#include <stdio.h>
main()
{
    char uptoflow(char ch);
    char ch;
    printf("\n enter the letter:\n");
    do{
    scanf("%c",&ch);
    printf("%c - %c \n",ch,uptolow (ch));
    }while(ch!='\n');
    printf("\n");
}


char uptolow(char ch1)
{

if(ch1>='A'&&ch1<='Z')
ch1=ch1+'a'-'A';
return(ch1);
}

错误提示：
        C:\Users\david\Desktop\c\A-a.cpp    In function 'int main()':
9    36    C:\Users\david\Desktop\c\A-a.cpp    [Error] 'uptolow' was not declared in this scope

注意函数名称的拼写  请逐一核对

1，main如果不声明为void 类型，则系统默认为int型，对应int型函数，需要添加return 0；
2.申明的函数和调用的函数不一致

#include <stdio.h>
char uptolow(char ch1);//需要先声明函数，增加这一行
main()
{
    char uptoflow(char ch);
    char ch;
    printf("\n enter the letter:\n");
    do {
        scanf("%c", &ch);
        printf("%c - %c \n", ch, uptolow(ch));
    } while (ch != '\n');
    printf("\n");
}


char uptolow(char ch1)
{

    if (ch1 >= 'A'&&ch1 <= 'Z')
        ch1 = ch1 + 'a' - 'A';
    return(ch1);
}

仔细看了下，下面这样改也行

#include <stdio.h>

main()
{
    char uptoflow(char ch);//函数名不一致
    char ch;
    printf("\n enter the letter:\n");
    do {
        scanf("%c", &ch);
        printf("%c - %c \n", ch, uptoflow(ch));//函数名不一致
    } while (ch != '\n');
    printf("\n");
}


char uptoflow(char ch1)//函数名不一致
{

    if (ch1 >= 'A'&&ch1 <= 'Z')
        ch1 = ch1 + 'a' - 'A';
    return(ch1);
}

谢谢

谢谢