What does this mean: K12_global(1:3:6,1:3:6)?

K12 = K12_global(1:3:6,1:3:6) 

I don't know how to interpret that indexing in matrices.

[Dyuman Joshi]

What exactly do you not know how to interpret?

Did you try searching on this forum for similar questions as mentioned in the 1st point of the help section?

Try dissecting the code into parts and see what each part does.

Answer 1

[Torsten]

Do you see what happens ?

The new matrix K12 is formed of the elements (1,1),(1+3,1),(1,1+3) and (1+3,1+3) of the original matrix K12_global.

K12_global = rand(9)

K12_global = 9×9

0.0194 0.0262 0.2509 0.2650 0.1267 0.3926 0.6566 0.5265 0.6897 0.7206 0.6288 0.2764 0.5182 0.3594 0.8476 0.4010 0.3064 0.1114 0.2628 0.2054 0.4492 0.5123 0.5923 0.5039 0.5320 0.6925 0.4534 0.4825 0.9864 0.0153 0.6897 0.4487 0.9993 0.0325 0.1075 0.2257 0.5787 0.9988 0.9926 0.4080 0.5705 0.6924 0.1036 0.6637 0.5976 0.0143 0.0776 0.9375 0.1070 0.0775 0.7704 0.9926 0.9436 0.0468 0.6609 0.6836 0.3231 0.5390 0.2477 0.8082 0.4072 0.4094 0.8814 0.7498 0.5859 0.3176 0.0981 0.0715 0.2600 0.2737 0.4692 0.8073 0.4474 0.8309 0.9656 0.1444 0.9107 0.9487 0.6329 0.2216 0.2917

K12 = K12_global(1:3:6,1:3:6)

K12 = 2×2

0.0194 0.2650 0.4825 0.6897

Answer 2

[Image Analyst]

Another example

indexes = 1 : 3 : 6

indexes = 1×2

1 4

The next index would be 7 since 4 + 3 = 7, but since it was told to stop at 6, the 7 does not get included.

K12_global  = reshape(1:49, 7, 7)

K12_global = 7×7

1 8 15 22 29 36 43 2 9 16 23 30 37 44 3 10 17 24 31 38 45 4 11 18 25 32 39 46 5 12 19 26 33 40 47 6 13 20 27 34 41 48 7 14 21 28 35 42 49

K12 = K12_global(1:3:6,1:3:6)

K12 = 2×2

1 22 4 25

Basically you can see it gets the intersection of the row indexes (1 & 4) with the column indexes (1 & 4).