Commentary: Control Flow

Steven Zeil

Last modified: Aug 31, 2017

Contents:

What distinguishes computers from simple calculators? The ability to organize calculations by making decisions about what calculations to perform and to some calculations over and over.

Read chapters 4 & 5, Control Structures, of Malik, if you have not already done so. (If you are working from a different C++ textbook, you will have to figure out the corresponding chapters.)

1 Conditionals and Loops

The concepts here should be very familiar to you. You should already know

what loops and conditional statements are and how they perform in a running program,
what kinds of things we use loops and conditional statement for.

If those kinds of ideas cause you to struggle, you should probably go back and review them, in your original programming language, write some sample programs involving them, until you are more comfortable with the ideas.

That really leaves just the syntax or appearance of these statements in C++ to worry about.

If you are a C programmer

If you are a Java programmer

If you are a Python programmer

This is where you have to come to grips with a fundamental “quirk” of Python. In Python, indentation mattered. You used indentation to indicate the fundamental structure of your code. As a Python programmer, you understand that

if x < 0:
    x = -x
    y  = true

is fundamentally different from

if x < 0:
    x = -x
y  = true

But Python is quite unusual in this respect. C++, like most programming languages, ignores indentation correctly. Most C++ programmers still indent their code, but that’s as an aid to any humans who might read it, not as an instruction to the compiler

For example, the C++ equivalent to the Python code

if x < 0:
    x = -x
    y  = true

would be:

if (x < 0)
{
    x = -x
    y  = true
}

But that could also be written as

if (x < 0)
{
    x = -x;
y  = true;
}

or even as

if (x < 0) { x = -x; y  = true; }

because indentation (and line breaks) are meaningless in C++.