' integer maths by TassyJim 07 Feb 2012
a$ = "13"
For i = 0 To 15 ' demo of the four operations
add( a$ , Str$( i ),res$ )
Print a$;" + ";i;" = ";res$
min( a$ , Str$( i ),res$ )
Print a$;" - ";i;" = ";res$
multy( a$ , Str$( i ),res$ )
Print a$;" * ";i;" = ";res$
div( a$ , Str$( i ),res$ )
Print a$;" / ";i;" = ";res$
Print
Next i

a$="1"
For i = 1 To 55 ' printing up to factorial 55.
b$=Str$(i)
multy(a$, b$, res$)
Print b$;"! = ";res$;" (";len(res$);" )"
a$=res$
Next i

End

Sub div( a$ , b$, c$ ) ' given a$ and b$, returns a/b in c$
Local f, i, d, t, try$, nr$, a1$, b1$, a2$, z$
a1$=a$ : b1$=b$ : c$="" ' make copies of a$ and b$ to preserve originals
If Val( b$ ) = 0 Then ' divide by zero
c$ = "error"
Else
For i = Len( a1$ ) - Len( b1$ ) To 0 Step -1
f = 0
nr$ = "0"
z$=String$(i,"0")
For d = 0 To 9
md( b1$ + z$ , d, try$ )
big( a1$ , try$, t )
If t Then
f = d
nr$ = try$
EndIf
Next d
c$ = c$ + Str$( f )
If f > 0 Then
min( a1$ , nr$, a2$ )
a1$ = a2$
EndIf
Next i
EndIf
trim (c$)
End Sub

Sub multy( a$ , b$, c$ ) ' given a$ and b$, returns the sum in c$
Local i, h$, a1$, b1$, t$, d$, z$
a1$=a$ : b1$=b$ : c$="" ' make copies of a$ and b$ to preserve originals
If Len( b1$ ) > Len( a1$ ) Then ' swap number for greater speed
h$ = a1$
a1$ = b1$
b1$ = h$
EndIf
For i = Len( a1$ ) To 1 Step -1
z$=String$(Len( a1$ )-i,"0")
md( b1$ , Val( Mid$( a1$ , i , 1 ) ) , t$)
add( c$ , t$+ z$ ,d$)
c$=d$
Next i
trim (c$)
End Sub

Sub md( a$ , nr, c$ ) ' multiply a$ by a single digit, result in c$
Local hold, carry, i
carry = 0 : c$=""
For i = Len( a$ ) To 1 Step -1
hold = Val( Mid$( a$ , i , 1 ) ) * nr + carry
carry = Int( hold / 10 )
c$ = Str$( hold Mod 10 ) + c$
Next i
If carry > 0 Then c$ = Str$( carry ) + c$
End Sub

Sub samelen (a$, b$) ' pads the shorter variable with leading zeros
dif=Abs(Len( a$ )-Len( b$ ))
If dif>0 Then
z$=String$(dif,"0")
If Len( a$ ) < Len( b$ ) Then
a$ = z$+a$
Else
b$ = z$+b$
EndIf
EndIf
End Sub

Sub add( a$ , b$, c$ ) ' given a$ and b$, returns the sum in c$
Local carry, hold, i,a1$, b1$
c$="" : a1$=a$ : b1$=b$
samelen a1$ , b1$
For i = Len( a1$ ) To 1 Step -1
hold = Val( Mid$( a1$ , i , 1 ) ) + Val( Mid$( b1$ , i , 1 ) ) + carry
carry = Int( hold / 10 )
x$=Str$( hold Mod 10 )
c$ = Right$( x$,1) + c$
Next i
If carry > 0 Then c$ = Str$( carry ) + c$
End Sub

Sub min( a$ , b$, c$ ) ' given a$ and b$, returns the difference in c$
Local a1$, b1$, h$, i, hold, borrow
c$="" : s$="" :a1$=a$ : b1$=b$ ' initialise variables and preserve a$ and b$
samelen a1$ , b1$
If a1$<b1$ Then ' if result is going to be negative, swap a1 & b1
h$ = a1$
a1$ = b1$
b1$ = h$
s$="-"
EndIf
For i = Len( a1$ ) To 1 Step -1
hold = Val( Mid$( a1$ , i , 1 ) ) - Val( Mid$( b1$ , i , 1 ) ) + 10 - borrow
borrow = 1- Int( hold / 10 )
c$ = Str$( hold Mod 10 ) + c$
Next i
trim c$
c$ = s$+c$ ' add the sign
End Sub

Sub big( a$, b$, biggest ) ' biggest = 1 if a$ >= b$
Local a1$, b1$ 'preserve a$ and b$
a1$ = a$ : b1$ = b$
biggest = 0
samelen (a1$ , b1$)
If a1$>=b1$ Then biggest = 1
End Sub

Sub trim (a$) ' given a$, leading zeros are stripped. a$ is altered.
Local s$
s$=""
If Left$(a$,1)="-" Then ' if the number is negative, preserve the sign
s$="-"
a$=Mid$(a$,2)
EndIf
Do While Left$(a$,1)="0" ' strip leading characters while they are "0"
a$=Mid$(a$,2)
Loop
If a$="" Then a$="0"
a$=s$+a$ ' replace the sign
End Sub