a. To give an unambiguous CFG for the language {w in every prefix of w the number of a's is at least the number of bs), we can use the following rules: S → aSb | A, A → aA | ε. Here, S is the start symbol, aSb generates words where the number of a's is greater than or equal to the number of b's, and.
A generates words where the number of a's is equal to the number of b's. The rule A → ε is necessary to ensure that words in which a and b occur in equal numbers are also generated.
b. For the language {w the number of a's and the number of b's in w are equal), we can use the rule S → AB, A → aA | ε, and B → bB | ε. Here, S is the start symbol, A generates words with an equal number of a's and b's, and B generates words with an equal number of b's and a's. Using these rules, we can generate any word in which the number of a's is equal to the number of b's.
c. To give an unambiguous CFG for the language {w the number of a's is at least the number of b's in w), we can use the following rules: S → aSbS | aS | ε. Here, S is the start symbol, and aSbS generates words in which the number of a's is greater than the number of b's, aS generates words in which the number of a's is equal to the number of b's, and ε generates the empty string. Using these rules, we can generate any word in which the number of a's is at least the number of b's.
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The unambiguous context-free grammars (CFGs) for the given languages:
a. {w in every prefix of w the number of a's is at least the number of b's}
S -> aSb | A
A -> ε | SaA
The start symbol S generates strings where each prefix has at least as many a's as b's. The production S -> aSb generates a string with one more a and b than its right-hand side. The production A -> ε generates the empty string, and A -> SaA generates a string with an equal number of a's and b's.
b. {w the number of a's and the number of b's in w are equal}
rust
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S -> aSb | bSa | ε
The start symbol S generates strings where the number of a's and b's are equal. The production S -> aSb adds an a and b in each step, and S -> bSa adds a b and a in each step. The production S -> ε generates the empty string.
c. {w the number of a's is at least the number of b's in w}
rust
Copy code
S -> aSb | aA | ε
A -> aA | bA | ε
The start symbol S generates strings where the number of a's is at least the number of b's. The production S -> aSb adds an a and a b to the string in each step, and S -> aA adds an a to the string. The non-terminal A generates a string with any number of a's followed by any number of b's. The production A -> aA adds an a to the string, A -> bA adds a b to the string, and A -> ε generates the empty string.
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