🔀 8. Decision Making

The programs so far have mainly followed one statement after another. To print Pass for one mark and Fail for another, a program must choose a different action depending on the data.

Input / current state
        ↓
     Condition
      /      \\
   true      false
    ↓          ↓
 Action A    Action B
        \\    /
         Continue

When C evaluates a controlling expression such as the expression inside if, zero means false and any nonzero value means true.

int x = -5;

if (x)
{
    puts("x is nonzero");
}

Use if when there is an action that should happen only if a condition is satisfied.

if (temperature > 40)
{
    puts("High temperature");
}

Evaluate temperature > 40
          ↓
       Is it true?
       /       \\
     YES        NO
      ↓          ↓
 print message  skip block
      \          /
       ↓        ↓
       continue after if

• The condition is evaluated first.
• If it is nonzero, the controlled statement/block executes.
• If it is zero, the block is skipped.
• Execution then continues with the statement after the if.

if (n % 2 == 0)
{
    puts("Even");
}
else
{
    puts("Odd");
}

n % 2 == 0
     ↓
  ┌──┴──┐
 true  false
  ↓       ↓
Even     Odd
  └──┬────┘
     ↓
  continue

For one execution of this statement, one branch is selected. The else block is the fallback when the if condition is false.

if (marks >= 90)
    grade = 'A';
else if (marks >= 75)
    grade = 'B';
else if (marks >= 60)
    grade = 'C';
else
    grade = 'D';

marks >= 90 ?  → A
      ↓ no
marks >= 75 ?  → B
      ↓ no
marks >= 60 ?  → C
      ↓ no
     else     → D

• Conditions are evaluated from top to bottom.
• The first true condition selects its branch.
• Once a branch is selected, the remaining conditions are skipped.
• If no condition is true, the final else executes, if present.

if (usernameValid)
{
    if (passwordValid)
    {
        puts("Login allowed");
    }
    else
    {
        puts("Wrong password");
    }
}

Username valid?
      ↓
   ┌──┴──┐
  NO    YES
  ↓       ↓
stop   Password valid?
          ↓
       ┌──┴──┐
      NO    YES
      ↓       ↓
 wrong     allowed

A nested decision checks another condition inside a selected branch. For example, first check whether a user is old enough, then check whether an ID is available.

if (a > 0)
    if (b > 0)
        puts("both positive");
    else
        puts("b is not positive");

Use braces to show exactly which statements belong to each branch:

if (a > 0)
{
    if (b > 0)
        puts("both positive");
}
else
{
    puts("a is not positive");
}

switch is useful when one expression is compared against a set of discrete constant values. Its controlling expression must have an integer type, such as int or char. Each case needs an integer constant expression, and duplicate case values are not allowed. Strings and floating-point values cannot be used directly as switch cases.

switch (choice)
{
    case 1:
        puts("Add");
        break;

    case 2:
        puts("Delete");
        break;

    default:
        puts("Invalid choice");
}

After a matching case is entered, execution continues through following statements until a control-flow transfer such as break occurs or the switch ends.

switch (grade)
{
    case 'A':
    case 'B':
        puts("Pass with strong grade");
        break;

    default:
        puts("Other grade");
}

grade == 'A' ─┐
              ├→ same block → break → exit switch
grade == 'B' ─┘

no match ───────────────→ default

When multiple labels intentionally share one block, fall-through is useful. When it happens accidentally because a break was forgotten, it becomes a bug.

int max = (a > b) ? a : b;

condition: a > b
       ↓
   ┌───┴───┐
 true     false
  ↓          ↓
  a          b
   \        /
    → value ←

The conditional operator is an expression. It evaluates the condition and produces one of two expression results.

goto jumps to a labeled statement within the same function. Execution continues from that label rather than from the next line.

if (resource2_failed)
    goto cleanup_resource1;

/* normal work */

cleanup_resource1:
    release_resource1();

A decision is not complete until you know what happens at the boundary values.

if (marks >= 40)
    puts("Pass");
else
    puts("Fail");

The difference between > 40 and >= 40 changes the result for exactly one important boundary value.