C general programming with strings

Question

If I am not allowed to use the <string.h> library, how can I easily compare values of a string. I have a data file with 6 possible values for one member of a structure. All I need to do is create a loop to count how many of each value is present in an array of structs. The problem is, I cannot figure out how to compare the value and thus when to increment the counter.

 for (i = 0; i < datasize; i++){
    if (struct.membervalue == given)
      givencount++;
    if (struct.membervalue == given2)  // But I can't compare them with the == 
      givencount2++ ;                  // because they are strings.
 }

EDIT: predefined enum that I MUST USE

 typedef enum { 
     penny = 1,
     nickel = 5,
     dime = 10,
     quarter = 25
 }changeT;

I have the value "penny" how do I compare to this or relate it?

Maybe this link can help stackoverflow.com/questions/4497680/c-strcmp-source-code — Oki Sallata
– Oki Sallata, Commented Jan 29, 2013 at 4:08
I believe there is an easier answer. I have access to enum types of each value, but I do not know how to translate my raw data into those enum types. I do not believe I am supposed to do any advanced string processing — SystemFun
– SystemFun, Commented Jan 29, 2013 at 4:14
@Vlad: I thought these were strings -- that's what you asked about. Are they actually enums and not strings at all? If so, == should work fine. — David Schwartz
– David Schwartz, Commented Jan 29, 2013 at 4:18
I have strings, but the exercises predefines an enum. So my data is in the format of a string, but I have a pre-defined enum type that has for its domain the value that my strings represent. — SystemFun
– SystemFun, Commented Jan 29, 2013 at 4:20

David Schwartz · Accepted Answer · 2013-01-29 16:13:50Z

2

bool isEqual(const char *string1, const char *string2)
{
    do
    {
        if (*string1 != *string2) return false;
        if (*string1 == 0) return true;
        ++string1;
        ++string2;
    } while (1);
}

Update: The enum doesn't change anything. You still have to identify the string "penny" before you can assign it the value for a penny.

edited Jan 29, 2013 at 16:13

answered Jan 29, 2013 at 4:09

David Schwartz

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4 Comments

SystemFun Over a year ago

That was my idea as well, but I think I am supposed to do something with enum types that I have been given.

David Schwartz Over a year ago

What types are membervalue and given?

David Schwartz Over a year ago

So why are you bringing enums into it?

SystemFun Over a year ago

They are predefined requirnments for the problem

Alex · Accepted Answer · 2013-01-29 07:37:02Z

1

You can try the following function:

int str_cmp(const unsigned char* str1, const unsigned char* str2)
{
    int result;

    do {
        result = (int)*str1 - (int)*str2;
        str1++;
        str2++;
    } while((!result) && (*str1|*str2))

    return result;
}

Output is a positive if str1>str2, negative if str1<str2 and zero if they are equal.

edited Jan 29, 2013 at 7:37

answered Jan 29, 2013 at 7:18

Alex

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1 Comment

qPCR4vir Over a year ago

str_cmp("","") - could be a problem?

Fredrick Gauss · Accepted Answer · 2013-01-29 07:34:09Z

0

Fastest one:

int strcmp(const char *s1, const char *s2) {  
  int ret = 0;  

  while (!(ret = *(unsigned char *) s1 - *(unsigned char *) s2) && *s2) 
    ++s1, ++s2;  

  if (ret < 0) {
    ret = -1;  
  } 
  else if (ret > 0) {
    ret = 1 ;
  }  

  return ret;
 }

answered Jan 29, 2013 at 7:34

Fredrick Gauss

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2 Comments

Alex Over a year ago

What is actual difference with my answer, except result -1,0,+1 ?

Fredrick Gauss Over a year ago

i dont know but they say this was the implementation of strcmp function in the string library as i remember. Maybe difference with do-while and bitwise op.

qPCR4vir · Accepted Answer · 2013-02-04 11:54:01Z

/*These variants could point to invalid memmory, but dont de-reference it.*/
int isEqual(const char *string1, const char *string2)
{
    while (*string1 == *string2++)      
        if ( 0 == *string1++  )      return 1;
    return 0;
 } 

/* This variant is NULL-resistent. For both NULL return true.*/
int isEqual(const char *string1, const char *string2)
{
    if ( !string1 || !string2   )      return  string1 == string2 ;

    while (*string1 == *string2++)      
        if ( 0 == *string1++  )      return 1;
    return 0;
 }

These are only the function to compare strings. In order to help more we need to see the code you are trying. It could be something like:

if (isEqual(data.membervalue, "penny" )   pennycount++;
else
if (isEqual(data.membervalue, "nickel")   nickelcount++;

And the enum you provided is not of great help to count. It is useful to calculate the "monetary" total.

int Total= penny * pennycount  + nickel * nickelcount ... ;

If all you need is the total, thing get simpler:

if (isEqual(data.membervalue, "penny" )   Total += penny;
else
if (isEqual(data.membervalue, "nickel")   Total += nickel;

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