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Annotation of 43BSD/contrib/icon/samples/prefix.icn, revision 1.1.1.1

1.1       root        1: #
                      2: #          I N F I X - T O - P R E F I X   C O N V E R S I O N
                      3: #
                      4: 
                      5: #  This program accepts infix expressions from standard input and
                      6: #  writes the corresponding prefix expressions to standard output.
                      7: 
                      8: procedure main()
                      9:    while write(prefix(read()))
                     10: end
                     11: 
                     12: procedure prefix(s)
                     13:    s := strip(s)
                     14:    return lassoc(s,'+-' | '*/') | rassoc(s,'^') | s
                     15: end
                     16: 
                     17: procedure strip(s)
                     18:    while s ? (="(" & s <- tab(bal(')')) & pos(-1))
                     19:    return s
                     20: end
                     21: 
                     22: procedure lassoc(s,c)
                     23:    local j
                     24:    s ? every j := bal(c)
                     25:    return form(s,\j)
                     26: end
                     27: 
                     28: procedure rassoc(s,c)
                     29:    local j
                     30:    return form(s,s ? bal(c))
                     31: end
                     32: 
                     33: procedure form(s,k)
                     34:    local a1, a2, op
                     35:    s ? {
                     36:       a1 := tab(k)
                     37:       op := move(1)
                     38:       a2 := tab(0)
                     39:       }
                     40:    return op || "(" || prefix(a1) || "," || prefix(a2) || ")"
                     41: end

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