C比较2个字符串变成内存分配问题

Question

Let me start off by saying, I do realize there are a lot of questions with the exact same title, but I didn't find what I was looking for in any of them. I tried to write the following code, in order to errorcheck the user's input, so he wouldn't give 2 variables the same name. Needless to say, it failed, and that is why I am here. While printing the strings I was comparing out as strings, using printf("%s", temp[j].name); was working fine, the character-by-character printing was outputting a series of characters that, from what I know, shouldn't be there. I would like to know what this could all be about, and if there is anyway to solve it, so I can actually compare the 2, without using something from string.h

Here is the code:

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

#define ARRAYLENGTH 20

typedef struct{
    char name[ARRAYLENGTH];
    char type[ARRAYLENGTH];
    char value[ARRAYLENGTH];
}variable;


int main(){
    int amount = 3;
    int i, j, k;
    variable * varray;
    variable * temp;
    int flag;
    int added = 1;

    varray = malloc(amount*sizeof(variable));
    if (varray == NULL){
        printf("error");
        return 1;
    }
        temp = malloc(amount*sizeof(variable));
    if (temp == NULL){
        printf("error");
        return 1;
    }
    printf("Give the name of variable # 1 \n");
    scanf("%s", varray[0].name);

    for (i = 1; i < amount; i++){
        flag = 0;
        while (flag == 0){
            printf("Give the name of variable # %d \n", i + 1);
            scanf("%s", temp[i].name);

            for (j = 0; j < added; j++){
                for (k = 0; temp[i].name[k] != '\0'; k++){
                    printf("%c,", temp[i].name[k]);
                }
                printf("\n");
                for (k = 0; temp[i].name[k] != '\0'; k++){
                    if (varray[j].name[k] != temp[i].name[k]){
                        flag = 1;
                        break;
                    }
                    if (varray[j].name[k] == temp[i].name[k]){
                        flag = 0;
                    }
                }
            }
            if (flag == 0){
                printf("The variable name you gave already exists, please choose another one. \n");
            }
            if (flag == 1){
                for (j = 0; j < ARRAYLENGTH; j++){
                    varray[i].name[j] = temp[i].name[j];
                }
            }
            if(flag == 1){
                added +=1;
            }
        }
    }
    for (i = 0; i < amount; i++){
        printf("%s \n", varray[i].name);
    }
    free(varray);
    free(temp);
}

The code compiles without problem, but when I tried to run it, I found that, no matter what my, as a user, input was, the flag would always be 1 in the end. The block of code

printf("\n");
for (k = 0; k < ARRAYLENGTH; k++){
    printf("%c,", temp[i].name[k]);
}
printf("\n");

And when the user input is the name John, outputs the following on Visual Studio 2013's Developer command prompt:

Give the name of variable # 1                                                                                                                         
John                                                                                                                                                  
Give the name of variable # 2                                                                                                                         
John                                                                                                                                                  
J,o,h,n,                                                                                                                                              
The variable name you gave already exists, please choose another one.                                                                                 
Give the name of variable # 2                                                                                                                         
George                                                                                                                                                
G,e,o,r,g,e,                                                                                                                                          
Give the name of variable # 3 
George                                                                                                                                                
G,e,o,r,g,e,                                                                                                                                          
G,e,o,r,g,e,                                                                                                                                          
The variable name you gave already exists, please choose another one.                                                                                 
Give the name of variable # 3                                                                                                                         
John                                                                                                                                                  
J,o,h,n,                                                                                                                                              
J,o,h,n,                                                                                                                                              
John                                                                                                                                                  
George                                                                                                                                                
John                 

What I am guessing this problem is about, is that the memory the system is allocating to temp and varray are already being used elsewhere. This errorcheck is crucial for a project I have to do, so I would appreciate any help I can get in solving this problem greatly. Thanks in advance,

LukeSykpe

Answer 1

The problem is with your printing logic.

The scanf function writes the user input into the array, followed by a terminating `\\0' character. It does not know the size of your array (20), so it doesn't touch the part of the array that it doesn't actually write.

Instead of this:

for (k = 0; k < ARRAYLENGTH; k++){

write:

for (k = 0; temp[i].name[k] != '\0'; k++) {

Note that you don't need to check for running off the end of the array here. Instead, make sure that the user string is not too big for your array. See this for how to do that.

Answer 2

Edit : This post is not to answer the original question, but to answer a follow-up question posted in comments. I tried to incorporate this into the previous answer, but the owner refused. So here it is.

The problem with your varray comparisons is that, with the code you are showing at least, varray is never initialized. So

if (varray[j].name[k] != temp[i].name[k])

Is a bit like taking a random byte in memory, assigning it to a variable and doing this :

if (RandomByteValue != temp[i].name[k])

Which 90% of the time will be true thus setting your flag to 1. Essentially, you're missing a

varray[i] = lastVariableGotFromUser

At the end of each main loop.

--- Edit : Added in minor corrections to general functionality ---

Try adding in this :

int added = 1;

Then change this :

for (j = 0; j < amount; j++){

with :

for (j = 0; j < added; j++){

and add in :

        if (flag == 1){
            // Your for loop
            added += 1;
        }

What was happening was that you iterated through fields of varray that were uninitialized, and contained random memory. With those modifications (If i didn't forget one, it should work. Try to always limit your loops to only the useful iterations. If you know you only have one variable added in, don't iterate through the 3 fields.

------- Last edit to correct a detail in his code -------

So, your whole :

for (k = 0; temp[i].name[k] != '\0'; k++){

Can be deleted. Now i also know that you don't want to use string.h, However, recoding a strcmp ain't all that complicated. Let's call it

int comp_str(str, str2) // Returns 1 if they don't match, zero if they do.

then just replace your whole for with :

if (comp_str(temp[i].name, varray[j].name) == 0) {
    flag = 0;
    break;
}
else
    flag = 1;

You only want to set the flag when a whole string has been analyzed. So pass it to another function, act upon the return value, and it works! Generally slice your code up. Easier to act/think on. (and also avoids having things like varray[j].name[k] != temp[i].name[k] which is long an not so pleasing to read, in your code.)