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SECTION 02 / 31
Create the dictionary before assigning entries
Start with d = {}. This declares an empty dictionary. Then d[7] = 76 creates a key 7 and stores 76. d[21] = 74 creates a second entry. Square brackets after the dictionary name contain a KEY. Curly braces display the dictionary itself.
![A separate illustrative update: assigning d[7] = 80 replaces its earlier 76. This does not change the original course record.](/images/computational-thinking/week-7/replace-value.png)
REAL COURSE DATA · KEYS, COUNTS AND CONNECTIONS
Create, replace and accumulate
Follow the dictionary as its keys and values change. Predict each lookup or update before revealing the next step.
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Predict the next key, whether it already exists, and the value that will be stored.
d = {}
d[7] = 76
d[21] = 74
answer = d[21] # 74
# Separate illustrative update, not a correction to the dataset:
d[7] = 80 # replace the previous value for key 7Assignment has two cases: a missing key creates an entry; a present key gets a replacement value. It does not keep two entries with the same key. To keep multiple values, deliberately store a list or another dictionary as the one value.
Try it yourself
After d={} then d[7]=76 then d[7]=80, how many entries exist?
Show explained answer
One entry: {7:80}.
Try it yourself
What is wrong with trying d[7]=76 before creating d?
Show explained answer
The variable has not been initialized as a dictionary. Initialize d={} first.
Try it yourself
Does d[7]=80 add 80 to 76?
Show explained answer
No. Assignment replaces 76 with 80. Addition would require d[7]=d[7]+80.