Control Structures

CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. The three ways instructions can flow through a program.

Flowchart Shape & Pseudocode for Each Structure

Structure	Flowchart Shape(s)	Pseudocode Form
Sequence	Rectangles connected top-to-bottom by plain arrows, no branching	`step1`
`step2`
`step3`
Selection (if-else)	Diamond (the test), with two outgoing arrows labelled Yes/No leading to separate rectangles	`IF cond THEN ... ELSE ... ENDIF`
Iteration (loop)	Diamond (the test) with a rectangle for the body, and an arrow looping back from the body to the diamond	`WHILE cond DO ... ENDWHILE`

If-else flowchart (textual): Decision: cond? (diamond) → Yes → Statement A (rectangle) → join | No → Statement B (rectangle) → join

While-loop flowchart (textual): Decision: cond? (diamond) → Yes → Loop body (rectangle) → arrow back up to the Decision | No → exit the loop, continue after it

Do-while flowchart (textual): Loop body (rectangle) runs first → Decision: cond? (diamond) → Yes → arrow back up to the body | No → exit — note the body sits before the diamond, which is exactly why it always runs at least once.

Sequential Structure

Statements execute strictly in the order they are written, top to bottom, one after another — the default control flow with no branching

Selection (Branching) Structures

if: executes a block only if a condition is true
if-else: executes one block if true, another if false
else if ladder: chains multiple conditions checked in order
switch-case: selects one branch among many based on the value of an expression

switch (grade) {
    case 'A': printf("Excellent"); break;
    case 'B': printf("Good"); break;
    default: printf("Needs improvement");
}

Without break, execution falls through to the next case regardless of its label — a classic exam trap
default is optional and can appear anywhere, but conventionally goes last

Iteration (Looping) Structures

Loop	Condition Check	Minimum Executions	Typical Use
for	Before each iteration	0	Known/fixed number of iterations
while	Before each iteration (pre-test)	0	Unknown iteration count, condition-driven
do-while	After each iteration (post-test)	1	Body must run at least once (e.g. menus)

for (int i = 0; i < 5; i++) { ... }   // init; condition; update

while (n > 0) { ... }                  // checks BEFORE running body

do { ... } while (n > 0);              // checks AFTER running body — runs at least once

Flowchart Shape & Pseudocode for Each Structure

Sequential Structure

Selection (Branching) Structures

Iteration (Looping) Structures

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