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101-09: choosing among many

This is lesson 101-09 of CU-101.2, checked against cint 0.2.0.

You will sort a number into one of several groups with switch, using ranges of numbers, and make sure every number lands somewhere.

the program

for tens in 4..=10 {
    I64 score = tens * 10;
    "score {score:>3}: ";
    switch (score) {
        case 90..=100: "A\n";
        case 80..=89: "B\n";
        case 70..=79: "C\n";
        case 60..=69: "D\n";
        default: "F\n";
    }
}

Run this program in ced

Run it:

score  40: F
score  50: F
score  60: D
score  70: C
score  80: B
score  90: A
score 100: A

what happened

The loop makes the scores. tens counts from 4 to 10, and score is ten times tens: 40, 50 and so on up to 100. A range in cint 0.2.0 counts by ones only, so a loop that needs steps of ten counts by ones and multiplies.

switch sorts one number into cases. switch (score) looks at score once and runs the lines of the case that holds it. Each case names a number or a range of numbers:

• case 90..=100: holds every score from 90 to 100.
• case 80..=89: holds 80 to 89, case 70..=79: holds 70 to 79 and case 60..=69: holds 60 to 69.
• default: holds every number no other case holds.

Only one case runs. When the lines of a case are done, the switch is done, and the program goes on after its closing brace. Here each case prints one letter and the line ends.

Every number must have a place. A switch without default must have a case for every possible value. With default at the end, every number has somewhere to go.

A switch can do what else if does. This program could be written as a chain of if and else if, as in lesson 101-06. A switch is shorter when every question is about the same number, and cint checks that its ranges do not overlap.

change it

1. Make the loop count from 0 to 10 instead. What do scores 0 to 30 get?
2. Add a grade E for scores from 50 to 59.
3. Give exactly 100 the grade A+. What else must change, and why?

error[C4021]: `switch` rule
  --> aplus.ci:6:14
   |
 6 |         case 90..=100: "A\n";
   |              ^
   |

when it goes wrong

Leave out default, and scores outside 60 to 100 have nowhere to go. cint refuses the program before it runs:

I64 score = 75;
switch (score) {
    case 90..=100: "A\n";
    case 80..=89: "B\n";
    case 70..=79: "C\n";
    case 60..=69: "D\n";
}

Run this program in ced

error[C4020]: `switch` rule
  --> nodefault.ci:2:1
   |
 2 | switch (score) {
   | ^
   |

A range in a case must be written with ..=. case 90..100: is refused with error C4022.

new words

switch, case, default

Next: lesson 101-10, functions.