CODEWITHSUNDEEP
cpointersstrcat
Favolas
Favolas

Reputation: 7243

c strcat with pointer

I'm trying to use pointers and strcat from C. This is part of my learning process.

The idea is the user inputs a string that contains digits and the output should return only the digits. So, if the user inputs te12abc the output should be 12.

This is my first try:

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

#define SIZE 10

int main()
{
    char palavra[SIZE];
    char palavra2[SIZE];
    char *pont = palavra;
    char *pont2 = palavra2;

    printf("Insert the string\n");
    scanf("%s", palavra);

    do{
        if (isdigit(*pont)){
            strcat(palavra2, *pont);
        }
        *pont++;
    }while (*pont != '\0');

    printf("\nThe number is:\n%s\n", palavra2);
    return 0;
}

I believe that the pointer is working as expected but can't understand why strcat is not working.

Made a second try that is the program finds a number, stores that char in one variable and only then try to use strcat with that variable. Here is the code:

int main()
{
    char palavra[SIZE];
    char palavra2[SIZE];
    char temp;
    char *pont = palavra;
    char * pont2 = &temp;

    printf("Insert the string\n");
    scanf("%s", palavra);

    do{
        if (isdigit(*pont)){
            temp = *pont;
            strcat(palavra2, pont2);
        }
        *pont++;
    }while (*pont != '\0');

    printf("\nThe number is:\n%s\n", palavra2);
    return 0;
}

Once again it gives me problems at strcat.

Made one last attempt but without pointer and still strcat does not work. Here is the code:

int main()
{
    int i = 0;
    char palavra[SIZE];
    char palavra2[SIZE];
    char temp;

    printf("Insert the string\n");
    scanf("%s", palavra);

    do{
        if (isdigit(palavra[i])){
            temp = palavra[i];
            strcat(palavra2, palavra[i]);
        }
        i++;
    }while (palavra[i] != '\0');

    printf("\nThe number is:\n%s\n", palavra2);
    return 0;
}

Can you point me to the right direction? Don't now what more can I do..

Regards,

favolas

Upvotes: 2

Views: 18259

Answers (4)

P.P
P.P

Reputation: 121427

Porblems with your code: (1st version)

1) You do strcat but *pont refers to single char, which is not a null-terminated string.

2) You do *pont++; But *pont is a value, not a pointer.

Do this change to the 1st version: It should be ok.

 do{
        if (isdigit(*pont)){
            *pont2=*pont;
             pont2++;
        }
        pont++; 
    }while (*pont != '\0');

*pont2='\0';

Upvotes: 3

Pavan Manjunath
Pavan Manjunath

Reputation: 28565

Your third try is almost correct.

Replace

strcat(palavra2, palavra[i]);

with

strncat(palavra2, palavra+i,1);

I am passing palavra+i and not palavra[i] cos the former is a progressing pointer and the latter is a character and strncat needs pointers

This is a nice example to illustrate how to concatenate a string and a character

Also, make sure you always initialize your variables

char palavra[SIZE]="";
char palavra2[SIZE]="";

Upvotes: 3

stefan bachert
stefan bachert

Reputation: 9624

Remove * (dereference),

        strcat(palavra2, pont);

strcat expects a char* not a char but this version appends the whole rest. you have to create a nul-terminated string.

And the * is useless

    *pont++;

This does the job

    pont++;

now all at once

int main()
{
  char palavra[SIZE];
  char palavra2[SIZE];
  char c2[2] = "a";
  char *pont = palavra;
  char *pont2 = palavra2;

  printf("Insert the string\n");
  scanf("%s", palavra);

  do{
    if (isdigit(*pont)){
      c2[0] = *pont;
      strcat(palavra2, c2);
    }
    pont++;
}while (*pont != '\0');

printf("\nThe number is:\n%s\n", palavra2);
return 0;

However, this is too complicated

int main()
{
  char palavra[SIZE];
  char palavra2[SIZE];


  printf("Insert the string\n");
  scanf("%s", palavra);

  char *pont = palavra;
  char *pont2 = palavra2;

  while (true) {
    char c = *pont ++;
    if (c == 0) break;
    if (isdigit(c)){
       *pont2++ = c;
    }
  };
  printf("\nThe number is:\n%s\n", palavra2);
  return 0;

Upvotes: 1

cnicutar
cnicutar

Reputation: 182734

You're making it harder than it has to be. You don't need strcat:

/* Untested. */   
char *pont = palavra;
char *pont2 = palavra2;

while (*pont) {
    if (isdigit(*pont))
        *pont2++ = *pont;

    pont++;
}
*pont2 = 0;

Upvotes: 3

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