Basics 5: dictionaries (key-value tables)

Lesson: tables of key → value

The modern key-value structure is the dict:

set freqs [dict create sys_clk 500 io_clk 100 ddr_clk 800]

dict get $freqs io_clk        ;# 100
dict set freqs spi_clk 25     ;# add/overwrite (name, no $: it modifies)
dict exists $freqs usb_clk    ;# 0
dict size $freqs              ;# 4

Iteration walks entries in insertion order (that is guaranteed, and this task depends on it):

dict for {port freq} $freqs {
    puts "$port runs at $freq MHz"
}

You will also meet the older array in real tool scripts (set counts(BUF_X1) 7, $counts(BUF_X1), array names counts). Arrays are fine for accumulators (you used one shape of this in the instance-counting interview problem), but they cannot be passed around as values; dicts can. New code: prefer dicts.

This problem is the direct rehearsal for the clock-creation interview question: same table, same loop, same unit conversion, just puts instead of create_clock.

Task

The starter defines:

set freqs [dict create sys_clk 500 io_clk 100 ddr_clk 800]

Loop with dict for and print each clock's period in nanoseconds (1000.0 ÷ megahertz), exactly:

sys_clk: 2.0 ns
io_clk: 10.0 ns
ddr_clk: 1.25 ns

Hint: if you get 2 instead of 2.0, you fell into the integer-division trap from Basics 2.