adding use case description

Author: Adinc02

usecase/use_case_description.md

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+# USE CASE: 3 Produce a Capital City Population Report
+## CHARACTERISTIC INFORMATION
+### Goal in Context
+As a data analyst, I want to list all cities by population so that I can compare urban populations worldwide from largest to smallest
+
+### Scope
+Population Reporting System
+
+### Level
+Primary task.
+
+### Preconditions
+The system contains accurate and current population data for all capital cities, including their country and geographic classification.
+
+### Success End Condition
+A city population report is generated, displayed in ascending order and available for analysis.
+
+### Failed End Condition
+No report is produced or the data is incomplete.
+
+### Primary Actor
+Data Analyst.
+
+### Trigger
+A request for city population analysis is initiated by the data analyst or a stakeholder.
+
+## MAIN SUCCESS SCENARIO
+1. Data analyst initiates a request to generate a city population report.
+2. System prompts for report parameters (e.g., global, continent, region, top-N cities).
+3. Data analyst selects desired filters and inputs top-N value if needed.
+4. System retrieves relevant city data including name, country, and population.
+5. System generates and displays the report in ascending order.
+6. Data analyst reviews and exports the report for further analysis or sharing.
+
+## EXTENSIONS
+3. Invalid filter or top-N input:
+    * System prompts for correction and re-entry.
+
+4. Data missing or incomplete:
+    * System notifies analyst of missing data and suggests alternative filters or data sources.
+
+## SUB-VARIATIONS
+
+- Report filtered by continent only.
+- Report filtered by region only.
+- Report showing top-N capital cities globally.
+- Report showing top-N capital cities within a continent.
+
+## SCHEDULE
+DUE DATE: Release 1.0