有限狀態機 - Finite-state machine
其中又可分為Mealy機和Moore機

所謂的有限狀態機對於工科的孩子來說大概最直觀的就是真值表吧(是嗎？)
當狀態為A條件為A時將指到狀態B

條件/狀態 | 狀態A | 狀態B
------------- | -------------
條件A | 狀態B | ...
條件B | ... | 狀態B
其中又以下圖為例

這邊以一個實際案例來模擬有限狀態機的行為

輸入一串隨意文字 以空白為間隔 以.為結束
以rate R2D2 48 2 time 5566.為例
rate會判為identifier
R2D2會判為identifier
48會判為number
2會判為number
time會判為identifier
5566會判為number
2t會判為Error
t#會判為Error

而實際轉換成C是將各個條件包成Function

int build_num(){
    char c;
    int state=1;
    do{
        scanf("%c",&c);
        if(c>='0' && c<='9'){
            printf("%c",c);
            state=1;
        }
        else if(c==' '){
            return state;
        }
        else{
            printf("%c",c);
            state=build_invalid();
            return 3;
        }
    }while(c!=' ');
return state;
}

int build_invalid(){
    char c;
    int state=3;
    do{
        scanf("%c",&c);
        if(c!=' '){
            printf("%c",c);
        }
    }while(c!=' ');
return state;
}

int build_id(){
    char c;
    int state=2;
    do{
        scanf("%c",&c);
        if(c>='a'&&c<='z'|| c>='A'&&c<='Z'){
            printf("%c",c);
        }
        else if(c>='0'&&c<='9'){
            printf("%c",c);
        }
        else if(c=='_'){
            printf("%c",c);
        }
        else if(c==' '){
            return 2;
        }
        else{
            printf("%c",c);
            build_invalid();
            return 3;
        }
    }while(c!=' ');
return state;
}

int main(){
    char c;
    int state=0;
    do{
        scanf("%c",&c);
        if(c>='0' && c<='9'){
            printf("%c",c);
            state=build_num();
            if(state == 1){
                printf(" - Number\n");
            }
            else if(state == 3){
                printf(" - Invalid\n");
            }
        }
        else if(c>='a' && c<='z' || c>='A' && c<='Z'){
            printf("%c",c);
            state=build_id();
            if(state == 2){
                printf(" - Identifier\n");
            }
            else if(state == 3){
                printf(" - Invalid\n");
            }
        }

    }while(c!='.');


}