How to convert a string to uppercase in C

Question

I'm trying to make all keywords from C into uppercase from a C file, but when I try to use the toupper function from stdlib it only accepts char type

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

const char *words[] = {
  "auto", "break", "case", "char", "continue", "do", "default", "const","double", "else", "enum", "extern", "for", "if", "goto", "float", "int", "long", "register", "return", "signed", "static", "sizeof", "short", "struct","switch","typedef","union","void",  "while","volatile", "unsigned"
};


int main()
{
  char line[200];
  size_t len = 0;

  FILE *f;
  f = fopen("main.c", "r");

  int line_no = 0;
  while (fgets(line, sizeof line, f))
  {
    ++line_no;
    for (size_t i = 0; i < (sizeof words)/sizeof *words; i++)
    {
      if (strstr(line, words[i]) != NULL && line_no > 8)
      {
        printf("%s", words[i]);
      }
    }
  }
  fclose(f);
}

That's right, it only works on characters. You need to call it in a loop to uppercase all the characters in a string. — Barmar
– Barmar, Commented Aug 6, 2022 at 0:46
Your sample code never calls toupper. What are you trying to do exactly? Do you want to convert all of line to uppercase, or do you just want to convert words[i] to uppercase? — William Pursell
– William Pursell, Commented Aug 6, 2022 at 0:49

score 1 · Accepted Answer · 2022-08-08 20:52:18Z

1

It's raining, and this challenge seemed interesting.

Here's a rough cut of what you might expand to suit your needs.

Instead of dealing with one line at a time (fgets), this simulates loading the entire file (fopen/ftell/fseek/fread) with a single long 'target' string.

It loops through each of the keywords, uppercase'ing when it encounters one. (You need to be careful... Notice the test that ensures "longval" is not made uppercase.)

Thanks for the fun!

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

const char *words[] = {
    "auto", "break", "case", "char", "continue", "do", "default", "const",
    "double", "else", "enum", "extern", "for", "if", "goto", "float", "int",
    "long", "register", "return", "signed", "static", "sizeof", "short",
    "struct","switch","typedef","union", "void", "while", "volatile", "unsigned"
};

void uc( char *buf, const char *word ) {
    int len = strlen( word ); // may need this...
    bool found = false;

    for( char *cp = buf; ( cp = strstr( cp, word ) ) != NULL; cp++ ) {
        char c = cp[len]; // one char beyond keyword length
        if( !isalpha( c ) && !isdigit( c ) && c != '_' ) {
            found = true;
            for( int i = 0; i < len; i++ )
                cp[i] = (char)toupper( cp[i] );
        }
    }
    if( found )
        printf( "\n\n**FOUND** '%s'\n\n%s", word, buf );
}

int main() {
    char *src = 
        "#include <stdio.h>\n\n"
        "int func( int x ) {\n    return (x * x);\n}\n\n"
        "int main( void ) {\n"
        "    int longval = 3;\n"
        "    while( ( longval = func( longval ) ) < 10000 )\n"
        "        printf( \"%d\\n\", longval );\n"
        "    return 0;\n"
        "}\n";

    // some compilers disallow modifying string literals
    char *copy = malloc( strlen( src ) + 1 );
    strcpy( copy, src );

    printf( "%s", copy );

    for( int i = 0; i < sizeof words/sizeof words[0]; i++ )
        uc( copy, words[i] );

    free( copy );
    return 0;
}

I make no claim that this is complete. My hope is that you will be able to understand what is needed.

Output of final printf(): (added as 'proof' of functionality). Notice that return, int, void and while have been UPPERCASE'd.

**FOUND** 'while'

#include <stdio.h>

INT func( INT x ) {
    RETURN (x * x);
}

INT main( VOID ) {
    INT longval = 3;
    WHILE( ( longval = func( longval ) ) < 10000 )
        printf( "%d\n", longval );
    RETURN 0;
}

edited Aug 8, 2022 at 20:52

answered Aug 6, 2022 at 3:02

user17592432

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5 Comments

David C. Rankin Over a year ago

You can call toupper() on any ASCII character. It will be left unchanged if not a lower-case alpha character.

MackProgram Over a year ago

I was trying to test your code, but it only returns a segmentation fault

user17592432 Over a year ago

@MackProgram Are you running this code, or might you have modified it to load the contents of a file into a buffer? If so, perhaps you need to add a '\0' byte to the end of the buffer that is 'size + 1' characters long... strstr() needs that '\0` to not run off into space...

MackProgram Over a year ago

I just copied and run the code, but I'm gonna try to do that

user17592432 Over a year ago

@MackProgram Just thought... Compiler differences... Code from my (old) C compiler does not prevent modifying a "string literal" (as used in my example)... Your C compiler may be newer and prevent that... I'm about to modify the example,. You could grab a copy of the revised version...

Locke · Accepted Answer · 2022-08-07 03:23:22Z

1

You need to convert the string one character at a time. Since string literals are stored in read-only memory, we need to allocate space for a new string and copy over the characters one by one. However, you will need to remember to free these after use. Alternatively, if we know all the strings are short, you could alternatively do this in an array on the stack.

char *toUppercase(char *lower) {
    int len = strlen(lower);

    // Allocate space for new string
    char *upper = (char *) malloc(sizeof(char) * (len + 1));

    // Add null terminator to string
    upper[len] = '\0';

    // Convert characters to uppercase one by one
    for (int i = 0; i < len; i++) {
        upper[i] = toupper(lower[i]);
    }

    return upper;
}

Edit: While I use int in this example, size_t is preferable to avoid the an integer overflow on large strings causing bytes to be accessed outside the bounds of the string (Effects strings with over 2,147,483,647 bytes on most systems depending on the compiler configuration. Strings under 32,768 bytes will not be effected regardless of compiler configuration).

edited Aug 7, 2022 at 3:23

answered Aug 6, 2022 at 0:54

Locke

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4 Comments

MackProgram Over a year ago

Thanks, do you know how can I replace the lowercase words to the uppercase ones /

chux Over a year ago

When lower[i] < 0, toupper(lower[i]) is not well defined. Better as toupper(*(unsigned char*) lower[i]).

chux Over a year ago

int len shoudl use size_t len = strlen(lower); to handle all string lengths.

Locke Over a year ago

@chux-ReinstateMonica I decided to use int instead of size_t to try to be more beginner friendly. While size_t would be objectively better, I was not confident that the asker understood the differences given the subject matter of the question so this seemed like a safer option for explaining the concept. However, by the time I finished writing the code block I had forgotten to leave a note about it.

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