Thread: Converting string to integer

  1. #1
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    Converting string to integer

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <string.h>
    void St2Int() {
        char inp[10];
        printf("Input String number\n");
        scanf("%s", inp);
        int *p;
        p = (int *)malloc(strlen(inp));
        p = (int *)(inp);
        int res = *p;
        printf("%d\n", res);
    }
    I am trying to store the input string value in a char pointer first, then, caste it to an integer pointer and then save it in an integer variable, but I am getting wrong output.
    My method is stupid but I am a beginner and curious why this code is not working.

  2. #2
    Registered User ssharish2005's Avatar
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    it could be as simple as this

    Code:
        char str[] = "100";
        int number;
    
        sscanf( str, "%d", &number );
        printf( "number is - %d\n", number );
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

  3. #3
    and the hat of int overfl Salem's Avatar
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    strtol() or strtoul() should be the preferred functions for converting strings to integers.

    These functions can detect numeric overflow, which sscanf cannot.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  4. #4
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    Quote Originally Posted by ssharish2005 View Post
    it could be as simple as this

    Code:
        char str[] = "100";
        int number;
    
        sscanf( str, "%d", &number );
        printf( "number is - %d\n", number );
    I didn't know about sscanf() function.
    But my question is, why is the variable res not storing the value p is pointing to, in fact, is p pointing to the input value at all?

  5. #5
    Hurry Slowly vart's Avatar
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    inout string ("100" contains following chars
    Code:
    {'1', '0', '0', 0}
    for example

    in hex it will be
    Code:
    {0x31, 0x30, 0x30, 0x00}
    This is completely different from hex representation of number 100
    Code:
    {0x64, 0x00, 0x00, 0x00}
    So just casting pointer to first array of bytes and telling it to interpret the contents as integer instead of string will not magically replace contents of the array.

    You need change the contents of the memory not just the way the memory byte are read
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  6. #6
    Registered User MutantJohn's Avatar
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    Quote Originally Posted by amit_s95 View Post
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <string.h>
    void St2Int() {
        char inp[10];
        printf("Input String number\n");
        scanf("%s", inp);
        int *p;
        p = (int *)malloc(strlen(inp));
        p = (int *)(inp);
        int res = *p;
        printf("%d\n", res);
    }
    I am trying to store the input string value in a char pointer first, then, caste it to an integer pointer and then save it in an integer variable, but I am getting wrong output.
    My method is stupid but I am a beginner and curious why this code is not working.
    I don't know if this is relevant or not but I don't think you even needed to malloc p anyway when all you do is immediately reassign the value of the pointer to a stack-based allocation and then store the value of the bytes there interpreted as an integer in a local variable. This would also create a memory leak that you can't fix because you overwrite the location of the heap allocation.

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