### Open Source Your Knowledge, Become a Contributor

Technology knowledge has to be shared and made accessible for free. Join the movement.

## Print stack first value

Let's add a useful, non-mathematical, unary operator: print. It would print stack's head as a decimal integer. The new code will be

- Print (letter P, code 80)

Note that this operator will not consume the first stacked value : it will print it bu value remains unchanged

## Let's start

- Memory: empty
- Cursor: first cell
- Input: space separated integers and operators

## Process

- Leave an empty space (current number on stack is null)
- While there is a char to read
- subtract 32 and set else flag
- if char is null : it's space
- reset else flag, move to the right

- subtract 10 (total 42), if char is null : it's multiply
- reset else flag, multiply the 2 operands

- subtract 1 (total 43), if char is null : it's addition
- reset else flag, sum the 2 operands

- subtract 2 (total 45), if char is null : it's subtraction
- reset else flag, subtract the 2 operands

- subtract 35 (total 80), if char is null : it's print
- reset else flag, print first operand

- otherwise
**add 32**(to get the digit value)- add current number 10 times to current digit to get new current number

- loop

## Code

```
>,[ leave empty space and read char
let's track our target values here
space 32
multiply 42
add 43
subtract 45
print 80
digit 48 to 57
>++++[-<-------->]+< subtract 32
[---------- subtract 10
[- subtract 1
[-- subtract 2
[>++++++[-<----->]+< subtract 35
[ it's a digit
>+++[-<++++++++>]< add 32 to get digit value (and reset else flag)
<[->++++++++++<] add current number 10 times to current digit
>[-<+>] replace current number by current digit
]>[- now unwind cases: it's print
<<<[->+>+<<]>[-<+>] copy number (to remain value unchanged after print)
>[>>>>++++++++++<<< print copied value
<[->+>>+>-[<-]<[->> as a decimal number
+<<<<[->>>+<<<]>]<< ** part 3 **
]>+[-<+>]>>>[-]>[-< ** part 4 **
<<<+>>>>]<<<<]<[>++ ** part 5 **
++++[<++++++++>-]<- ** part 6 **
.[-]<] ** part 7 **
++++++++++.[-]> print newline (and reset newline char)
]<]>[- next case: it's subtraction
<<<[-<->]> do the subtraction
]<]>[- next case : it's add
<<<[-<+>]> do the addition
]<]>[- next case : it's multiply
<<<[->>+<<]>>[-<<< do the multiplication
[->+>+<<]>[-<+>]>> ** part 2 **
]<<<[-]>>[-<<+>>] ** part 3 **
]<]>[- next case : it's space
> move to next stack entry
] end of switch/case
<,] read char and loop
```

## Final state

- Memory: stack of integers
- Cursor: after stack
- Input: empty (read)
- Output: unchanged

## Test program

The program can be tested on its own.

You can try with input `6 8 * P 7 7 * P 1 P 7 7 * P + P`

for example. It should print 48, 49, 1, 49 and 50

Suggested playgrounds

Open Source Your Knowledge: become a Contributor and help others learn. Create New Content