/excellent).
### Exercise 4
Print numbers from 0 to 9 using a **for** loop.
### Exercise 5
Print numbers from 1 to 10.
### Exercise 6
Use a **while** loop to print numbers from 0 to 4.
### Exercise 7
Predict the output:
x = 3
if x > 5:
print("A")
else:
print("B")
```
x = 3
if x > 5:
print("A")
else:
print("B")
```
### Exercise 8
Explain why this code is wrong:
while x < 10:
print(x)
```
while x < 10:
print(x)
```
### Exercise 9
Write a loop that sums numbers from 1 to 5.
### Exercise 10
Explain why control flow is essential in data analysis.
---
## Solutions
### Exercise 1
if x > 0:
print("Positive")
```
if x > 0:
print("Positive")
```
### Exercise 2
if n % 2 == 0:
print("Even")
else:
print("Odd")
```
if n % 2 == 0:
print("Even")
else:
print("Odd")
```
### Exercise 3
if score >= 90:
print("Excellent")
elif score >= 60:
print("Passed")
else:
print("Failed")
```
if score >= 90:
print("Excellent")
elif score >= 60:
print("Passed")
else:
print("Failed")
```
### Exercise 4
for i in range(10):
print(i)
```
for i in range(10):
print(i)
```
### Exercise 5
for i in range(1, 11):
print(i)
```
for i in range(1, 11):
print(i)
```
### Exercise 6
x = 0
while x < 5:
print(x)
x += 1
```
x = 0
while x < 5:
print(x)
x += 1
```
### Exercise 7
# Output: B
```
# Output: B
```
### Exercise 8
# Indentation error: the print statement must be indented
```
# Indentation error: the print statement must be indented
```
### Exercise 9
total = 0
for i in range(1, 6):
total += i
print(total)
```
total = 0
for i in range(1, 6):
total += i
print(total)
```
### Exercise 10
# Control flow allows decisions, iteration, and conditional data processing
```
# Control flow allows decisions, iteration, and conditional data processing
```
---
## Next Lesson Preview
In Lesson 6, we will introduce:
- functions
- parameters and return values
- why functions are essential for reusable analytical pipelines
## This is where Python code becomes modular and scalable.