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