Error in case construct with string comparison (scheme)
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(define aaa ;;val:string
(lambda(x) ;;x:string
(case
((string=? (substring x 0 1) "+") (aaa(substring x 1)))
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
((char=?(string-ref x 0)#.) (-404))
(else
(if (= (findpoint x) -1)
"a"
"b"
)
)
)
)
)
Hello, I have a problem with DrRacket:
When I try to run this code, it gives me the error:
case: expected a symbol (without its quote) or a number as a choice, but found a string
referring to the line (5):
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
This line was actually supposed to look like this,
((string=? (substring x 0 1) "+")(string-append("-" (aaa(substring x 1)))))
but I thought that using two strings "a" "b" would ease to spot the problem.
I don't understand why i get this error, since both "a" "b" are strings and not symbols or lists, and also I can't understand why I don't get this error on the previous line
CONTEXT:
the procedure is supposed check if the first character of a string is a +/-/., and then do thing through recursion using else (again, "a" "b" are examples)
scheme
asked Nov 26 '18 at 18:34
ArzoArzo
86
add a comment |
(define aaa ;;val:string
(lambda(x) ;;x:string
(case
((string=? (substring x 0 1) "+") (aaa(substring x 1)))
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
((char=?(string-ref x 0)#.) (-404))
(else
(if (= (findpoint x) -1)
"a"
"b"
)
)
)
)
)
Hello, I have a problem with DrRacket:
When I try to run this code, it gives me the error:
case: expected a symbol (without its quote) or a number as a choice, but found a string
referring to the line (5):
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
This line was actually supposed to look like this,
((string=? (substring x 0 1) "+")(string-append("-" (aaa(substring x 1)))))
but I thought that using two strings "a" "b" would ease to spot the problem.
I don't understand why i get this error, since both "a" "b" are strings and not symbols or lists, and also I can't understand why I don't get this error on the previous line
CONTEXT:
the procedure is supposed check if the first character of a string is a +/-/., and then do thing through recursion using else (again, "a" "b" are examples)
scheme
asked Nov 26 '18 at 18:34
ArzoArzo
86
add a comment |
(define aaa ;;val:string
(lambda(x) ;;x:string
(case
((string=? (substring x 0 1) "+") (aaa(substring x 1)))
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
((char=?(string-ref x 0)#.) (-404))
(else
(if (= (findpoint x) -1)
"a"
"b"
)
)
)
)
)
Hello, I have a problem with DrRacket:
When I try to run this code, it gives me the error:
case: expected a symbol (without its quote) or a number as a choice, but found a string
referring to the line (5):
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
This line was actually supposed to look like this,
((string=? (substring x 0 1) "+")(string-append("-" (aaa(substring x 1)))))
but I thought that using two strings "a" "b" would ease to spot the problem.
I don't understand why i get this error, since both "a" "b" are strings and not symbols or lists, and also I can't understand why I don't get this error on the previous line
CONTEXT:
the procedure is supposed check if the first character of a string is a +/-/., and then do thing through recursion using else (again, "a" "b" are examples)
scheme
asked Nov 26 '18 at 18:34
ArzoArzo
86
(define aaa ;;val:string
(lambda(x) ;;x:string
(case
((string=? (substring x 0 1) "+") (aaa(substring x 1)))
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
((char=?(string-ref x 0)#.) (-404))
(else
(if (= (findpoint x) -1)
"a"
"b"
)
)
)
)
)
Hello, I have a problem with DrRacket:
When I try to run this code, it gives me the error:
case: expected a symbol (without its quote) or a number as a choice, but found a string
referring to the line (5):
((string=? "a" "b")(string-append("-" (aaa(substring x 1)))))
This line was actually supposed to look like this,
((string=? (substring x 0 1) "+")(string-append("-" (aaa(substring x 1)))))
but I thought that using two strings "a" "b" would ease to spot the problem.
I don't understand why i get this error, since both "a" "b" are strings and not symbols or lists, and also I can't understand why I don't get this error on the previous line
CONTEXT:
the procedure is supposed check if the first character of a string is a +/-/., and then do thing through recursion using else (again, "a" "b" are examples)
scheme
scheme
asked Nov 26 '18 at 18:34
ArzoArzo
86
asked Nov 26 '18 at 18:34
ArzoArzo
86
asked Nov 26 '18 at 18:34
ArzoArzo
86
asked Nov 26 '18 at 18:34
ArzoArzo
86
asked Nov 26 '18 at 18:34
ArzoArzo
86
86
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
It seems like you are mixing cond, the flat if-elseif-else, and case, which is similar to a switch statement.
How to use cond:
(cond ((equal? 1 2) 'consequent)
((odd? 1) 'consequent2) ; as many terms as you want. It stops at the first true
(else 'alternative)) ; or else it evaluates the alterntaive
; ==> consequent2
Vs how to use case
(case 'value4
((value1 value2) 'consequent)
((value3 value4) 'consequent2)
(else 'default))
; ==> consequent2
Now that case statement is just sugar. Your Scheme implementation will make it into something similar to this:
(cond ((and (eqv? 'value4 'value1) (eqv? 'value4 'value2)) 'consequent)
((and (eqv? 'value4 'value3) (eqv? 'value4 'value4)) 'consequent2)
(else 'default))
Thus notice the values to match in a case are treated as if they are quoted. Eg. you values can not be variables since they will only match their symbol.
If you want to use case I would have done this:
(define (first-letter-operator? str)
(case (string-ref str 0)
((#+ #- #.) #t)
(else #f)))
(first-letter-operator? "+345634") ; ==> #t
(first-letter-operator? "hello") ; ==> #f
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
It seems like you are mixing cond, the flat if-elseif-else, and case, which is similar to a switch statement.
How to use cond:
(cond ((equal? 1 2) 'consequent)
((odd? 1) 'consequent2) ; as many terms as you want. It stops at the first true
(else 'alternative)) ; or else it evaluates the alterntaive
; ==> consequent2
Vs how to use case
(case 'value4
((value1 value2) 'consequent)
((value3 value4) 'consequent2)
(else 'default))
; ==> consequent2
Now that case statement is just sugar. Your Scheme implementation will make it into something similar to this:
(cond ((and (eqv? 'value4 'value1) (eqv? 'value4 'value2)) 'consequent)
((and (eqv? 'value4 'value3) (eqv? 'value4 'value4)) 'consequent2)
(else 'default))
Thus notice the values to match in a case are treated as if they are quoted. Eg. you values can not be variables since they will only match their symbol.
If you want to use case I would have done this:
(define (first-letter-operator? str)
(case (string-ref str 0)
((#+ #- #.) #t)
(else #f)))
(first-letter-operator? "+345634") ; ==> #t
(first-letter-operator? "hello") ; ==> #f
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
add a comment |
It seems like you are mixing cond, the flat if-elseif-else, and case, which is similar to a switch statement.
How to use cond:
(cond ((equal? 1 2) 'consequent)
((odd? 1) 'consequent2) ; as many terms as you want. It stops at the first true
(else 'alternative)) ; or else it evaluates the alterntaive
; ==> consequent2
Vs how to use case
(case 'value4
((value1 value2) 'consequent)
((value3 value4) 'consequent2)
(else 'default))
; ==> consequent2
Now that case statement is just sugar. Your Scheme implementation will make it into something similar to this:
(cond ((and (eqv? 'value4 'value1) (eqv? 'value4 'value2)) 'consequent)
((and (eqv? 'value4 'value3) (eqv? 'value4 'value4)) 'consequent2)
(else 'default))
Thus notice the values to match in a case are treated as if they are quoted. Eg. you values can not be variables since they will only match their symbol.
If you want to use case I would have done this:
(define (first-letter-operator? str)
(case (string-ref str 0)
((#+ #- #.) #t)
(else #f)))
(first-letter-operator? "+345634") ; ==> #t
(first-letter-operator? "hello") ; ==> #f
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
add a comment |
It seems like you are mixing cond, the flat if-elseif-else, and case, which is similar to a switch statement.
How to use cond:
(cond ((equal? 1 2) 'consequent)
((odd? 1) 'consequent2) ; as many terms as you want. It stops at the first true
(else 'alternative)) ; or else it evaluates the alterntaive
; ==> consequent2
Vs how to use case
(case 'value4
((value1 value2) 'consequent)
((value3 value4) 'consequent2)
(else 'default))
; ==> consequent2
Now that case statement is just sugar. Your Scheme implementation will make it into something similar to this:
(cond ((and (eqv? 'value4 'value1) (eqv? 'value4 'value2)) 'consequent)
((and (eqv? 'value4 'value3) (eqv? 'value4 'value4)) 'consequent2)
(else 'default))
Thus notice the values to match in a case are treated as if they are quoted. Eg. you values can not be variables since they will only match their symbol.
If you want to use case I would have done this:
(define (first-letter-operator? str)
(case (string-ref str 0)
((#+ #- #.) #t)
(else #f)))
(first-letter-operator? "+345634") ; ==> #t
(first-letter-operator? "hello") ; ==> #f
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
It seems like you are mixing cond, the flat if-elseif-else, and case, which is similar to a switch statement.
How to use cond:
(cond ((equal? 1 2) 'consequent)
((odd? 1) 'consequent2) ; as many terms as you want. It stops at the first true
(else 'alternative)) ; or else it evaluates the alterntaive
; ==> consequent2
Vs how to use case
(case 'value4
((value1 value2) 'consequent)
((value3 value4) 'consequent2)
(else 'default))
; ==> consequent2
Now that case statement is just sugar. Your Scheme implementation will make it into something similar to this:
(cond ((and (eqv? 'value4 'value1) (eqv? 'value4 'value2)) 'consequent)
((and (eqv? 'value4 'value3) (eqv? 'value4 'value4)) 'consequent2)
(else 'default))
Thus notice the values to match in a case are treated as if they are quoted. Eg. you values can not be variables since they will only match their symbol.
If you want to use case I would have done this:
(define (first-letter-operator? str)
(case (string-ref str 0)
((#+ #- #.) #t)
(else #f)))
(first-letter-operator? "+345634") ; ==> #t
(first-letter-operator? "hello") ; ==> #f
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
edited Nov 26 '18 at 19:20
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
answered Nov 26 '18 at 19:15
SylwesterSylwester
35.4k22956
35.4k22956
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
add a comment |
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
Wow, I mixed up the two things, and i didn't notice... Sorry for the waste of time and thanks for the help!
– Arzo
Nov 26 '18 at 20:01
add a comment |
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