A character from a string

A string is read by address too: a character index instead of a slot number, a character code instead of a value

mediumYou need: The memory cell, Print Char

The third thing read can read is an ordinary string. The address here is a character index, and the answer is its code, a number:

read code "mlog" 0

code gets 109 — that is m. Not a letter, a number: mlog has no separate “character” kind at all.

steptick 0

Read0

=

Print Char1

char 

Operation2

=

+

Jump -> 03

if <

Print Flush4

Stop5

@counter0number

codenullnull

indexnullnull

The program walks the string character by character and puts them back together with printchar. The board shows mlog — the same string, only assembled one character at a time.

Why, if a string cannot even be joined

Strings in mlog are almost immovable: they cannot be concatenated, compared or trimmed. The only things you can do with one are print it whole and take it apart character by character. Hence the uses:

• print text letter by letter — a crawling line on a board, a letter per iteration;
• break a string into codes and store them in memory, to reassemble them in another order later;
• fetch a single character out of a prepared set: read code "0123456789" digit turns a number into the code of its digit.

Note

The other side of it is printchar: it takes a code and appends a character to the buffer. read takes apart, printchar puts together, and both work in numbers. In detail in the lesson “Print Char”.

The bounds are the same as memory’s

Address	What happens
0 … length minus one	the character’s code
length and above	emptiness
negative	emptiness
fractional	truncated: 2.9 is character 2

There is no way to find out a string’s length — the language has no “how many characters” instruction. So a loop over a string either knows the length in advance or runs until the first emptiness:

read code "mlog" index
jump 5 strictEqual code null
printchar code
op add index index 1
jump 0 always
printflush message1

Note

An ordinary equal code 0 would work here too — emptiness equals zero in it — but it says something else: “the code equals zero”. A strict comparison (=== in the block, strictEqual in the text) asks exactly what is needed: “is there a character at all”.

Remember

• read on a string gives the character’s code by index, not the character itself.
• The addresses behave like memory’s: from zero, fractional truncated, emptiness past the end.
• A string’s length cannot be asked for: a loop runs until the first emptiness or knows it in advance.
• printchar puts the characters back together.

Task

Change the string "mlog" in the example to "logic" and the number in the jump from 4 to 5: the board shows logic. Any letters read the same way — non-Latin ones just have bigger codes.

Then put "mlog" back and leave 5 in the jump: one extra character that is not in the string goes into the buffer. Nothing breaks — printchar passes emptiness by in silence, and the line stays as it was.