Lists and loops: work through many prices

Prices come in runs: a close for every day, a row for every trade. A list holds a run of values under one name, in order, and a loop does the same piece of work on each one.

Positions in a list count from zero, so the first close is closes[0]. A negative position counts back from the end, so closes[-1] is the last. A slice such as closes[:3] takes a part of the list.

There are two loops. A for loop visits every value and is the one you will use most. A while loop repeats until a test stops being true, which suits the question in the script: how many days passed before something happened? The closing prices are invented.

The code

# Lists and loops: keep many values under one name and do the same thing to each.
# No broker, no account, no internet. The closing prices are invented.

closes = [101.2, 102.8, 102.1, 103.9, 105.0, 104.4]

print("How many:", len(closes))
print("First:", closes[0], "Last:", closes[-1])
print("The first three:", closes[:3])
print("The last two:", closes[-2:])

# A for loop visits each value in turn.
highest = closes[0]
for close in closes:
    if close > highest:
        highest = close
print("Highest:", highest, "and max() agrees:", max(closes))

# range() gives positions, so each close can be compared with the one before it.
changes = []
for i in range(1, len(closes)):
    changes.append(round(closes[i] - closes[i - 1], 2))
print("Day to day changes:", changes)

# enumerate() gives the position and the value together.
for day, change in enumerate(changes, start=2):
    word = "up" if change > 0 else "down"
    print("Day", day, word, abs(change))

# A while loop repeats until its test is False.
up_days = 0
position = 0
while position < len(changes) and changes[position] > 0:
    up_days += 1
    position += 1
print("Up days before the first fall:", up_days)

# A list can grow.
closes.append(106.3)
print("After append:", len(closes), "values, the last is", closes[-1])

What you should see

How many: 6
First: 101.2 Last: 104.4
The first three: [101.2, 102.8, 102.1]
The last two: [105.0, 104.4]
Highest: 105.0 and max() agrees: 105.0
Day to day changes: [1.6, -0.7, 1.8, 1.1, -0.6]
Day 2 up 1.6
Day 3 down 0.7
Day 4 up 1.8
Day 5 up 1.1
Day 6 down 0.6
Up days before the first fall: 1
After append: 7 values, the last is 106.3

Run with Python 3.12 on Windows. No network and no account.

Check yourself

Answer each question from the code above before you open it. The answers were written from the script on this page and what it printed.

Exercise

Add a for loop that counts the days on which the close fell, and print the count as Down days. Put it just before the line that appends a value.

How many: 6
First: 101.2 Last: 104.4
The first three: [101.2, 102.8, 102.1]
The last two: [105.0, 104.4]
Highest: 105.0 and max() agrees: 105.0
Day to day changes: [1.6, -0.7, 1.8, 1.1, -0.6]
Day 2 up 1.6
Day 3 down 0.7
Day 4 up 1.8
Day 5 up 1.1
Day 6 down 0.6
Up days before the first fall: 1
Down days: 2
After append: 7 values, the last is 106.3

# Exercise solution: the lesson script with the one change the exercise asks for.
# Lists and loops: keep many values under one name and do the same thing to each.
# No broker, no account, no internet. The closing prices are invented.

closes = [101.2, 102.8, 102.1, 103.9, 105.0, 104.4]

print("How many:", len(closes))
print("First:", closes[0], "Last:", closes[-1])
print("The first three:", closes[:3])
print("The last two:", closes[-2:])

# A for loop visits each value in turn.
highest = closes[0]
for close in closes:
    if close > highest:
        highest = close
print("Highest:", highest, "and max() agrees:", max(closes))

# range() gives positions, so each close can be compared with the one before it.
changes = []
for i in range(1, len(closes)):
    changes.append(round(closes[i] - closes[i - 1], 2))
print("Day to day changes:", changes)

# enumerate() gives the position and the value together.
for day, change in enumerate(changes, start=2):
    word = "up" if change > 0 else "down"
    print("Day", day, word, abs(change))

# A while loop repeats until its test is False.
up_days = 0
position = 0
while position < len(changes) and changes[position] > 0:
    up_days += 1
    position += 1
print("Up days before the first fall:", up_days)

down_days = 0
for change in changes:
    if change < 0:
        down_days += 1
print("Down days:", down_days)

# A list can grow.
closes.append(106.3)
print("After append:", len(closes), "values, the last is", closes[-1])

Your progress

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Where this sits in the path

Stage 1 of 4: First steps. Values, decisions, loops and functions, and the habit of testing what you wrote. No files and no account.

1. 1First Python script
2. 2Variables and types
3. 3If and comparisons
4. 4Lists and loopsYou are here
5. 5Functions and sizing
6. 6Test your functions

The whole path, every stage