Lists & Arrays

One number was a field; a few flags were bools; many things in order is a LIST — the data structure behind your card deck, your playtest rows, and the grid jam's board. This is the reference for the whole list family, AP pseudocode and C# side by side.

The vocabulary, both languages

AP pseudocodeC# on a ListWhat it does
aList[i]list[i]Access index i. AP counts from 1; C# counts from 0 — the exam's favorite off-by-one
APPEND(aList, item)list.Add(item)Add at the end
REMOVE(aList, i)list.RemoveAt(i)Remove at index (or Remove(item))
INSERT(aList, i, item)list.Insert(i, item)Squeeze into position i, shifting the rest right
LENGTH(aList)list.CountHow many items
FOR EACH item IN aListforeach (var item in list)Visit every item, no index needed
FOR i FROM 1 TO LENGTHfor (int i = 0; i < list.Count; i++)Visit every INDEX — use when position matters

Array vs List — same idea, one difference that matters

An array has a fixed size set at creation (int[] scores = new int[10];); a List grows and shrinks on demand. Underneath, a List IS an array that copies itself into a bigger one when full — which is why adding is usually instant and occasionally a little slow. Choose: a hand of changing size wants a List; a fixed 8×8 board can be either (the grid jam uses a 2D array, 12.1b). The exam uses "list" for both and only tests operations, never capacity management.

The four patterns you will write forever

1. Traverse to total

AP:                                C#:
total ← 0                          int total = 0;
FOR EACH item IN aList             foreach (int item in list)
    total ← total + item                total += item;

2. Traverse with a filter (selection inside iteration)

AP:                                C#:
result ← []                        var result = new List<int>();
FOR EACH item IN aList             foreach (int item in list)
    IF (item > threshold)             if (item > threshold)
        APPEND(result, item)               result.Add(item);

3. Find the best (the AI score loop)

AP:                                C#:
best ← aList[1]                    int best = int.MinValue;
FOR EACH item IN aList             foreach (int item in list)
    IF (item > best)                   if (item > best)
        best ← item                       best = item;

This is the card AI's whole personality (10.2d): score every option, keep the winner. "Best" starts at an impossible value so the first real item always replaces it.

4. Swap (the shuffle's spine)

AP:                                C#:
temp ← aList[i]                    var temp = list[i];
aList[i] ← aList[r]                list[i] = list[r];
aList[r] ← temp                    list[r] = temp;

Fisher-Yates shuffles the whole deck from exactly this: walk from the end, pick a random earlier-or-equal index, swap. Every card gets one fair chance at every slot (10.2c).

Two bugs, one rule

Bug 1 — remove while traversing forward: deleting item i shifts every later item left, and your loop's next step skips one entirely. Fix: traverse BACKWARD (from the end to the start) so the shift happens behind you: 

for (int i = list.Count - 1; i >= 0; i--)
{
    if (list[i].cost > 5) list.RemoveAt(i);   // safe: shifts are behind
}

Bug 2 — the bounds: the last item is at index Count − 1, not Count. Accessing list[Count] is the classic IndexOutOfRange crash. The exam version: a list of 30 students has valid indices 1–30 in AP and 0–29 in C#. Write both numbers when counting matters — you have seen this trap in every MC set (11.4a).

Lists of lists — the grid

// 2D array — the grid jam's board
int[,] grid = new int[8, 8];
grid[row, col] = 1;                 // ROW first — always read aloud

// list-of-lists — the AP shape
List<List<int>> grid2 = new List<List<int>>();
grid2[r][c]                          // same convention, row first

Rows before columns is pure convention, but the convention is everywhere; on a square board a swap is invisible, so test on a rectangle (12.1b). Symmetry on a grid is one expression: mirrorCol = W − 1 − col.

Where you meet lists this year

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