Decisions: make the program choose the right path

The goal is 30 minutes, and you study for exactly 30. Should the program say “Goal met”? The equality case will tell us whether the condition matches the rule.

Predict a goal check

We want the message Goal met. whenever completed study minutes are at least the goal. Read the program and predict both output lines. The two numbers are equal on purpose: a boundary often reveals whether a condition matches the requirement.

using System;

int completedMinutes = 30;
int goalMinutes = 30;
bool reachedGoal = completedMinutes >= goalMinutes;

if (reachedGoal)
{
    Console.WriteLine("Goal met.");
}
else
{
    Console.WriteLine("Keep going.");
}

Console.WriteLine("Check complete.");

A condition is a yes/no question

A comparison produces a bool result. Use > for greater than, < for less than, >= for at least, <= for at most, == for equal, and != for not equal. The single equals sign assigns a value; the double equals sign compares values. Parentheses enclose the condition in an if statement.

completedMinutes >= goalMinutes ?
          /                 \
       true                 false
    Goal met.             Keep going.
          \                 /
             Check complete.

Braces mark the statements belonging to each block. Indentation makes the structure easier to read, but braces determine its boundaries. A statement after the whole if/else runs after either branch completes normally. This is why Check complete. is printed for both a successful goal and an unfinished goal.

Test the boundary, not only one happy example

Keep goalMinutes at 30. Predict the first message for completedMinutes values 29, 30 and 31 before running each edit. Expect Keep going., Goal met., and Goal met. respectively. A condition that works for 31 can still be wrong at 30.

Add a third case

For planned time, our rule has three cases: negative is invalid, zero means no plan yet, and positive means a plan exists. An else if adds another condition. Conditions are checked in order; the first matching branch runs, and the remaining branches in that chain are skipped.

Write your attempt before reading this complete solution. Start with plannedMinutes set to 0:

using System;

int plannedMinutes = 0;

if (plannedMinutes < 0)
{
    Console.WriteLine("Minutes cannot be negative.");
}
else if (plannedMinutes == 0)
{
    Console.WriteLine("No study time planned yet.");
}
else
{
    Console.WriteLine("Study time is planned.");
}

Change the value to -1 and then 5. Expect the negative warning and the planned-time message, respectively. Keep these three values in a tiny test table beside your code. When a program has more branches, a table helps you see which paths you have actually tried.

Explain back: What is different about = and ==? Why do 29, 30 and 31 make a better goal test set than 31 alone?

Reference: Microsoft — selection statements.