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101-12: lists

This is lesson 101-12 of CU-101.2, checked against cint 0.2.0.

You will keep a week of numbers under one name, reach each one by its position and see what cint does when a position does not exist.

the program

void main() {
    I64[7] temps = [12, 15, 11, 18, 21, 19, 16];
    "there are {len(temps)} days\n";
    "Monday was {temps[0]}, Sunday was {temps[6]}\n";
    temps[2] = 13;
    "Wednesday, corrected: {temps[2]}\n";
    "the day after Sunday: {temps[7]}\n";
}

Run this program in ced

Run it:

there are 7 days
Monday was 12, Sunday was 16
Wednesday, corrected: 13
fault[E_BOUNDS]: index out of bounds
  --> temps.ci:7:34
   |
 7 |     "the day after Sunday: {temps[7]}\n";
   |                                  ^ index.checked.i64
   |
   = operation: index.checked.i64
   = operand-count: 1
   = operand: I64 7
   = exact:   none
   = limit:   I64 7
   = revision: 9eb95af35c842e3f063ac00a7cb32e13cd838a461e5eb04b6d46c5eaf40c8c4b
   = source-map: none
   = address: none
   = stack-depth: 0
   = fuel-consumed: 1 (not part of the fault record)
   = state: the program stopped; stdout holds what it wrote before the fault

what happened

A list holds several numbers under one name. I64[7] temps makes a list of seven I64 numbers called temps. In cint a list of a fixed length is called an array. The [12, 15, 11, 18, 21, 19, 16] after the = gives the seven numbers to start with, in order: a week of noon temperatures in degrees Celsius, from Monday to Sunday.

Positions count from 0. temps[0] is the first number, 12, and temps[6] is the seventh and last, 16. The number in the square brackets is called the index. Other programming languages, such as C and Python, count positions from 0 too. An array of 7 has the indexes 0 to 6.

len gives the length. len(temps) is 7. A loop over 0..len(temps) visits every index, 0 to 6, as lesson 101-07 explained for ...

One number can change. temps[2] = 13; replaces the third number, Wednesday's 11, with 13. The rest stay as they were.

There is no eighth day. temps[7] asks for an index the array does not have. cint stopped the program with an index out of bounds fault at line 7, at temps[7]. The report says the index was 7 and the limit is 7, meaning the index must be less than 7. The three lines before it printed, and nothing after it did.

change it

1. Print the average temperature of the week with a loop that adds up every day.
2. Find the warmest day and print its index. You need a variable that remembers the best so far, as highest did in lesson 101-08.
3. Delete the last print line, so the program finishes, then print all seven days, one to a line, with a loop.

    I64 total = 0;
    for day in 0..len(temps) {
        total += temps[day];
    }
    "average: {total / len(temps)}\n";

when it goes wrong

The index can come from a variable, and cint checks it the same way when the program runs:

void main() {
    I64[7] temps = [12, 15, 11, 18, 21, 19, 16];
    I64 day = 7;
    "the day after Sunday: {temps[day]}\n";
}

Run this program in ced

fault[E_BOUNDS]: index out of bounds
  --> index.ci:4:34
   |
 4 |     "the day after Sunday: {temps[day]}\n";
   |                                  ^ index.checked.i64
   |
   = operation: index.checked.i64
   = operand-count: 1
   = operand: I64 7
   ...

In C, reading past the end of an array is undefined behavior: the program can quietly give back whatever number is stored next to it, or crash. In cint, every index is checked.

The starting numbers must match the length. I64[7] temps = [12, 15]; gives two numbers for seven places, and cint refuses it with error C2011. In cint 0.2.0 an array can only be made inside a function, so this lesson's program starts from main, as lesson 101-10 explained.

new words

array, index, out of bounds

Back to 101: first programs.