Printing string pointers in c

Question

So, essentially I have two files:

File 1:

//
//  main.c
//  frederickterry
//
//  Created by Rick Terry on 1/15/15.
//  Copyright (c) 2015 Rick Terry. All rights reserved.
//

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int size (char *g) {
    int ofs = 0;

    while (*(g+ofs) != '\0') {
        ++ofs;
    }

    return ofs;
}

int parse(char *g) {
    // Setup
    char binaryConnective;
    int negated = 0;

    // Looking for propositions
    int fmlaLength = size(g);
    if(fmlaLength == 0) {
        return 1;
    }


    if(fmlaLength == 1) {
        if(g[0] == 'p') {
            return 1;
        } else if (g[0] == 'q') {
            return 1;
        } else if (g[0] == 'r') {
            return 1;
        } else {
            return 0;
        }
    }
    // Now looking for negated preposition

    if(fmlaLength == 2) {
        char temp[100];
        strcpy(temp, g);
        if(g[0] == '-') {
            negated = 1;
            int negatedprop = parse(g+1);
            if(negatedprop == 1) {
                return 2;
            }
        }
    }

    // Checking if Binary Formula
    char arrayleft[50];
    char arrayright[50];
    char *left = "";
    char *right = "";
    int numLeft = 0;
    int numRight = 0;
    int bclocation = 0;
    int binarypresent = 0;



    if(fmlaLength != 1 && fmlaLength != 2) {
        if(g[0] == '-') {
            int negatedBinary = parse(g+1);
            if(negatedBinary == 1 || negatedBinary == 2 || negatedBinary == 3) {
                return 2;
            } else {
                return 0;
            }
        }
        int i = 0;
        int l = 0;
        int p = strlen(g);
        for(l = 0; l < strlen(g)/2; l++) {
            if(g[l] == '(' && g[p-l-1] == ')') {
                i++;
            }
        }


        for(int q = i; q < strlen(g); q++) {
            if(g[q] == '(') {
                numLeft++;
            } else if(g[q] == ')') {
                numRight++;
            }
            arrayleft[q] = g[q];

            //printf("%c", arrayleft[i]);
            //printf("%s", left);


            if((numRight == numLeft) && (g[q+1] == 'v' || g[q+1] == '>' || g[q+1] == '^')) {
                arrayleft[q+1] = '\0';
                bclocation = q+1;
                binaryConnective = g[q+1];
                binarypresent = 1;
                //                    printf("The binary connecive is: %c\n", binaryConnective);
                break;
            }

        }
        if(binarypresent == 0) {
            return 0;
        }

        int j = 0;
        for(int i = bclocation+1; i < strlen(g)-1; i++) {
            arrayright[j] = g[i];
            j++;
        }

        arrayright[j] = '\0';

        left = &arrayleft[1];
        right = &arrayright[0];
        //printf("Printed a second time, fmla 1 is: %s", left);
        int parseleft = parse(left);
        //        printf("Parse left result: %d\n", parseleft);
        if(parseleft == 0) {
            return 0;
        }
        int parseright = parse(right);

        if(parseright == 0) {
            return 0;
        }
        //        printf("Parse right result: %d\n", parseleft);
        if(negated == 1) {
            return 2;
        } else {
            return 3;
        }
    }

    return 0;
}

int type(char *g) {
    if(parse(g) == 1 ||parse(g) == 2 || parse(g) == 3) {
        if(parse(g) == 1) {
            return 1;
        }
        /* Literals, Positive and Negative */
        if(parse(g) == 2 && size(g) == 2) {
            return 1;
        }
        /* Double Negations */
        if(g[0] == '-' && g[1] == '-') {
            return 4;
        }
        /* Alpha & Beta Formulas */
        char binaryConnective;
        int numLeft = 0;
        int numRight = 0;
        int bclocation = 0;
        int binarypresent = 0;

        int i = 0;
        if(g[0] == '(') {
            i++;
        }


        if(g[0] == '-') {
            i++;
            if(g[1] == '(') {
                i++;
            }
        }

        for(i; i < strlen(g); ++i) {
            if(g[i] == '(') {
                numLeft++;
            } else if(g[i] == ')') {
                numRight++;
            }

            if(numRight == numLeft) {
                if(g[i+1] == 'v' || g[i+1] == '>' || g[i+1] == '^') {
                    bclocation = i+1;
                    binaryConnective = g[i+1];
                    binarypresent = 1;
                    break;
                }
            }
        }

        /* Connective established */
        if(binaryConnective == '^') {
            if(g[0] == '-') {
                return 3;
            } else {
                return 2;
            }
        } else if(binaryConnective == '>') {
            if(g[0] == '-') {
                return 2;
            } else {
                return 3;
            }
        } else if (binaryConnective == 'v') {
            if(g[0] == '-') {
                return 2;
            } else {
                return 3;
            }
        }
    }
    return 0;
}

char bin(char *g) {
    char binaryConnective;
    char arrayLeft[50];
    int numLeft = 0;
    int numRight = 0;
    int bclocation = 0;

    int i = 0;
    if(g[0] == '(') {
        i++;
    }

    if(g[0] == '-') {
        i++;
        if(g[1] == '(') {
            i++;
        }
    }

    for(i; i < strlen(g); ++i) {
        if(g[i] == '(') {
            numLeft++;
        } else if(g[i] == ')') {
            numRight++;
        }
        int j = 0;
        arrayLeft[j++] = g[i];

        if(numRight == numLeft) {
            if(g[i+1] == 'v' || g[i+1] == '>' || g[i+1] == '^') {
                arrayLeft[i+1] = '\0';
                bclocation = i+1;
                binaryConnective = g[i+1];
                return binaryConnective;
            }
        }
    }
    return binaryConnective;
}

char *partone(char *g) {
    char binaryConnective;
    char arrayLeft[50];
    char arrayRight[50];
    int numLeft = 0;
    int numRight = 0;
    int bclocation = 0;

    int i = 0;
    if(g[0] == '(') {
        i++;
    }

    if(g[0] == '-') {
        i++;
        if(g[1] == '(') {
            i++;
        }
    }
    int j = 0;
    for(i; i < strlen(g); ++i) {
        if(g[i] == '(') {
            numLeft++;
        } else if(g[i] == ')') {
            numRight++;
        }
        arrayLeft[j] = g[i];

        if(numRight == numLeft) {
            if(g[i+1] == 'v' || g[i+1] == '>' || g[i+1] == '^') {
                arrayLeft[j+1] = '\0';
                bclocation = i+1;
                binaryConnective = g[i+1];
                break;
            }
        }
        j++;
    }
    int m = 0;
    for(int k = bclocation+1; k < strlen(g)-1; k++) {
        arrayRight[m] = g[k];
        m++;
    }

    arrayRight[m] = '\0';

    char* leftSide = &arrayLeft[0];
    //    printf("%s\n", leftSide);
    //    printf("%s\n", rightSide);
    int k = 0;
    k++;
    return leftSide;
}

char *parttwo(char *g) {
    char binaryConnective;
    char arrayLeft[50];
    char arrayRight[50];
    int numLeft = 0;
    int numRight = 0;
    int bclocation = 0;

    int i = 0;
    if(g[0] == '(') {
        i++;
    }

    if(g[0] == '-') {
        i++;
        if(g[1] == '(') {
            i++;
        }
    }
    int j = 0;
    for(i; i < strlen(g); ++i) {
        if(g[i] == '(') {
            numLeft++;
        } else if(g[i] == ')') {
            numRight++;
        }
        arrayLeft[j] = g[i];

        if(numRight == numLeft) {
            if(g[i+1] == 'v' || g[i+1] == '>' || g[i+1] == '^') {
                arrayLeft[j+1] = '\0';
                bclocation = i+1;
                binaryConnective = g[i+1];
                break;
            }
        }
        j++;
    }
    int m = 0;
    int n = size(g) - 1;
    if(g[strlen(g)-1] != ')') {
        n++;
    }
    for(int k = bclocation+1; k < n; k++) {
        arrayRight[m] = g[k];
        m++;
    }

    arrayRight[m] = '\0';

    char* leftSide = &arrayLeft[0];
    char* rightSide = &arrayRight[0];
    //    printf("%s\n", leftSide);
    //    printf("%s\n", rightSide);

    return rightSide;
}

char *firstexp(char *g) {
    char* left = partone(g);
    char leftArray[50];
    int i = 0;
    for(i; i < strlen(left); i++) {
        leftArray[i] = left[i];
    }
    leftArray[i] = '\0';
    char binConnective = bin(g);
    int typeG = type(g);
    if(typeG == 2) {
        if(binConnective == '^') {
            return &leftArray;
        } else if(binConnective == '>') {
            return &leftArray;
        }
    } else if(typeG == 3) {
        if(binConnective == 'v')
            return &leftArray;
    }
    char temp[50];
    for(int i = 0; i < strlen(leftArray); i++) {
        temp[i+1] = leftArray[i];
    }
    temp[0] = '-';
    char* lefttwo = &temp[0];

    if(typeG == 2) {
        if(binConnective == 'v') {
            return lefttwo;
        }
    } else if(typeG == 3) {
        if(binConnective == '>' || binConnective == '^') {
            return lefttwo;
        }
    }

    return "Hello";
}

char *secondexp(char *g) {
//    char binaryConnective = bin(g);
//    char* right = parttwo(g);
//    char rightArray[50];
//    int i = 0;
//    for(i; i< strlen(right); i++) {
//        rightArray[i+1] = right[i];
//    }
//    rightArray[i] = '\0';
//    int typeG = type(g);
//    if(type(g) == 2) {
//        if(binaryConnective == '^') {
//            return &rightArray;
//        }
//    } else if(type(g) == 3) {
//        if(binaryConnective == 'v' || binaryConnective == '>') {
//            return &rightArray;
//        }
//    }


        return "Hello";
}

typedef struct tableau tableau;
\

\
struct tableau {

    char *root;

    tableau *left;

    tableau *right;

    tableau *parent;

    int closedbranch;

};


int closed(tableau *t) {
    return 0;
}

void complete(tableau *t) {
}

/*int main(int argc, const char * argv[])
{
    printf("Hello, World!\n");
    printf("%d \n", parse("p^q"));
    printf("%d \n", type("p^q"));
    printf("%c \n", bin("p^q"));
    printf("%s\n", partone("p^q"));
    printf("%s\n", parttwo("p^q"));
    printf("%s\n", firstexp("p^q"));
    printf("Simulation complete");
    return 0;
}*/

File 2:

#include <stdio.h>
#include <string.h>   /* for all the new-fangled string functions */
#include <stdlib.h>     /* malloc, free, rand */
#include "yourfile.h"

int Fsize = 50;

int  main()

{ /*input a string and check if its a propositional formula */
    char *name = malloc(Fsize);
    printf("Enter a formula:");
    scanf("%s", name);
    int p=parse(name);
    switch(p)
    {case(0): printf("not a formula");break;
        case(1): printf("a proposition");break;
        case(2): printf("a negated formula");break;
        case(3): printf("a binary formula");break;
        default: printf("what the f***!");
    }
    printf("\n");
    if (p==3)
    {
        printf("the first part is %s and the second part is %s", partone(name), parttwo(name));
        printf(" the binary connective is %c \n", bin(name));
    }


    int t =type(name);
    switch(t)
    {case(0):printf("I told you, not a formula");break;
        case(1): printf("A literal");break;
        case(2): printf("An alpha formula, ");break;
        case(3): printf("A beta formula, ");break;
        case(4): printf("Double negation");break;
        default: printf("SOmewthing's wrong");
    }
    if(t==2) printf("first expansion fmla is %s, second expansion fmla is %s\n", firstexp(name), secondexp(name));
    if(t==3) printf("first expansion fmla is %s, second expansion fmla is %s\n", firstexp(name), secondexp(name));




    tableau tab;
    tab.root = name;
    tab.left=0;
    tab.parent=0;
    tab.right=0;
    tab.closedbranch=0;


    complete(&tab);/*expand the root node then recursively expand any child nodes */
    if (closed(&tab)) printf("%s is not satisfiable", name);
    else printf("%s is satisfiable", name);

    return(0);
}

If you look at the first file, you'll see a method called * firstexp(char * g).

This method runs perfectly, but only if another method called * secondexp(char * g) is commented out.

If * secondexp(char * g) is commented out, then *firstexp runs like this:

Enter a formula:((pvq)>-p)
a binary formula
the first part is (pvq) and the second part is -p the binary connective is > 
A beta formula, first expansion fmla is -(pvq), second expansion fmla is Hello
((pvq)>-p) is satisfiableProgram ended with exit code: 0

otherwise, if *secondexp is not commented out, it runs like this:

Enter a formula:((pvq)>-p)
a binary formula
the first part is (pvq) and the second part is -p the binary connective is > 
A beta formula, first expansion fmla is \240L, second expansion fmla is (-
((pvq)>-p) is satisfiable. Program ended with exit code: 0

As you can see, the outputs are completely different despite the same input. Can someone explain what's going on here?

Answer 1

In your parttwo() function you return the address of a local variable

return rightSide;

where rightSide is a pointer to a local variable.

It appears that your compiler gave you a warning about this which you solved by making a pointer to the local variabe arrayRight, that may confuse the compiler but the result will be the same, the data in arrayRight will no longer exist after the function returns.

You are doing the same all over your code, and even worse, in the secondexp() function you return a the address of a local variable taking it's address, you are not only returning the address to a local variabel, but also with a type that is not compatible with the return type of the function.

This is one of many probable issues that your code may have, but you need to start fixing that to continue with other possible problems.

Note: enable extra warnings when compiler and listen to them, don't try to fool the compiler unless you know exactly what you're doing.

Answer 2

In the commented-out parts of secondexp and in parttwo, you return the address of a local variable, which you shouldn't do.

You seem to fill a lot of ad-hoc sized auxiliary arrays. These have the problem that they might overflow for larger expressions and also that you cannot return them unless you allocate them on the heap with malloc, which also means that you have to free them later.

At first glance, the strings you want to return are substrings or slices of the expression string. That means that the data for these strings is already there.

You could (safely) return pointers into that string. That is what, for example strchr and strstr do. If you are willing to modify the original string, you could also place null terminators '\0' after substrings. That's what strtok does, and it has the disadvantage that you lose the information at that place: If you string is a*b and you modify it to a\0b, you will not know which operator there was.

Another method is to create a struct that stores a slice as pointer into the string and a length:

struct slice {
    const char *p;
    int length;
};

You can then safely return slices of the original string without needing to worry about additional memory.

You can also use the standard functions in most cases, if you stick to the strn variants. When you print a slice, you can do so by specifying a field width in printf formats:

printf("Second part: '%.*s'\n", s->length, s->p);