Lesson 4: Program Control Structures

By default, statements execute in sequence. Control structures change this:

• Selection (if, if-else, switch) — choose which code to execute based on a condition.
• Repetition (while, for, do) — repeat code while a condition holds.

Relational and Equality Operators

<   less than          >   greater than
<=  less than/equal    >=  greater than/equal
==  equal to           !=  not equal to

Logical Operators

!   NOT (unary)
&&  AND (binary)
||  OR (binary)

In C: false = 0, true = any non-zero value.

Operator Associativity

+ - ++ -- !       right to left
* / %             left to right
+ -               left to right
< <= > >=         left to right
== !=             left to right
&&                left to right
||                left to right
= += -= *= /=     right to left

The if Statement

if (condition)
    statement;

If the condition is true (non-zero), the statement executes; otherwise skipped.

The if-else Statement

if (condition)
    statement1;
else
    statement2;

Nested if-else:

if (x == 50) {
    if (y >= 120) {
        sum = x + y;
        printf("Sum: %d", sum);
    } else {
        diff = x - y;
        printf("Difference: %d", diff);
    }
} else {
    printf("Next time");
}

The switch Statement

switch (expression) {
    case constant1:
        statement;
        break;
    case constant2:
        statement;
        break;
    default:
        statement;
}

• switch expression must evaluate to int or char (not float or string).
• case values must be constants.
• break exits the switch; without it, execution "falls through" to the next case.
• default handles unmatched values.

Grouping cases (intentional fall-through):

switch (QUIZ) {
    case 10:
    case 9:  printf("A"); break;   // 9 or 10 → A
    case 8:  printf("B"); break;
    case 7:  printf("C"); break;
    default: printf("F");
}

break

Two uses:

1. Exits a switch after a matching case.
2. Immediately exits the innermost loop.

while (1) {
    scanf("%lf", &x);
    if (x < 0.0)
        break;
    printf("%f\n", sqrt(x));
}

continue

Skips the rest of the current loop iteration and jumps to the next iteration.

do {
    scanf("%d", &num);
    if (num < 0)
        continue;
    printf("%d", num);
} while (num != 100);

while Loop

Condition checked before each iteration. If false initially, body never runs.

while (condition)
    statement;

// Example:
while (number != 0) {
    scanf("%d", &number);
    sum += number;
}

for Loop

for (initialization; condition; increment)
    statement;

1. Initialization — runs once before the loop.
2. Condition — checked before each iteration.
3. Increment — runs after each iteration.

for (x = 100; x != 65; x += 5) {
    z = sqrt(x);
    printf("Square root of %d is %f", x, z);
}

Multiple variables (comma operator):

for (x = 0, y = 0; x + y < 10; x++) { ... }

do-while Loop

Body runs at least once — condition checked after each iteration.

do {
    statement;
} while (condition);

// Example:
do {
    printf("Enter a number: ");
    scanf("%d", &a);
    sum = sum + a;
} while (a != 0);
printf("The sum is %d", sum);

Tracing a for loop step by step:

• In C a condition is false when it is 0 and true when it is any non-zero value.
• if and if-else choose between paths. switch jumps to the case that matches an int or char value, and without break it falls through into the next case.
• A while loop checks its condition before each pass, so it can run zero times. A do-while loop checks after, so its body always runs at least once.
• A for loop runs its initialization once, checks the condition before every pass, and runs the increment after every pass.
• break leaves the loop (or switch) immediately; continue skips the rest of this pass and goes to the next one.

Change the grade value and see which branch runs. Then modify the loop limits.