{"id":445,"date":"2025-05-19T19:32:46","date_gmt":"2025-05-19T19:32:46","guid":{"rendered":"https:\/\/buhave.com\/courses\/?p=445"},"modified":"2025-05-20T12:56:49","modified_gmt":"2025-05-20T12:56:49","slug":"control-flow-2","status":"publish","type":"post","link":"https:\/\/buhave.com\/courses\/python\/control-flow-2\/","title":{"rendered":"Control Flow"},"content":{"rendered":"if, elif, and else statements<\/h2>\n
These statements let your program make decisions based on conditions \u2014 they\u2019re the core of control flow in Python.<\/p>\n
Basic Syntax<\/h3>\n
if condition:<\/em>
\n# code block if condition is True<\/em>
\nelif another_condition:<\/em>
\n# code block if elif is True<\/em>
\nelse:<\/em>
\n# code block if none are True<\/em><\/p>\n\n- Each condition is checked in order<\/li>\n
- Indentation (typically 4 spaces) defines code blocks<\/li>\n
- You can have one if, zero or more elif, and zero or one else<\/li>\n<\/ul>\n
Simple Example:<\/strong><\/p>\ntemperature = 25<\/em><\/p>\nif temperature > 30:<\/em>
\nprint(“It’s hot outside.”)<\/em>
\nelif temperature >= 20:<\/em>
\nprint(“It’s a nice day.”)<\/em>
\nelse:<\/em>
\nprint(“It\u2019s cold outside.”)<\/em><\/p>\nOutput:<\/strong> “It’s a nice day.”<\/p>\nFlow Explanation:<\/strong><\/p>\n\n- If the if condition is True, it runs that block and skips the rest.<\/li>\n
- If the if is False, Python checks the elif (if any).<\/li>\n
- If none of the conditions are True, the else block runs.<\/li>\n<\/ul>\n
Real-Life Example: Age Check<\/strong><\/p>\nage = int(input(“Enter your age: “))<\/em><\/p>\nif age < 13:<\/em>
\nprint(“You are a child.”)<\/em>
\nelif age < 18:<\/em>
\nprint(“You are a teenager.”)<\/em>
\nelse:<\/em>
\nprint(“You are an adult.”)<\/em><\/p>\nNesting if Statements<\/strong><\/p>\nYou can nest conditions inside others:<\/p>\n
score = 85<\/em><\/p>\nif score >= 50:<\/em>
\nprint(“You passed!”)<\/em>
\nif score >= 80:<\/em>
\nprint(“Great job!”)<\/em>
\nelse:<\/em>
\nprint(“You failed.”)<\/em><\/p>\nCommon Mistakes to Avoid<\/strong><\/p>\n\n\n\nMistake<\/strong><\/th>\nFix<\/strong><\/th>\n<\/tr>\n<\/thead>\n\n\nMissing colon :<\/code><\/td>\nAdd a colon at end of condition
\nif x == 5<\/code> \u2192 if x == 5:<\/code><\/td>\n<\/tr>\n\n| Improper indentation<\/td>\n | Use consistent 4-space indentation
\nprint(\"Hello\")<\/code> \u2192 \u00a0\u00a0\u00a0\u00a0print(\"Hello\")<\/code><\/td>\n<\/tr>\n\nUsing =<\/code> instead of ==<\/code><\/td>\n=<\/code> is assignment, ==<\/code> is comparison
\nif x = 5:<\/code> \u2192 if x == 5:<\/code><\/td>\n<\/tr>\n\nForgetting to cast input()<\/code> to int<\/code><\/td>\nUse int(input())<\/code> if comparing numbers
\nage = input()<\/code> \u2192 age = int(input())<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nQuick Quiz<\/strong><\/p>\nWhat will this print?<\/p>\n x = 10<\/em><\/p>\nif x > 20:<\/em>
\nprint(“A”)<\/em>
\nelif x > 5:<\/em>
\nprint(“B”)<\/em>
\nelse:<\/em>
\nprint(“C”)<\/em><\/p>\nAnswer: “B”<\/p>\n Summary Table:<\/strong><\/p>\n\n\n\nStatement<\/strong><\/th>\nPurpose<\/strong><\/th>\n<\/tr>\n<\/thead>\n\n\nif<\/code><\/td>\n| Starts a conditional check<\/td>\n<\/tr>\n | \nelif<\/code><\/td>\nChecks another condition if the previous if<\/code> or elif<\/code> fails<\/td>\n<\/tr>\n\nelse<\/code><\/td>\nRuns if no if<\/code> or elif<\/code> conditions are met<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nTruthy and falsy values<\/h2>\nIn Python, every value has an inherent Boolean value \u2014 either True or False \u2014 even if it’s not literally the TrueorFalse` Boolean type.<\/p>\n These values are evaluated when used in conditions, such as in if statements.<\/p>\n What are “Truthy” and “Falsy”?<\/h3>\n\n- Truthy: A value that evaluates to True when used in a Boolean context.<\/li>\n
- Falsy: A value that evaluates to False.<\/li>\n<\/ul>\n
Example:<\/strong><\/p>\nif “hello”:<\/em>
\nprint(“This is truthy.”)<\/em><\/p>\nOutput:<\/strong> This is truthy.<\/p>\n\n- Because a non-empty string (“hello”) is considered truthy.<\/li>\n<\/ul>\n
Falsy Values in Python<\/strong><\/p>\nThese values are considered False in Boolean contexts:<\/p>\n \n\n\nType<\/strong><\/th>\nFalsy Value<\/strong><\/th>\n<\/tr>\n<\/thead>\n\n\n| Boolean<\/td>\n | False<\/code><\/td>\n<\/tr>\n\n| NoneType<\/td>\n | None<\/code><\/td>\n<\/tr>\n\n| Numeric<\/td>\n | 0<\/code>, 0.0<\/code>, 0j<\/code><\/td>\n<\/tr>\n\n| Sequence\/Collection<\/td>\n | ''<\/code> (empty string),
\n[]<\/code> (empty list),
\n()<\/code> (empty tuple),
\n{}<\/code> (empty dict),
\nset()<\/code> (empty set)<\/td>\n<\/tr>\n\n| Custom object<\/td>\n | Any object with __bool__()<\/code> or __len__()<\/code> returning False<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n# All of these evaluate as False<\/em>
\nif not “”:<\/em>
\nprint(“Empty string is falsy”)<\/em><\/p>\nif not 0:<\/em>
\nprint(“Zero is falsy”)<\/em><\/p>\nif not None:<\/em>
\nprint(“None is falsy”)<\/em><\/p>\nif not []:<\/em>
\nprint(“Empty list is falsy”)<\/em><\/p>\nTruthy Values in Python<\/strong><\/p>\nEverything else not listed as falsy is truthy.<\/p>\n Examples:<\/strong><\/p>\n“hello” # non-empty string<\/em>
\n42 # any non-zero number<\/em>
\n[1, 2, 3] # non-empty list<\/em>
\n{“a”: 1} # non-empty dictionary<\/em>
\nTrue # obviously!<\/em><\/p>\nThese will all make if statements run:<\/strong><\/p>\nif [1, 2, 3]:<\/em>
\nprint(“Truthy list!”)<\/em><\/p>\nif “Python”:<\/em>
\nprint(“Truthy string!”)<\/em><\/p>\nBoolean Context Examples:<\/strong><\/p>\nx = []<\/em><\/p>\nif x:<\/em>
\nprint(“List has items.”)<\/em>
\nelse:<\/em>
\nprint(“List is empty.”)<\/em><\/p>\nOutput:<\/strong> List is empty.<\/p>\nUsing bool() to Check Value Truthiness<\/strong><\/p>\nYou can test how Python evaluates any value using bool():<\/p>\n print(bool(0)) # False<\/em>
\nprint(bool(“Hi”)) # True<\/em>
\nprint(bool([])) # False<\/em>
\nprint(bool([0])) # True (non-empty list!)<\/em><\/p>\nSummary:<\/strong><\/p>\n\n\n\nValue Type<\/strong><\/th>\nTruthy Example<\/strong><\/th>\nFalsy Example<\/strong><\/th>\n<\/tr>\n<\/thead>\n\n\n| String<\/td>\n | \"Python\"<\/code><\/td>\n\"\"<\/code><\/td>\n<\/tr>\n\n| Number<\/td>\n | 42<\/code><\/td>\n0<\/code><\/td>\n<\/tr>\n\n| List<\/td>\n | [1, 2]<\/code><\/td>\n[]<\/code><\/td>\n<\/tr>\n\n| Dictionary<\/td>\n | {\"a\": 1}<\/code><\/td>\n{}<\/code><\/td>\n<\/tr>\n\n| Boolean<\/td>\n | True<\/code><\/td>\nFalse<\/code><\/td>\n<\/tr>\n | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |