Thanks you On Tuesday, January 2, 2018 at 6:09:23 PM UTC, gneuner2 wrote: > > On 1/2/2018 12:43 PM, Sanjeeb Sinkhada wrote: > > hi could anyone help me with this question? > > Write a function that will take a list of pairs and return just the > > pair comprising the first element of the first pair and the second > > element of the last pair. > > Eg. > > > (reduce-pairs ’()) > > NIL > > > (reduce-pairs ’((1 2) (3 4))) > > (1 4) > > > (reduce-pairs ’((1 2) (3 4) (5 6))) > > (1 6) > > > > I have got this so far > > (define reduce-pair(λ(x y)(cond > > ((pair? x)x) > > (cons( car x) > > (cons ( list-tail (second y))))))) > > but its completely wrong > > thanks in advance > > Yup, it definitely is wrong. <grin> This smells like homework and the > homework policy in this forum is to help but not answer directly. > > > First, your examples have a single list argument, but your function is > defined to have 2 arguments. Why? > Second, what should happen if the list contains only 1 pair or contains > elements that are not pairs? > Third, are you supposed to solve the problem by walking the list, or can > you use any pre-defined list functions? > > Think about how you would get the first and last elements of the list. > Then think about how you get the first and second values of those > pairs. Then think about how to make a new pair from those values. If > you can use any predefined function, then you're at least half way there > already ... you just need to put it together. > > George >
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