Flow of Control

2.1 Core concepts

• Flow of control is defined as the order of execution of statements in a program. It can be implemented using control structures. Python supports two types of control structures: selection and repetition. The default execution order (statement by statement from top to bottom) is called sequence. (NCERT §6.1, p. 121)
• Selection (if..else) is used when a decision must be made between two or more alternative paths. The concept of selection is implemented using the if..else statement in Python. (NCERT §6.2, p. 122)
• The syntax of the simple if statement is: if condition: followed by an indented block of statement(s). If the condition is true, the indented statements execute; if false, they are skipped. (NCERT §6.2, p. 123)
• The if..else statement allows two alternative blocks: the if block executes when the condition is true, and the else block executes when the condition is false. (NCERT §6.2, p. 123)
• The if..elif..else chain (where elif means else..if) is used when multiple conditions must be checked in sequence. As soon as one condition is true, its indented block executes and the rest of the if statement terminates. The number of elif clauses depends on the number of conditions. (NCERT §6.2, p. 124)
• A nested if is an if..else placed inside another if or elif block. Python allows many levels of such nesting. (NCERT §6.2, p. 126)
• Indentation in Python refers to the leading whitespace (spaces or tabs) at the beginning of a statement. The same level of indentation associates statements into a single block of code. Python's interpreter checks indentation strictly and raises a syntax error if it is incorrect. The common practice is to use a single tab for each level. Unlike most other languages, Python does not use curly brackets for blocks. (NCERT §6.3, p. 126)
• Repetition (also called iteration) means executing a set of statements repeatedly. This is implemented using looping constructs. Python provides two looping constructs: for and while. The condition is checked based on the value of a control variable. When the condition becomes false, the loop terminates. The programmer must ensure an exit condition exists; otherwise an infinite loop results. (NCERT §6.4, p. 127–128)
• The for loop iterates over a range of values or a sequence (numeric values, characters of a string, elements of a list or tuple). It is used when the number of iterations is known in advance. Syntax: for <control-variable> in <sequence/range>: followed by the indented body. When all items in the range are exhausted, control transfers to the statement after the loop. (NCERT §6.4.1, p. 128)
• The range() function is a built-in Python function with syntax range([start], stop[, step]). It generates a sequence of integers from start up to (but not including) stop, incrementing by step. Default start is 0; default step is 1. step can be a positive or negative integer but not zero. All parameters must be integers. (NCERT §6.4.1(B), p. 130)
• The while loop executes its body repeatedly as long as the control condition is true. The condition is evaluated before each iteration. If the condition is initially false, the body is not executed even once. The body must contain statements that eventually make the condition false to avoid an infinite loop. (NCERT §6.4.2, p. 131)
• The break statement immediately exits (terminates) the current loop and resumes execution at the statement following the loop body. It alters the normal flow of execution inside a loop. (NCERT §6.5.1, p. 133)
• The continue statement skips the remaining statements of the current iteration and jumps back to the beginning of the loop for the next iteration. Unlike break, continue does not exit the loop — it only skips the rest of the current pass. (NCERT §6.5.2, p. 135)
• Nested loops: A loop contained within another loop is called a nested loop. Python imposes no restriction on how many loops can be nested or on levels of nesting. Any combination of for and while loops can be nested. For each single iteration of the outer loop, the inner loop runs through its complete cycle. (NCERT §6.6, p. 136–137)

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 6.2 (p. 122): Flowchart depicting decision making — shows a diamond (decision) shape with num1 > num2?; the "Yes" branch computes diff = num1 - num2, the "No" branch computes diff = num2 - num1. Illustrates the if..else structure visually.
• Figure 6.4 (p. 128): Flowchart of the for loop — Start → Initialisation → Test Expression → (True) Body of For Loop → back to Test Expression; (False) → Exit for Loop → Statement following the loop → Stop.
• Figure 6.5 (p. 131): Flowchart of the while loop — Start → Initialisation → Test Expression → (True) Body of while Loop → back to Test Expression; (False) → Statements following while loop → Stop.
• Figure 6.5 / break flowchart (p. 133): Flowchart showing break — within the loop body, if the break condition is encountered, control immediately exits the loop to the statement following it.
• Figure 6.6 (p. 135): Flowchart of continue — when continue is encountered, control jumps back to the loop condition check (not to the statement after the loop), skipping remaining statements of that iteration.

2.4 Common confusions / NTA trap points

• break vs continue: Students confuse the two. break exits the loop entirely; continue only skips the rest of the current iteration and re-checks the loop condition. NTA frequently presents code snippets and asks what is printed — identifying whether break or continue is present is critical.
• range() stop value is exclusive: range(1, 6) produces 1, 2, 3, 4, 5 — NOT 6. NTA constructs distractors that include the stop value in the output. Also, range(10) starts at 0, not 1.
• while loop with initially-false condition: If the condition is false from the start, the body executes zero times. NTA asks "how many times is the body executed?" for such cases.
• elif vs else: elif checks a new condition; else catches all remaining cases. Using elif condition without a final else means nothing executes if all conditions are false — NTA tests this with output-tracing questions.
• Indentation errors (NCERT §6.3, p. 126). A statement with wrong indentation belongs to a different block. NTA may show code where a print is at the wrong indent level inside a loop or if, and ask for the output — the answer changes entirely based on indentation.
• range() step cannot be zero (NCERT §6.4.1(B), p. 130). range(0, 5, 0) raises ValueError. NTA distractor may claim step=0 produces an infinite sequence.
• for loops do NOT need to know iteration count at compile time (NCERT §6.4.1, p. 128). They iterate over any sequence; the programmer must know the iteration count, but Python figures it out at runtime.
• break only exits the innermost loop (NCERT §6.5.1, p. 133). In nested loops, break does not exit all enclosing loops. NTA traps assume break "exits all loops".
• else clause on loops is outside the NCERT syllabus here. Stick to NCERT — for..else / while..else are not covered. Avoid that in CUET answers.
• Negative step requires start > stop (NCERT §6.4.1(B), p. 130). range(0, 9, -1) yields an empty sequence; range(9, 0, -1) yields 9, 8, ..., 1.
• Iteration variable persists after the loop (NCERT §6.4.1). After for i in range(5):, i is 4 outside the loop body. NTA may use this in output-tracing.