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copied!<pre><code>for (i = 0; str1[i] && str2[i]; i++) </code></pre> <p>is the same as</p> <pre><code>for (i = 0; (str1[i] != 0) && (str2[i] != 0); i++) </code></pre> <p>which is the same as</p> <pre><code>for (i = 0; (str1[i] != '\0') && (str2[i] != '\0'; i++) </code></pre> <p>Basically if any expression is used in a conditional statement, then the value is checked for boolean - true or false. If it's not a boolean - say it's an integral type, then 0 is <code>false</code> anything else is <code>true</code>.</p> <p>Here <code>str[i]</code> is a <code>char</code> - which is an integral type. So if str[i] is 0, then it evaluates to false, else it evaluates to true.</p> <p>For eg.</p> <pre><code>char c = 'A'; if(c) </code></pre> <p>is the same as</p> <pre><code>if (c != 0) </code></pre> <p>which is the same as</p> <pre><code>if (c != '\0') </code></pre> <p>c is 'A' (which in ascii is 65). 65 != 0, hence it evaluates to true & the <code>if</code> will be entered.</p> <p>if instead, you have</p> <pre><code>char c = 0; </code></pre> <p>or</p> <pre><code>char c = '\0'; </code></pre> <p>then c evaluates to 0. Hence if(c) evaluates to false & the if is not entered.</p> <p>You can extend the same logic to str[i] which is an <code>char</code>.</p> <p>If you have <code>str1[]="rate"</code>, it's the same as <code>str1[0] = 'r', str1[1] = 'a', str1[2] = 't', str1[3] = 'e', str1[4] = 0</code>.</p> <p>About the <code>count1[str1[i]]++;</code></p> <p>It's a count of how many times each character occurs - for eg. if the char set is ascii, then at the end of string traversal, count['A'] (which is the same as count[65]) will contain the number of times 'A' occurred in the string.</p> <p>It will work only if each member of the count arrays are initialized to <code>0</code> somewhere (or they are global).</p> <p>consider </p> <pre><code>str1[] = "ABAB"; </code></pre> <p><code>count[str1[0]]</code> is same as <code>count['A']</code> which is same as <code>count[65]</code> (if char set is ascii).</p> <p>The <code>++</code> will cause count['A'] to become 1</p> <p>When <code>i</code> becomes 1, <code>count[str1[1]]++</code> causes <code>count['B']</code> to become 1. <code>i</code> = 2, then <code>count['A']</code> becomes 2. <code>i</code> = 3, then <code>count['B']</code> becomes 2.</p>

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