الدرس 04 · Basics

Control flow: if, else and while

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A program runs its lines one after another, top to bottom, until a control-flow statement changes the order. Follow a twelve-line countdown as an arrow marks the line that runs: a while loop repeats its body while n > 0 is true, an if/else picks one of two lines to print on each pass, and the loop stops when its condition turns false.

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01n holds 3. Is the condition n > 0 true or false, and what does a while loop do with the answer?

3 > 0 is true. A while loop tests its condition before every pass and runs the body each time the answer is true.

02In if (n == 1) ... else ..., n holds 2. What runs?

2 == 1 is false, so the line under the if is skipped and the else part runs. An if/else always runs exactly one of its two paths.

03What is the difference between = and == in C++?

n = n - 1; stores a new value in n. n == 1 asks whether n equals 1 and gives a bool, true or false.

04What would happen if the line n = n - 1; were removed from the countdown?

A loop stops only when its condition turns false. Without a line that changes n, 3 > 0 stays true forever: an infinite loop that prints 3 again and again.

05The countdown prints 3, 2, last and Go!. How many times does the while condition n > 0 get tested?

It is tested before each of the three passes (n = 3, 2 and 1, all true) and once more when n is 0. That last test is false, which ends the loop.

المصطلحات الأساسية

المسرد

Control flow: The order in which a program's statements run. By default top to bottom; if, else and loops change it.
Condition: An expression whose answer is true or false, such as n > 0 or n == 1. if and while use one to decide what runs next.
bool: The C++ type with exactly two values, true and false. Comparisons such as > and == give a bool.
if / else: A choice: if the condition is true, the first statement runs; otherwise the else statement runs. Exactly one of them runs.
while loop: A loop that tests its condition before every pass and runs its body again and again while the condition is true.
Loop body: The statements inside a loop's { }, run once on every pass.
Iteration: One pass through a loop's body. The countdown's loop makes three iterations.
Infinite loop: A loop whose condition never becomes false, so it never stops. Usually a bug, often a forgotten update like n = n - 1.

تفاصيل إضافية

الملاحظات والمصادر

An if or else without braces controls only the one statement after it, as on lines 6 and 8. Many style guides suggest always writing braces { } so that adding a second line later cannot slip outside the if.
= stores a value (assignment); == compares two values and gives true or false. Writing if (n = 1) by mistake stores 1 in n; compilers warn about it with -Wall.
Reaching the closing } of main without a return statement is the same as return 0;, so this program ends with exit status 0 (success). This rule applies only to main.
The same countdown can be written as a for loop, which keeps the start, the test and the step on one line: for (int n = 3; n > 0; n = n - 1) { ... }. C++ also has do-while, switch, break and continue.
The program on the card is real, standard C++. Build it with g++ -std=c++20 -Wall -Wextra countdown.cpp -o countdown (or clang++), run ./countdown and it prints 3, 2, last and Go! on four lines.

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