Conditionals & Loops

ECON 3209 · Week 2, Lecture 1 · Kerala Agricultural University

Department of Development Economics, KAU

Autumn 2026

Learning Outcomes

By the end of this lecture, you will be able to:

  1. Use if, elif, and else to make economic decisions in code
  2. Write for and while loops to process repeated calculations
  3. Control loops with break and continue
  4. Trace how indentation determines program logic in Python
  5. Apply control flow to Kerala agricultural and cooperative banking examples

Why Control Flow Matters

  • Economists rarely analyse one observation at a time
  • We often classify villages, crops, borrowers, or districts using rules
  • We also repeat the same calculation across many years or households
  • Control flow tells Python when to run code and how many times
  • It is the bridge from simple arithmetic to real data analysis

A loan officer deciding whether to approve, reject, or review an application is using the same logic as an if/elif/else block.

Conditional Statements: if, elif, else

  • if checks the first condition
  • elif means “else if” and handles additional cases
  • else runs when no earlier condition is true
  • The colon : starts a code block
  • Indentation shows which lines belong to each branch

Example: Classifying NPA Risk

  • The same variable can lead to different outputs
  • Conditions are checked from top to bottom
  • Put the most specific or most urgent rule first
  • Once Python finds a true branch, it skips the rest

Combining Conditions

  • Use and when all conditions must be true
  • Use or when any one condition is enough
  • Use not to reverse a condition
  • Complex rules are common in credit screening and crop classification
  • Parentheses improve readability when conditions become long

Loan approved if:

\[(\text{income} \geq 30000) \; \text{and} \; (\text{credit score} \geq 700) \; \text{and} \; (\text{default history} = \text{False})\]

Example: Screening Borrowers

  • This is a compound boolean expression
  • Each sub-condition returns True or False
  • The entire if statement depends on the combined result
  • Clear rules make economic decisions reproducible

Repetition with for Loops

  • A for loop repeats code over a sequence
  • Typical sequences are lists, strings, ranges, or arrays
  • Use for when you know what items you want to iterate over
  • In economics, this is useful for districts, crops, or years
  • Python reads the loop body one item at a time

Example: Summing District Deposits

  • Start with an accumulator such as total = 0
  • Update it inside the loop
  • After the loop ends, the final value is available
  • This pattern appears everywhere in statistics and accounting

range() and Counting Loops

  • range(5) gives 0, 1, 2, 3, 4
  • range(2021, 2026) gives 2021 to 2025
  • The stop value is not included
  • range() is ideal for fixed-number repetition
  • Use it when looping over time periods or index positions

while Loops

  • A while loop continues as long as a condition remains true
  • Use it when the stopping point is not known in advance
  • while loops are common in simulations and iterative finance problems
  • Make sure something changes inside the loop
  • Otherwise you create an infinite loop

break and continue

  • break exits the loop immediately
  • continue skips the rest of the current iteration
  • These commands help clean messy real-world datasets
  • Use them carefully so your logic stays readable

Common Loop Mistakes

  • Forgetting the colon : after if, for, or while
  • Mixing tabs and spaces in indentation
  • Updating the wrong variable inside a loop
  • Writing a while loop with no progress toward stopping
  • Reusing unclear variable names like x, y, and z

Debugging tip: read your loop aloud — “for each district, do this calculation” — and then check whether the code matches that sentence.

Exercise

A cooperative bank reviews annual loan recovery rates: 92, 88, 95, 76, 90.

  1. Use a for loop to print each rate.
  2. If a rate is below 80, print Needs field visit.
  3. At the end, print how many rates are below 90.

Summary

  • ✅ if/elif/else lets Python choose between alternatives
  • ✅ for loops repeat code across known sequences
  • ✅ while loops repeat until a condition becomes false
  • ✅ break stops a loop; continue skips one iteration
  • ✅ Indentation is part of Python syntax, not just style
  • ✅ Control flow is essential for classifying, filtering, and aggregating economic data

Next Lecture

Functions & Scope

  • We will package repeated logic into reusable functions
  • You will learn about parameters, return values, and variable scope
  • This will help you write cleaner and more modular econometrics code